551 
 
 ,ESE LIBRARY 
 
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 ^ 
 
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THE 
 INTERNATIONAL SCIENTIFIC SERIES. 
 
 VOL. LIX. 
 
WE A T H E E 
 
 A POPULAR EXPOSITION 
 
 OF THE 
 
 NATURE OF WEATHER CHANGES 
 FROM DAY TO DAY 
 
 BY THE 
 
 HON. RALPH ABERCROMBY 
 
 FELLOW OF THE ROYAL METEOROLOGICAL SOCIETY, LONDON; 
 
 MEMBER OF THE SCOTTISH METEOROLOGICAL SOCIETY; AND AUTHOR OF 
 
 "PRINCIPLES OF FORECASTING BY MEANS OF WEATHER CHARTS" 
 
 SECOND EDITION 
 
 LONDON 
 KEG AN PAUL, TRENCH & CO., 1, PATERNOSTER SQUARE 
 
 1888 
 
7 
 
 (77r rights of translation and of reproduction are reserved.) 
 
PREFACE. 
 
 THE object of this work is the same as that of other 
 volumes of the International Scientific Series, to which 
 it belongs viz. to place before the general reader a short 
 but clear picture of the modern /aspects of the science of 
 which it treats. \ 
 
 With this view, the more elementary parts of the 
 subject weather science have been treated of in the 
 first three chapters of the book, while the more difficult 
 questions are reserved for the later portion of the work. 
 Though this method of treatment involves a certain 
 amount of repetition, the author hopes that the work 
 may thus prove acceptable to a large number of readers 
 who would have been deterred from the perusal of a more 
 formal treatise. 
 
 This book is not intended to be in any way an 
 encyclopaedia of meteorology, or a mere repertory of 
 facts. Our endeavour has been to sketch the great 
 principles of the science as a whole, and to give a clear 
 
 b 
 
VI PREFACE 
 
 picture of the general conclusions as to the actual nature 
 of weather to which meteorologists have been led. 
 
 Many books have been written on storms and climate, 
 but none on everyday weather. The whole of this work 
 is devoted to weather, in the tropics as well as in the 
 temperate zone. 
 
 This volume is not a mere compilation of existing 
 knowledge, for the results of many of the author's original 
 and unpublished researches are included in its pages. 
 Such, for instance, as the explanation of many popular 
 prognostics ; the elucidation of the general principles of 
 reading the import of cloud-forms ; the classification of 
 those cases in which the motion of the barometer fails to 
 foretell correctly the coming weather ; and the character 
 of that kind of rainfall which is not indicated in any way 
 by isobaric maps. 
 
 Most of the charts are derived from the publications 
 of various meteorological offices ; but almost all the 
 diagrams have been drawn for this work, or have only 
 appeared in some of the author's papers. 
 
 Every endeavour has been made to do justice to the 
 discoverers of any new principle, but it has not been 
 considered necessary to give references to all the original 
 authorities in a popular work. 
 
 To those who have only known meteorology as a dull 
 branch of statistics, the perusal of these pages may 
 perhaps open a new prospect in science, and a new vision 
 to the mind. 
 
PREFACE. Vll 
 
 The author wishes specially to acknowledge the 
 assistance which he has received from the Meteorological 
 Office in London, and the United States Signal Office, by 
 the supply of the material contained in some of their 
 various publications ; and also the courtesy of the Council 
 of the Koyal Meteorological Society of London, in lending 
 him the blocks which have illustrated several of his own 
 papers. 
 
 His thanks are also due to Mr. E. Ellery, of Melbourne 
 Observatory, and Mr. W. E. Cooke, of Adelaide Observa- 
 tory, for information and diagrams to illustrate Australian 
 weather and forecasts ; and to Mr. H. F. Blanford, 
 meteorological reporter for the Government of India, for 
 information and material relating to the nature of the 
 monsoons. 
 
CONTENTS. 
 
 PART I. ELEMENTARY. 
 CHAPTER I. 
 
 INTRODUCTORY. 
 
 PAGE 
 
 Myths ... ... ... ... ... ... ... 3 
 
 Prognostics ... ... ... ... ... ... 4 
 
 The barometer ... ... ... ... ... ... 4 
 
 Statistics ... ... ... ... ... ... 6 
 
 Synoptic charts ... ... ... ... ... ... 7 
 
 Theoretical developments ... ... ... ... 11 
 
 Plan of the book ... ... ... ... ... ... 12 
 
 CHAPTER II. 
 
 WEATHER-PROGNOSTICS. 
 
 Introduction ... ... ... ... ... ... 16 
 
 Early explanations ... ... ... ... ... 17 
 
 Modern developments ... ... ... ... ... 18 
 
 Synoptic charts ... ... ... ... ... 18 
 
 Relation of wind and weather to isobars ... ... ... 23 
 
 The seven fundamental shapes of isobars ... ... ... 25 
 
 Cyclone-prognostics ... ... ... ... ... 27 
 
Xll CONTENTS. 
 
 PAGE 
 
 Variations in velocity and gradient ... ... ... 187 
 
 Relation of direction to gradient ... ... ... 191 
 
 Inclination of wind to isobars ... ... ... ... 192 
 
 Calms ... ... ... ... ... ... 194 
 
 Winds in southern hemisphere ... ... ... ... 194 
 
 General remarks ... ... ... ... ... 199 
 
 Relation of force to velocity ... ... ... ... 202 
 
 CHAPTER VII. 
 
 HEAT AND COLD. 
 
 Diurnal isotherms ... ... ... ... ... ... 204 
 
 How diurnal modify general isotherms ... ... ... 208 
 
 Temperature-disturbance of a cyclone ... ... ... 213 
 
 Sources of heat ... ... ... ... ... 217 
 
 Sources of cold ... ... ... ... ... ... 220 
 
 The "Blizzard" and the "Barber" ... ... ... 223 
 
 Examples of daily temperature-changes over Europe ... ... 226 
 
 Forecasting temperature ... ... ... *.. 230 
 
 Primary and secondary effects of heat ... ... ... 231 
 
 CHAPTER VIII. 
 
 SQUALLS, THUNDERSTORMS, AND NON-ISOBARIC RAINS. 
 
 Simple squalls ... ... ... ... ... ... 234 
 
 Thunder- squalls ... ... ... ... ... 235 
 
 Barometer in squalls and thunderstorms ... ... ... 236 
 
 Line-squalls ... ... ... ... ... ... 240 
 
 Thunderstorms associated with line-squalls ... ... ... 245 
 
 Thunderstorms with secondaries ... ... ... 254 
 
 General remarks ... ... ... ... ... ... 257 
 
 Non-isobaric rains ... ... ... ... ... 259 
 
 The south-west monsoon ... ... ... ... 259 
 
CONTENTS. Xlll 
 
 CHAPTER IX. 
 
 PAMPEROS, WHIRLWINDS, AND TORNADOES. 
 
 PAGE 
 
 Pamperoa... ... ... ... ... ... ... 263 
 
 Whirlwinds ... ... ... ... ... ... 267 
 
 Tornadoes ... ... ... ... ... ... 267 
 
 Eelation of whirlwinds to cyclones ... ... ... 277 
 
 CHAPTER X. 
 
 LOCAL VARIATION OP WEATHER. 
 
 Nature and principles ... ... ... ... ... 280 
 
 Local cloud ... ... ... ... ... ... 282 
 
 rain ... ... ... ... ... ... 284 
 
 Mountain rain ... ... ... ... ... 286 
 
 Valley rain ... ... ... ... ... ... 287 
 
 Localization of hailstorms ... ... ... ... 288 
 
 Tidal showers ... ... ... ,.. ... ... 291 
 
 CHAPTER XL 
 
 DIURNAL VARIATION OP WEATHER. 
 
 Independence of diurnal variations and general changes ... 294 
 
 Diurnal temperature ... ... ... ... ... 294 
 
 cloud ... ... ... ... ... ... 299 
 
 rain ... ... ... ... ... ... 301 
 
 wind ... ... ... ... ... ... 304 
 
 velocity ... ... ... ... ... 304 
 
 direction ... ... ... ... ... 306 
 
 General view of the subject ... ... ... ... 310 
 
 CHAPTER XII. 
 
 ANNUAL AND SECULAR VARIATIONS. 
 
 Seasonal appearance of the sky ... ... ... ... 312 
 
 Recurrent types of weather ... ... ... ... 312 
 
LIST OF ILLUSTRATIONS. 
 
 FIG. PAGB 
 
 1. Fundamental shapes of isobars ... ... . ... 25 
 
 2. Cyclone-prognostics ... ... ... ... 28 
 
 3. Weather sequence in a cyclone ... ... ... ... 40 
 
 4. "Weather in secondary (synoptic) ... ... ... 43 
 
 5. Weather sequence in secondary ... ... ... ... 46 
 
 6. Anticyclone prognostics ... ... ... ... 48 
 
 7. Wedge-shaped isobar prognostics ... ... ... 54 
 
 8. Straight isobar prognostics ... ... ... ... 60 
 
 9. Halo prognostic failure ... ... ... ... 66 
 
 10. 66 
 
 11. Cumulus and cirrus ... ... ... ... ..* 73 
 
 12. Festooned cumulus ... ... ... ... 78 
 
 13. Cumulus, degraded cumulus, and line cumulus ... ... 80 
 
 14. Formation of cloud-stripes ... ... ... ... 85 
 
 15. Cloud-perspective ... ... ... ... ... 87 
 
 16. Surface and highest currents over cyclones and anticyclones 93 
 
 17. Converging striated cirrus-stripes ... ... ... 97 
 
 18. Fleecy cirro-cumulus ... ... ... ... 104 
 
 19. Strato- cumulus ; roll cumulus ... ... ... ... 110 
 
 20. Vertical gradients oVTSr cyclone and anticyclone ... 139 
 
 21. Cyclone weather ... ... ... ... ... 142 
 
 22. Anticyclone weather ... ... ... ... 142 
 
 23. Weather in Y-depression ... ... ... ... 144 
 
 24. Wind in a "col" ... ... ... ... ... 148 
 
 25. A meteogram ... ... ... ... ... 152 
 
PART I. 
 ELEMENTARY. 
 
WEATHER 
 
 CHAPTEK L 
 
 INTRODUCTORY. 
 
 THE earliest records of weather among every nation are 
 to be found in those myths, or popular tales, which, while 
 describing rain, cloud, wind, and other natural phenomena 
 in highly figurative language, refer them to some super- 
 natural or personal agency by way of explanation. 
 
 The most interesting thing about these mythical 
 stories is the remarkable fidelity with which they reflect 
 the climate of the country that gave them birth. For 
 example, from the mythologies of Greece and Scandinavia 
 we can almost construct an account of the climate of 
 those two countries by simply translating the figurative 
 phraseology of their legends into the language of modern 
 meteorology. 
 
 Many survivals of mystic speech are still found among 
 popular prognostics, and especially in cloud names. 
 
 In England and Sweden " Noah's Ark " is still seen 
 in the sky, while in Germany the " Sea-Ship " still turns 
 
4 WEATHER. 
 
 its head to the wind before rain. In Scotland the " Wind- 
 Dog " and the " Boar's Head " are still the dread of the 
 fisherman, while such names as " Goat's Hair " and 
 "Mare's Tails" recall some of the shaggy monsters of 
 antiquity. 
 
 PROGNOSTICS. 
 
 At a rather later period of intellectual development, 
 the premonitory signs of good or bad weather become 
 formulated into short sayings, or popular prognostics. A 
 large number of these are still current in every part of 
 the world, but their quality and value is very varied. 
 Some represent the astrological attitude of mind, by 
 referring weather changes to the influence of the stars or 
 phases of the moon ; others, on the contrary, are very 
 valuable, and, in conjunction with other aids to weather- 
 forecasting, prognostics will never be entirely superseded, 
 especially for use on board ship. Till within a very 
 recent period, their science and explanation had hardly 
 advanced since they were first recorded. In many cases 
 the prognostics came true"; when they failed, no explana- 
 tion could be suggested why they did so ; neither could 
 any reason be given why the same weather was not always 
 preceded by the same signs. A halo sometimes precedes 
 a storm ; why does it not always do so ? Why is rain 
 sometimes preceded by a soft sky, and sometimes by hard 
 clouds ? 
 
 THE BAROMETER. 
 
 About one hundred and fifty years ago the barometer 
 was invented. Very soon after that discovery, observation 
 
INTRODUCTORY. 5 
 
 showed that, in a general way, the mercury fell before 
 rain and wind, and rose for finer weather. Also that bad 
 weather was more common when the whole level of the 
 barometer was low, independent of its motion one way or 
 the other, than when the level was high. But as with 
 prognostics, so with these indications, many failures 
 occurred. Sometimes rain would fall with a high or 
 rising barometer, and sometimes there would be a fine 
 day with a very low or falling glass. No reason could be 
 given for these apparent exceptions, and the whole science 
 of barometric readings seemed to be shrouded in mystery. 
 
 STATISTICS/ 
 
 The science of probabilities came into existence about 
 the commencement of this century, and developed the 
 science of statistics. By this method the average readings 
 of meteorological instruments, such as the height of the 
 barometer or thermometer, or the mean direction and 
 force of the wind, at any number of places were calculated, 
 and the results were sometimes plotted on charts so as to 
 show the distribution of mean pressure, temperature, etc., 
 over the world. 
 
 By this means a great advance was made. Besides 
 giving a numerical value to many abstract quantities, the 
 plotting of such lines as the isothermals of Dove con- 
 clusively showed that many meteorological elements 
 hitherto considered capricious were really controlled by 
 general causes, such as the distribution of land and sea. 
 
 Still more fruitful were these charts as the parents of 
 the more modern methods of plotting the readings of the 
 
6 WEATHER. 
 
 barometer over large areas at a given moment, instead of 
 the mean value for a month or year. We shall refer to 
 the results which have been thus obtained more fully 
 presently. Then by tabulating statistics of the relative 
 frequency of different winds at sea, many ocean voyages 
 notably those across the " doldrums," or belt of calms 
 near the equator were materially shortened. 
 
 Statistics also of the annual amount of rainfall became 
 of commercial value as bearing on questions of the 
 economic supply of water for large towns, and much 
 valuable information was acquired as to the dependence 
 of mortality on different kinds of weather. Of more 
 purely scientific interest were the variations of pressure, 
 temperature, wind, etc., depending on the time of day, or 
 what are technically known as diurnal variations, which 
 were brought to light by these comparisons. 
 
 This branch of the subject is known as " Statistical 
 Meteorology," and has advanced very little since it was 
 first developed by Dove and Kaeintz. 
 
 When the attempt was made to apply statistics to 
 weather-changes from day to day, it was found that 
 average results were useless. The mean temperature for 
 any particular day of the year might be 50, if deduced 
 from the returns of a great many years, but in any par- 
 ticular year it might be as low as 40, or as high as 60. 
 The first application of the method was made by the 
 great Napoleon, who requested Laplace to calculate when 
 the cold set in severely over Russia. The latter found 
 that on an average it did not set in hard till January, 
 The emperor made his plans accordingly ; a sharp spell 
 of cold came in December, and the army was lost. 
 
INTKODUCTOKY. 7 
 
 It has now been thoroughly recognized that statistics 
 give a numerical representation of climate, but little or 
 none of weather, and that large masses of figures have 
 been accumulated, to which it is difficult to attach any 
 physical significance. The misuse of statistics has done 
 much to bring the science of meteorology into disrepute. 
 
 SYNOPTIC CHAKTS. 
 
 But within the last twenty years a new treatment of 
 weather problems has been introduced, known as the 
 synoptic method, by which the whole aspect of meteor- 
 ology has been changed. By this method, a chart of a 
 large area of the earth's surface is taken, and after mark- 
 ing on the map the height of the barometer at each 
 place, lines are drawn through all stations at which the 
 barometer marks a particular height. Thus a line would 
 be drawn through all places where the pressure was 
 30*0 inches, another through all where it was 29*8 inches, 
 and so on at any intervals which were considered neces- 
 sary. These lines are called " isobars," because they mark 
 out lines of equal pressure. When these charts were 
 first introduced, the estimation of the value of the mean 
 pressure was so great that, instead of drawing lines 
 where pressure was equal at the moment, they were 
 drawn through those places where the pressure was 
 equally distant from the mean of the day for each place. 
 These lines were called " is-abnormals ; " that is, equal 
 from the mean. This was, however, soon abandoned, for 
 reasons which will be explained farther on in this work. 
 After the isobars have been put in, lines are usually 
 
8 WEATHER. 
 
 drawn through all places where the temperature is equal 
 .at the moment. These are called " isotherms," or lines of 
 equal temperature. Then arrows to mark the velocity 
 and direction of the wind are inserted ; and finally 
 .letters, or other symbols, to denote the appearance of the 
 sky, the amount of cloud, or the occurrence of rain or 
 snow. Such a chart is called a "synoptic chart," 
 because it enables the meteorologist to take a general 
 view, as it were, over a large area. Sometimes they 
 are called " synchronous charts," because they are com- 
 piled from observatioDS taken at the same moment of 
 time. 
 
 When these came to be examined, the following 
 important generalizations were discovered : 
 
 1. That in general the configuration of the isobars 
 assumed one of seven well-defined forms. 
 
 2. That, independent of the shape of the isobars, the 
 wind always took a definite direction relative to the trend 
 of those lines, and the position of the nearest area of 
 low pressure. 
 
 3. That the velocity of the wind was always nearly 
 proportional to the closeness of the isobars. 
 
 4. That the weather that is to say, the kind of cloud, 
 rain, fog, etc. at any moment was related to the shape, 
 and not the closeness, of the isobars, some shapes en- 
 closing areas of fine, others of bad, weather. 
 
 5. That the regions thus mapped out by isobars were 
 constantly shifting their position, so that changes of 
 weather were caused by the drifting past of these areas 
 of good or bad weather, just as on a small scale rain falls 
 as a squall drives by. The motion of these areas was 
 
. 
 INTRODUCTORY. 9 
 
 found to follow certain laws, so that forecasting weather- 
 changes in advance became possible. 
 
 6. That sometimes in the temperate zone, and 'habitu- 
 ally in the tropics, rain fell without any appreciable 
 change in the isobars, though the wind conformed to 
 the general law of these lines. 
 
 Observation also showed that, though the same shapes 
 of isobars appear all over the world, the details of weather 
 within them, and the nature of their motion, are modi- 
 fied by numerous local, diurnal, and 'annual variations. 
 Hence modern weather science consists in working out 
 for each country the details of the character and motion 
 of the isobars which are usually found over it; just as 
 the geologist finds crumplings and denudation all over 
 the world, and works out the history of the physical 
 appearance of his own scenery by studying the local 
 development of these agencies. 
 
 So far the science rests on pure observation that such 
 and such wind or weather comes with such and such a 
 shape of isobars. But it has been found, still farther, that 
 the seven fundamental shapes of isobars are, as it were, 
 the product of so many various ways in which an 
 atmosphere circulating from the equator to the poles 
 may move. Just as the motion of a river sometimes 
 forms descending eddies or whirlpools, sometimes back- 
 waters in which the water is rising upwards, or yet at 
 other times ripples in which the circulation is very com- 
 plex, so it now appears that the general movement of the 
 atmosphere from the equator to the pole sometimes breaks 
 up into a rotating and descending movement round that 
 configuration of isobars known as an anticyclone, some- 
 
10 WEATHER. 
 
 times into a rotating and ascending movement round that 
 known as a cyclone, or at other times quite in a different 
 way during certain kinds of squalls and thunderstorms. 
 
 Isobars, therefore, represent the effect on our barometers 
 of the movements of the air above us, so that by means of 
 isobars we trace the circulation and eddies of the atmosphere. 
 
 By carrying the general laws of physics into the 
 conception of a circulating gas, we find that a cold mixed 
 atmosphere of air and vapour descending into a warmer 
 sqil would remain clear and bright ; while a similar 
 atmosphere rising into cooler strata would condense some 
 of its vapour into rain or cloud. It is by reasoning of 
 this nature that the origin of some of the most beautiful 
 and complex forms of clouds has been discovered. 
 
 Following out these lines of research, a new science 
 of meteorology has grown up, which entirely alters the 
 attitude of mind with which we regard weather-changes, 
 and gives rise to an entirely new method of weather- 
 forecasting that far surpasses all previous efforts, and 
 which explains and develops all that was known before. 
 
 On the one hand, the new method not only explains 
 why certain prognostics are usually signs of good or bad 
 weather, and the reason why the indications sometimes 
 fail ; but also the reason why rain, for instance, is some- 
 times foretold by one prognostic, and sometimes by a 
 totally different one. 
 
 On the other hand, it not only gives a more extended 
 meaning to all the statistics which partially represent the 
 climate of a place, and to the relation of the diurnal to 
 the general changes of weather ; but it also enables new- 
 inferences to be drawn, which had hitherto been im- 
 
INTRODUCTORY. 11 
 
 possible from some observations, and explains why other 
 sets of figures must always remain without any physical 
 significance. 
 
 THEORETICAL DEVELOPMENTS. 
 
 We may notice here an attempt which has been made 
 by one school of meteorologists to deduce all weather 
 a priori from changes in the radiative energy of the sun ; 
 that is to say, that from a knowledge of greater or less heat 
 being emitted by the sun, they would treat the consequent 
 alteration of weather as a direct hydrodynamical problem. 
 Given an earth surrounded by fifty miles of damp air, and 
 a sun at varying altitude, and of varying radiative energy, 
 deduce from that all the diverse changes of weather. This 
 is doubtless a very tempting ideal, for there is no doubt 
 that the sun's heat is the prime mover of all atmospheric 
 circulation ; but when we have explained what the nature 
 of weather-changes is, we shall see that there is little 
 hope that this method will ever lead to satisfactory 
 results. 
 
 Other meteorologists, who lay less stress on the vary- 
 ing power of the sun, have taken up the indications of 
 synoptic charts, and endeavoured to construct a mathe- 
 matical theory of cyclones and the general circulation of 
 the atmosphere. Ferrel, Mohn, Gulberg, Sprung, and 
 others, have all started with the analysis of the motion of 
 a free mass of air on the earth's surface, first given by 
 Professor Ferrel, and worked out, from that and other 
 general principles, schemes of the nature and propagation 
 of cyclones, and of the general distribution of pressure 
 over the world. Though, as will be seen hereafter, the 
 
1 WEATHER. 
 
 science of weather-forecasting can never be treated mathe- 
 matically, still the labours of these writers form a distinct 
 branch of meteorology, and the author regrets that the 
 scope of this work precludes him from giving a chapter 
 which would summarize in a popular manner the results 
 that they have obtained. 
 
 This is the deductive portion of meteorology. We 
 shall confine this work entirely to the inductive branch 
 of the science ; and, independent of any theoretical con- 
 siderations, show the observed association of different 
 groups of phenomena, and the generalizations that have 
 been arrived at by observation only. 
 
 PLAN OF THE BOOK. 
 
 Though a vast amount of work has been given to 
 synoptic meteorology in all parts of the world, still the 
 results obtained by different investigators remain buried 
 in the scattered transactions of innumerable societies, and 
 no book at present exists which contains a methodical 
 statement of what has been achieved. Many isolated 
 principles have been discovered, but no attempt has been 
 made to lay down the broad principles of the science of 
 weather as a whole. The object of this work is to supply 
 that want by putting before the public a short popular 
 account of all the principal results which have been dis- 
 covered in recent years by means of synoptic charts, and 
 of their bearing not only in modifying all our^ views as to 
 the nature of weather at all, but also in explaining all 
 that was previously known. We shall especially en- 
 deavour to explain the general principles involved, draw- 
 ing our illustrations from all countries, so as to show 
 
INTRODUCTORY. 13 
 
 what, is general and what is local, and to give a truly 
 International character to this work; while a few ex- 
 amples of British weather will be given in some detail, 
 so as to demonstrate how the minutest weather-changes 
 are subordinate to general laws. 
 
 The plan of this book will be as follows. We shall 
 commence with a chapter on popular weather-prognostics, 
 so as to introduce some of the simpler portions of synoptic 
 meteorology. Clouds and cloud-prognostics will form a 
 chapter by themselves, so as to exhibit the great develop- 
 ment which has recently been made in the interpretation 
 of their indications. So far, we shall confine our atten- 
 tion to weather of the northern temperate zone only. 
 
 This will take up about one-third of the work, and 
 exhaust the more -popular portions of the subject. We 
 shall then have to plunge more deeply into the details 
 of isobars, and explain how they are all the products of 
 different forms of atmospheric circulation. From them 
 we shall pass to the consideration of barograms and 
 meteograms generally, and show especially how the 
 changes in the shape of the isobars, as seen on two suc- 
 cessive charts, indicate the sequence of weather as ob- 
 served in any one place. This, which is the fundamental 
 point of all synoptic meteorology, is also unfortunately 
 the most difficult to grasp, and can only be fully realized 
 after considerable practise. Once, however, that its im- 
 port is fully mastered, the remainder of the work will 
 seem comparatively simple. 
 
 We shall then discuss the relation of both the velocity 
 and direction of the wind to isobars, and after that the 
 influence of different shapes of isobars in modifying the 
 
14 WEATHER. 
 
 distribution of heat and cold from day to day in various 
 parts of the world. 
 
 Squalls, thunderstorms, and non-isobaric rains will next 
 engage our attention ; and a short chapter on Pamperos, 
 whirlwinds, and Tornados will naturally follow next. We 
 shall then consider the local influence of the configuration 
 of the earth's surface on weather, and devote a whole long 
 chapter to the diurnal phenomena of weather, with special 
 reference to the manner in which they modify the weather 
 that characterizes each shape of isobars. 
 
 From this we shall easily be led to comprehend the 
 nature of the annual fluctuations of weather, and of those 
 of a longer period, such as the supposed connection 
 between sunspots and rainfall. 
 
 Having thus explained what we may call the com- 
 ponents of weather, we shall be ready to understand 
 the nature of sequences or spells of weather, even in 
 the most variable climates. Our illustrations will be 
 drawn chiefly from that portion of the northern hemi- 
 sphere which lies between the Urals and the Eocky 
 Mountains, but we shall also include some examples from 
 the monsoon districts of India, and from Australia, so as 
 to explain the nature of day to day weather in the tropics 
 and in the southern hemisphere. 
 
 When this is done, we shall have completed our 
 description of the nature of weather, and will then turn 
 to the question of forecasting. This falls readily into 
 two distinct problems: 1. To show all that a solitary 
 observer, with a barometer and his eye-observations on 
 clouds and prognostics, can do in the way of forecasting. 
 In this chapter we shall explain fully why the baro- 
 
INTRODUCTORY. 15 
 
 meter sometimes appears to fail, and also how much the 
 older knowledge can be increased by a knowledge of 
 synoptic charts. The space at our disposal will not, how- 
 ever, permit us to explain the modern developments of 
 the principles of handling ships in hurricanes, which 
 would naturally come in this chapter. 
 
 2. To show what a meteorologist can do, seated in 
 a central bureau, with telegraphic communication in all 
 directions, and who, after making a synoptic chart, and 
 combining it with every other modern aid, issues tele- 
 graphic forecasts to all parts of the country. This is the 
 highest problem of meteorology. 
 
16 WEATHER. 
 
 CHAPTER II. 
 
 WEATHER-PROGNOSTICS. 
 
 INTRODUCTION. 
 
 THE second stage in the history of meteorology, after the 
 mythic phase has been passed, is the collection of 
 numerous observations on the appearance of the sky, the 
 movements of animals, etc., before rain or fine weather 
 into the form of short sayings, which are usually known 
 as popular prognostics. For instance, halos round the 
 sun, or swallows flying low, are known all over the world 
 very frequently to precede rain. On the other hand, a 
 copious deposition of d'ew, or a white silvery moon, are 
 equally widely known as precursors of fine weather. 
 
 One of the earliest collections of prognostics is found in 
 the " Diosemeia " of Aratus, a Greek who flourished in 
 Macedonia and Asia Minor about 270 B.C. The principal 
 interest attached to his work is that many of his prog- 
 nostics were incorporated by Virgil in his Georgics, and 
 that from them through the medium of the Latin 
 monks, during the revival of learning in the Middle Ages 
 a very considerable number have been translated into 
 
WEATHER-PROGNOSTICS. 17 
 
 modern European languages, and are in current use at the 
 present time. 
 
 EARLY EXPLANATIONS. 
 
 From classic times, down to the commencement of 
 this century, it can hardly be said that this branch of 
 meteorology made any advance. Few, if any, new prog- 
 nostics had been discovered, and neither their physical 
 explanation nor their meteorological significance had 
 been found out. But about eighty years ago, some 
 physical explanations were given. It was found that the 
 air always contained a certain quantity of uncondensed 
 vapour, and means were invented for measuring this 
 amount accurately. From this, the nature and conditions 
 of the formation of dew were discovered, and also that 
 before many cases of rain the air became more charged 
 with vapour. This latter fact gave the explanation of 
 several rain-prognostics. For instance, when walls sweat, 
 stones grow black, and clouds form on hilltops, rain may 
 be expected almost all the world over. 
 
 But even when these reasons had been discovered, the 
 science flagged. A large number of rain-prognostics 
 could not be shown by any means to depend on an increase 
 of moisture, and, as vapour cannot grow in the air, some 
 explanation was needed to account for its variable quan- 
 tity. And even when, in a general way, the prognostic 
 had been explained, no clue whatever had been found for 
 what we may call the meteorological significance. What 
 was the relation of the damp to the rain ? Why did the 
 prognostic sometimes fail? Why are there many rain- 
 prognostics associated with a tolerably dry air ? Why is 
 
 c 
 
18 WEATHER. 
 
 not all rain preceded by the same set of prognostics? 
 To all these questions no answer could be given. Prog- 
 nostics had almost fallen into disrepute ; they were con- 
 sidered no part of science, and had been supposed to be 
 only suitable for rustics and sailors. 
 
 MODERN DEVELOPMENTS. 
 
 So the subject remained till the introduction of 
 synoptic charts. Then it was soon seen that in Temperate 
 regions the broad features of weather depend on the 
 shape of the isobaric lines, and later on it was shown 
 the author believes, mainly by himself that nearly all 
 prognostics have a definite place in some shape of isobars, 
 and that all the above questions, formerly insoluble, 
 receive a ready explanation. It has also been demon- 
 strated that prognostics can never be superseded for use 
 on board ship, and that even in the highest developments 
 of weather-forecasting by means of electric telegraph, 
 prognostics often afford most valuable information. But 
 before we attempt to explain how this is done, we must 
 introduce the reader into the elements of synoptic meteor- 
 ology. 
 
 SYNOPTIC CHARTS. 
 
 Synoptic meteorology is that part of the science which 
 deals with the results obtained by constructing synoptic 
 charts. Formerly, all meteorology was deduced from the 
 changes which took place in the instrumental readings 
 at any one place during any interval of time, say one 
 day. For instance, a great deal had been discovered as 
 
WEATHER-PROGNOSTICS. 19 
 
 to the connection between a falling or rising barometer 
 and the accompanying rain or wind. Synoptic charts, on 
 the contrary, are constructed by taking the readings of 
 any instrument (say the barometer), or any observations 
 on the sky or the weather (say where rain is falling, or 
 cloud or blue sky is seen), at a large number of places 
 at the same moment (say 8 a.m. at Greenwich). A map 
 of the area or district from which the observations have 
 been received is then taken, the barometer-readings are 
 marked down over their respective places, and then lines 
 are drawn through all the stations where the pressure is 
 equal. For instance, through all the places where the 
 pressure is 29*9 inches (760 mm.), and again at convenient 
 intervals, generally of about two-tenths of an inch, say 
 29-7 ins. (755 mm.), 29'5 ins. (750 mm.), and so on. These 
 lines are called isobaric lines, or more shortly isobars 
 that is, lines of equal atmospheric weight or pressure. 
 This method of showing the distribution of pressure by 
 isobars is exactly analogous to that of marking out hills 
 and valleys by means of contour lines of equal altitude. 
 
 Similarly, the places which report rain, cloud, blue sky, 
 etc., are marked with convenient symbols to denote these 
 phenomena. In Great Britain, a system known as Beau- 
 fort's weather-notation is exclusively used. It is as follows: 
 This will be useful, as it is employed in all our charts. 
 
 BEAUFOKT'S NOTATION OF WEATHER. 
 
 SYMBOL. 
 
 b Blue sky, whether with clear or hazy atmosphere. 
 
 c Clouds (detached). 
 d Drizzling rain. 
 / Fog. 
 g Gloomy, very. 
 
20 WEATHER. 
 
 SYMBOL. 
 h Hail. 
 I Lightning. 
 
 m Misty, hazy atmosphere. 
 
 o Overcast, the whole sky being covered with au impervious cloud, 
 p Passing, temporary showers. 
 q Squally. 
 
 r Rain, continued rain, 
 s Snow. 
 t Thunder. 
 
 u Ugly, threatening appearance of the weather. 
 v Visibility, whether the sky be cloudy or not. 
 w Dew. 
 
 We should remark here that, though in common 
 parlance the word " weather " is used collectively for the 
 sum of every meteorological element, wind, rain, heat, 
 cold, etc., in this work, and in all synoptic charts, 
 "weather" is used in a more restricted sense to denote 
 whether the actual appearance of the sky is blue, cloudy 
 or otherwise, and whether rain, snow, hail, etc., are falling. 
 
 Then arrows are placed over each observing station, 
 with a number of barbs and feathers which roughly 
 indicate the force of the wind. By an international 
 convention, the arrows always fly with the wind ; that is to 
 say, they do not face the wind like the pointer of a wind- 
 vane. The scale of force usually adopted is that of Beau- 
 fort, which is given opposite. It will be observed that this 
 is a practical scale, based on the amount of canvas a ship 
 can carry. At sea this is certainly a better gauge than 
 any instrumental readings, though there is always a 
 certain disagreement in the estimate of different observers. 
 For land-observations, and those unacquainted with ships, 
 an equivalent of miles per hour and metres per second 
 is given. 
 
WEATHER-PKOGNOSTICS. 
 
 21 
 
 BEAUFORT'S SCALE OF WIND. 
 
 FORCE. 
 
 0. Calm. 
 
 1. Light air. 
 
 2. Light breeze. \ 
 
 3. Gentle breeze. 
 
 I 
 
 4. Moderate breeze. > 
 
 5. Fresh breeze. > 
 
 6. Strong breeze. 
 
 7. Moderate gale. 
 
 8. Fresh gale. 
 
 9. Strong gale. 
 
 10. Whole gale. 
 
 11. Storm. 
 
 12. Hurricane. 
 
 Or just sufficient to give steerage 
 way 
 
 Or that in which a 
 well - conditioned 
 man-of-war, with 
 all sail set, and clean 
 full, would go in 
 smooth water, from 
 n ,,, /Royals, etc. 
 
 towMfh Wfc"" 
 
 1 to 2 knots 
 
 fn 4. 
 
 5 to 6 knots 
 
 topsails 
 
 by. 
 
 le-reefed topsails, 
 etc 
 
 IClose - reefed topsails 
 
 and courses 
 
 Or that with which she could 
 scarcely bear close-reefed main- 
 topsail and reefed foresail 
 Or that which would reduce her to 
 
 storm-stay sails ... 
 Or that which no canvas could with- 
 stand 
 
 VELOCITY. 
 
 Miles Metres 
 per per 
 hour, second. 
 
 3 1-34 
 
 8 3-6 
 
 13 5-82 
 
 18 8-1 
 
 23 10-3 
 28 12-5 
 
 34 15-2 
 
 40 17-9 
 
 48 21-5 
 
 56 25-0 
 
 65 29-0 
 75 33-5 
 90 40-0 
 
 When all this is done, we can see at a glance whether 
 or how wind, rain, cloud, and blue sky are connected with 
 the shape of the isobars. In fact, a synoptic chart gives us, 
 as it were, a bird's-eye view of the weather at the particular 
 moment for which the chart is constructed, over the whole 
 district from which reports have been received. Suppose, 
 now, that after an interval of twenty-four hours another 
 
22 WEATHER. 
 
 chart is constructed from observations taken over the 
 same area, then we generally find that the shape of the 
 isobars and the position of the areas of high and low 
 pressure have considerably changed, and with them the 
 positions of those areas where the weather is good or bad. 
 For instance, suppose that at 8 a.m. on one morning we 
 find pressure low over Ireland and high over Denmark, 
 with rain over Ireland, cloud over England, and blue sky 
 in Denmark ; and that by 8 a.m. on the following day we 
 find that the low-pressure area has advanced to Denmark, 
 and that a new high pressure has formed over Ireland, 
 with rain in Denmark, broken sky in England, and blue 
 sky in Ireland; suppose, too, that the record of the 
 weather, say in London, for those twenty-four hours had 
 been as follows : cloudy sky, followed by rain, after which 
 the sky broke ; how can an inspection of the two charts 
 help us to explain the weather as observed in London during 
 that day ? 'Our bird's-eye view would show that the rain- 
 area which lay over Ireland in the morning had drifted 
 during the day over England, including London, and 
 covered Denmark by next morning. It would also tell 
 us that the position* of the rain was identified with, 
 and moved along with the low pressure. This is the 
 fundamental idea of all synoptic meteorology, but one 
 which can only be thoroughly grasped after a consider- 
 able experience in tracing actual cases. It is so different 
 looking at the "ups" and "downs" of the barometer 
 when they are marked on a diagram, and then at any 
 two synoptic charts which refer to the same period, that 
 it is very difficult at first to see any connection at alL 
 In fact, deductions from barograms as such barometric 
 
WEATHER-PROGNOSTICS. 23 
 
 traces are called and deductions from synoptic charts 
 are so apparently unconnected that they have hitherto 
 been almost treated as different branches of meteorology. 
 One main feature of this book will be our endeavour to 
 collate these deductions together, and to show how 
 changes in the charts for a large district are simultane- 
 ously shown by fluctuation in the instrumental readings 
 at any one place. It must be borne in mind, however, 
 that the whole aim and object of meteorology is to ex- 
 plain weather as it occurs at any place ; that is, what 
 successive changes each individual observer will ex- 
 perience. Synoptic charts are only a means to this end. 
 
 KELATIONS OF WIND AND WEATHER TO ISOBARS. 
 
 Such, then, is a synoptic chart. Many thousands 
 have been constructed for all parts of the world, and by 
 comparing them the following important generalizations 
 have been arrived at : 
 
 1. That in general the configuration of the isobars 
 takes one of seven well-defined forms. 
 
 2. That, independent of the shape of the isobars, the 
 wind always takes a definite direction relative to the 
 trend of these lines, and the position of the nearest area 
 if low pressure. 
 
 3. That the velocity of the wind is always nearly pro- 
 portional to the closeness of the isobars. 
 
 4. That the weather that is to say, the kind of cloud, 
 rain, fog, etc. at any moment depends on the shape, and 
 not the closeness, of the isobars, some shapes being 
 associated with good and others with bad weather. 
 
WEATHER. 
 
 5. That the regions thus mapped out by the isobars 
 were constantly shifting their position, so that changes 
 of weather were caused by the drifting past of these areas 
 of good or bad weather, just as on a small scale rain falls 
 as a squall drives by. The motion of these areas was 
 found to follow certain laws, so that forecasting weather 
 changes in advance became a possibility. 
 
 6. That in the temperate zones sometimes, and habitu- 
 ally in the tropics, rain fell without any appreciable 
 change in the isobars, though the wind conformed more 
 regularly to the general law of these lines. This class of 
 rainfall will be called throughout this work " non-isobaric 
 rain." 
 
 It will be convenient to take first the broad features 
 of the relation of wind to isobars, which are as follows : 
 
 First as regards direction. The wind in all cases is 
 not exactly parallel to the isobar, but inclined towards 
 the nearest low pressure at an angle of from 30 to 40. If 
 you stand with your back to the wind, the lowest pressure 
 will always be on your left hand in the northern hemi- 
 sphere, and on your right in the southern hemisphere. 
 This is what is commonly known as "Buys Ballot's 
 Law." 
 
 Then as to velocity. All we need say here is that 
 the velocity is roughly proportional to the closeness of 
 the isobars, and that the measure of the closeness is 
 called the barometric gradient, for in our chapter on 
 wind and calm we will give all necessary details on this 
 branch of the subject. 
 
 The upshot of these two principles is, that if you give 
 a meteorologist a chart of the world with the isobars only 
 
WEATHER-PROGNOSTICS. 
 
 25 
 
 marked on it, he can put in very approximately the 
 direction and force of the wind all over the globe. 
 
 When we have explained the relation of weather to 
 the shapes of isobars, we shall see that he could also write 
 down very nearly the kind of weather which would be 
 -experienced everywhere. 
 
 THE SEVEN FUNDAMENTAL SHAPES OF ISOBARS. 
 
 Then as to the shapes themselves. 
 
 In Fig. 1 we give in a diagrammatic form the broad 
 
 Cyclone 
 
 FIG. 1. The seven fundamental shapes of isobars. 
 
 features only of the distribution of pressure over the 
 North Atlantic, Europe, and the eastern portions of the 
 United States on February 27, 1865. Coast-lines are 
 omitted so as not to confuse the eye, so also are lines of 
 latitude and longitude; but the foot-note at the bottom 
 of the figure represents the equator, and the top of the 
 diagram would be on the Arctic Circle. All pressures 
 
26 
 
 WEATHER. 
 
 of and under 29 '9 ins. (760 mm.) are shown with dotted 
 lines, so that the eye sees at a glance the broad dis- 
 tribution of high or low pressure. The whole seven 
 fundamental shapes of isobars will be found there. 
 
 Looking at the top of the diagram, we see two nearly 
 circular areas of low pressure, round which the isobars are 
 rather closely packed. Such areas, or rather the con- 
 figurations of isobars which enclose them, are called 
 "cyclones," from a Greek word meaning a circle, because 
 they are nearly circular, and, as we shall see presently, the 
 wind blows nearly in a circle round their centre. 
 
 Just south of one of the cyclones, the isobar of 29'9 ins. 
 (760 mm.) forms a small sort of nearly circular loop, 
 enclosing lower pressure ; this -is called a " secondary 
 cyclone," because it is usually secondary or subsidiary 
 to the primary cyclones above described. 
 
 Further to the left the same isobar of 29'9 ins. bends 
 itself into the shape of the letter V, also enclosing low 
 pressure; this is called a "Y-shaped depression," or, 
 shortly, a V." 
 
 Between the two cyclones the isobar of 29*9 ins. pro- 
 jects upwards, like a wedge or an inverted letter V., but 
 this time encloses high pressure ; this shape of lines i& 
 called a " wedge." 
 
 Below all these we see an oblong area of high pressure, 
 round which the isobars are very far apart ; this is called 
 an " anticyclone," because it is the opposite to a cyclone 
 in everything wind, weather, pressure, etc. 
 
 Between every two anticyclones we find a furrow, 
 neck, or " col " of low pressure analogous to the col which 
 forms a pass between two adjacent mountain-peaks. 
 
WEATHER-PKOGNOSTICS. 27 
 
 Lastly, as marked in the lower edge of the diagram, 
 isobars sometimes run straight, so that they do not include 
 any kind of area, but represent a barometric slope 
 analogous to the sloping sides of a long hill. 
 
 We may forestall succeeding chapters so far as to say 
 that the cyclones, secondaries, V's, and wedges are usually 
 moving towards the east at the rate of about twenty miles 
 an hour ; but that the anticyclones, on the contrary, are 
 usually stationary for days, and sometimes for months 
 together. 
 
 We should also note that, though the general principles 
 of prognostics and the broad features of the weather in 
 each of these shapes of isobars are the same all over the 
 world, the minute details which we intend to give now 
 apply to Great Britain and the temperate zones only. 
 
 We will now take the five more important shapes 
 separately, and detail the kind of wind and weather which 
 is experienced in different parts of each of them. From 
 this we shall be led to the explanation of the nature of 
 popular prognostics. The account of " V's " and " cols " 
 will be reserved for our chapter on isobars, as no special 
 prognostics are grouped round these two forms. 
 
 CYCLONE- PROGNOSTICS. 
 
 We will begin with these as they are by far the most 
 important. In Fig. 2 we give a diagram on which we 
 have written in words the kind of weather which would 
 be found in every portion of a typical cyclone; arrows 
 also show the direction of the wind relative to the isobars 
 and to the centre. 
 
28 WEATHER 
 
 First let us look at the isobars. We find that they 
 are oval, and that they are not quite concentric, but the 
 centre of the inner one we will call the centre of the 
 cyclone. Now observe the numbers attached to the isobars ; 
 the outer one is 3OO ins. (762 mm.), the inner one 29'0 
 ins. (737 mm.). But suppose the outer one was the same, 
 
 Blue 
 
 Cirrus 
 FIG. 2. Cyclone prognostics. 
 
 but the inner one was 29*5 (755 mm.) We should then 
 have two cyclones, differing in nothing but depth'; that is, 
 in the closeness of the isobars, or the steepness of the 
 barometric slope. Observation has shown that under 
 these circumstances the general character of the weather 
 and the direction of the wind everywhere would be the 
 same ; the only difference would be that the wind would 
 
WEATHER-PROGNOSTICS. 29 
 
 blow a hard gale in the first, and only a moderate breeze 
 in the second case ; and that what was a sharp squall in 
 the one would be a quiet shower in the other. This is 
 one of the fundamental principles of synoptic meteorology 
 that the character of the weather and direction of the 
 wind depend entirely on the shape of the isobars, while 
 the force of the wind and intensity of the character of 
 the weather depend only on the closeness of the isobars. 
 
 The difference in the details of the weather in a 
 cyclone, or any other isobaric shape which are due to 
 difference in ,the steepness of the isobars, is called a 
 difference in the intensity of the weather. Hence, when 
 we speak of a cyclone as being intense, we mean that it 
 has steep isobars somewhere. The word " intensity " will 
 occur very often in these pages, for when we come to talk 
 about the general sequence of weather from day to day, 
 we shall find that there is no difference betiueen tlie cyclones 
 ivhich cause storms and those ivhich cause ordinary weather 
 except intensity. This is another of the fundamental 
 principles of meteorology. 
 
 Returning now to our cyclone, the whole of the por- 
 tion in front of the centre facing the direction towards 
 which it moves is called its front, and the whole of this 
 portion may obviously be divided into a right and left 
 front. The other side of the centre is, of course, the rear 
 of the cyclone. Then, as the whole cyclone moves along 
 its course, it is evident that the barometer will be falling 
 more or less at every portion of the front, and rising more 
 or less everywhere in rear, so that there must be a line of 
 places somewhere across the cyclone, where the barometer 
 has touched its lowest point and is just going to rise. 
 
30 WEATHER. 
 
 This line is called the " trough " of the cyclone, because 
 if we look at the barometer-trace at any one place, the 
 " ups " and " downs " suggest the analogy of waves, so 
 that the lowest part of a trace may be called a " trough." 
 Or we may look at the cyclone as a circular eddy, moving 
 in a given direction, and so far presenting some analogy 
 to a wave. Here we are face to face with the primary 
 difficulty of understanding synoptic charts. When we 
 look at any chart of a cyclone which represents the state 
 of things existing at some one moment, there is little to 
 suggest the idea of a trough, because the latter depends 
 on the motion of the cyclone, which cannot be shown on a 
 chart. Perhaps the following illustration may help to 
 explain the nature of the trough. Suppose the cyclone 
 represented the inside of a conical crater, if we walked 
 along the line that marks the path of the centre from the 
 word " FRONT " to the word " REAR " on the diagram, we 
 should pass over the centre of the crater, and be walking 
 downhill all the time till we reached the bottom, and up- 
 hill afterwards. But now, if we walked across the crater 
 on any other line parallel to this one, say from the word 
 " pale " to the word " COOL," we shall equally walk down- 
 hill till we arrive at the point occupied by the letter q in 
 the word "squalls." At q we should still be on the side 
 of the crater, and some distance from the centre, but 
 after passing q we should begin to walk uphill. 
 
 W T hen we have once realized the meaning of the 
 trough, we shall never fall into the very common error of 
 thinking that because our barometer has begun to rise, 
 the centre of the cyclone has necessarily passed over 
 us. It is probably only the trough, but we will explain 
 
WEATHER-PROGNOSTICS. 31 
 
 afterwards how we can tell whether it is the centre or 
 not. 
 
 So far for the shape and names of the different por- 
 tions of the cyclone. Now for the wind. A glance at the 
 arrows will show that, broadly speaking, the wind rotates 
 round the centre in a direction opposite to the motion of 
 the hands of a watch. That is to say, that in the extreme 
 front, following the outer isobar, the wind is from the 
 south-east ; further round, it is from the east-north-east ; 
 still further, from the north-north-west ; then from about 
 west ; and, finally, from the south-west. Then we note 
 that in front the wind is slightly incurved towards the 
 centre, and therefore blows somewhat across the isobars, 
 while in rear it has little or no incurvature, and blows 
 nearly parallel to the isobars. The velocity or force of 
 the wind will depend on the closeness of the isobars. In 
 the diagram they are much closer set in rear than in 
 front of the cyclone, and therefore the wind is strongest 
 behind the centre. 
 
 In our chapter on clouds we shall have to go much 
 more minutely into the nature of wind, both on the sur- 
 face and in the upper currents ; but here we wish to confine 
 our attention as far as possible to the weather and 
 appearance of the sky. 
 
 For the same reason, the details of gradients will not 
 be developed till we come to our chapter on wind and 
 calms. 
 
 The weather in a cyclone is somewhat complicated. 
 Some characteristic features depend on the position of 
 the trough, and have nothing to do with the centre. For 
 instance, the weather and sky over the whole front of the 
 
32 WEATHER. 
 
 cyclone that is, all that lies in front of the trough is- 
 characterized by a muggy, oppressive feel of the air, and 
 a dirty, gloomy sky of a stratiform type, whether it is 
 actually raining or only cloudy. On the other side, the 
 whole of the rear is characterized by a sharp, brisk feel of 
 the air, and a hard, firm sky of cumulus type. 
 
 But, on the contrary, other characteristic features are 
 related to the centre, and have little to do with the 
 trough. The rotation of the wind, though slightly 
 modified near the trough, is in the main related to the 
 centre, and the broad features of the weather in a cyclone 
 are a patch of rain near the centre, a ring of cloud sur- 
 rounding the rain, and blue sky outside the whole system. 
 The centre of the rain-area is rarely concentric with the 
 isobars. It usually extends further in front thaji in rear, 
 and more to the south than to the north, but is still 
 primarily related to the centre. 
 
 This will be readily seen by reference to the diagram ; 
 there the drizzle and driving rain extend some distance to 
 the right front, while almost directly behind the centre 
 patches of blue sky become visible. Thus a cyclone has, 
 as it were, a double symmetry : that is to say, one set of 
 phenomena, such as warmth, cloud character, etc., which 
 are symmetrically disposed in front and rear of the trough ; 
 and another set, such as wind and rain, which are sym- 
 metrically arranged round the centre. There is reason to 
 believe that what we may call the circular symmetry of a 
 cyclone is due to the rotation of the air, while the pro- 
 perties which are related to the trough are due to the 
 forward motion of the whole system. 
 
 As this is a somewhat difficult conception, perhaps 
 
WEATHER-PKOGNOSTICS. 33 
 
 the following analogy may not be out of place. Let us 
 consider the twofold distribution of the population of 
 London. As regards density, we find a comparatively thinly 
 populated district in the centre of London that is, in the 
 City proper. Bound this there is a tolerably symmetrical 
 ring of very densely populated streets, outside of which 
 the population thins away towards the suburbs. But at 
 the same time London is divided into very well-defined 
 halves of comparative poverty and wealth the east and 
 west ends respectively. This is a far more strongly 
 marked distinction than any which is found between the 
 north and south sides of London, in spite of a river that 
 might have been supposed to make a natural boundary. 
 This distinction into an east and west end is always attri- 
 buted to the general march of the population westwards. 
 Thus the front and rear of the moving population have a 
 symmetry independent of the density of the population 
 round a centre. 
 
 Returning now to the details of weather in a cyclone, 
 we have marked on the diagram the kind of weather 
 and cloud which would be found in different parts of 
 a cyclone. The first thing which will strike us is that 
 the descriptive epithets applied to the sky contain the 
 phraseology of the most familiar prognostics. At the 
 extreme front we see marked " pale moon," " watery sun/' 
 which means that in that portion of a cyclone the moon 
 or sun will look pale or watery through a peculiar kind of 
 sky. But all over the world a pale moon and watery sun 
 are known as prognostics of rain. Why are they so ? 
 The reason we can now explain. Since a cyclone is 
 usually moving, after the front part where the sky gives 
 
34 WEATHER. 
 
 a watery look to the sun has passed over the observer, the 
 rainy portion will also have to come over him before he 
 experiences the blue sky on the other side of the cyclone. 
 
 Suppose the cyclone stood still for a week, then the 
 observer would see a watery sky for a week, without any 
 rain following. Suppose the cyclone came on so far as to 
 bring him under a watery sky, and then died out or 
 moved in another direction, then, after seeing a watery 
 sky, no rain would fall, but the sky would clear. The 
 prognostic would then be said to fail, but the word is only 
 partially applicable. The watery sky was formed and seen 
 by the observer, because he was in the appropriate portion 
 of the cyclone, and so far the prognostic told its story 
 correctly viz. that the observer was in the front of the 
 rainy area of a cyclone. The prognostic failed in its 
 ordinary indication because the cyclone did not move on 
 as usual, but died out, and therefore never brought its 
 rainy portion over the observer. This is the commonest 
 source of the so-called failure of a rain-prognostic in 
 Great Britain. The reason why all rain is not preceded 
 by a watery sky is because there are other sources of rain 
 besides a cyclone, which are preceded by a different set of 
 weather-signs. Such is the whole theory of prognostics. 
 
 The same reasoning which applies to a watery sky 
 holds good for every other cyclone-prognostic. We shall 
 have explained why any prognostic portends rain when 
 we have shown that the kind of sky or other appearance 
 which forms the prognostic belongs to the front of the rainy 
 portion of a cyclone. Conversely we shall have explained 
 why any prognostic indicates finer weather when we have 
 shown that the kind of sky belongs to the rear of a 
 
WEATHER-PROGNOSTICS. 35 
 
 Cyclone. It will be convenient, therefore, to describe the 
 weather in different parts of a cyclone, and the appro- 
 priate prognostics together. 
 
 First, to take those prognostics which depend on 
 qualities common to the whole front of the cyclone, viz. 
 a falling barometer, increased warmth and damp, with 
 a, muggy, uncomfortable feel of the air, and a dirty sky. 
 
 From the increasing damp in this part of a cyclone, 
 while the sky generally is pretty clear, cloud forms round 
 and "caps" the tops of hills, which has given rise to 
 numerous local sayings* The reason is that a hill always 
 deflects the air upwards. Usually the cold caused by 
 ascension and consequent expansion is not sufficient to 
 lower the temperature of the air below the dew-point ; 
 but when very damp, the same amount of cooling will 
 bring the air below the dew-point, and so produce con- 
 densation. 
 
 From the same excessive damp the following may be 
 explained : 
 
 "When walls are more than usually damp, rain is 
 expected." 
 
 The Zuni Indians in New Mexico say that " When the 
 locks of the ISTavajos grow damp in the scalp-house, surely 
 it will rain." From this we may assume that scalps are 
 slightly hygroscopic, probably from the salt which they 
 contain. 
 
 Also, owing to excessive moisture, clouds appear soft 
 and lowering, and reflect the glare of ironworks and the 
 lights of large towns. 
 
 With the gloomy, close, and muggy weather, some 
 people are troubled with rheumatic pains and neuralgia, 
 
36 WEATHER. 
 
 old wounds and corns are painful, animals and birds are 
 restless, and drains and ditches give out an offensive 
 smell. 
 
 A glance at the diagram will show that the barometer 
 falls during the whole of the front of the cyclone. There- 
 fore the explanation of the universally known fact that 
 the barometer generally falls for bad weather is, that both 
 rain and wind are usually associated with the front of a 
 cyclone. When we discuss secondaries, we shall find 
 a kind of rain for which the barometer does not fall ; and 
 in our chapter on forecasting for solitary observers we 
 shall explain why it sometimes rains while the barometer- 
 is rising, and why there is sometimes fine weather while 
 the mercury is falling. 
 
 Now to take prognostics which belong to different 
 portions of the cyclone-front. 
 
 By reference to Fig. 2 it will be seen that in the out- 
 skirts of the cyclone-front there is a narrow ring of halo- 
 forming sky. Hence the sayings 
 
 " Halos predict a storm (rain and wind, or snow and 
 wind) at no great distance, and the open side of the halo 
 tells the quarter from which it may be expected." 
 
 " Mock suns predict a more remote and less certain 
 change of weather." 
 
 With regard to the open side of the halo indicating 
 the quarter from which the storm may be expected, it 
 does not appear that this can be used as a prognostic with 
 any certainty. It, however, most probably originated in 
 the fact that halos are usually seen in the south-west 
 or west, when the sun or moon is rather low, the lower 
 portion of the halo being cut off by the gloom on the 
 
WEATHER-PROGNOSTICS. 37 
 
 horizon, and that European storms generally come from 
 those quarters : a heavy bank of cloud will often lie in 
 that direction. 
 
 Inside the halo sky comes the denser cloud which gives 
 the pale watery sun and moon. Still nearer the centre we 
 find rain, first in the form of drizzle, then as driving rain. 
 In the left front we find ill-defined showers and a dirty sky. 
 
 We have now come to the trough of the cyclone. The 
 line of the trough is often associated with a squall or 
 heavy shower, commonly known as " a clearing shower." 
 This is much more marked in the portion of the trough 
 which lies to the south of the cyclone's centre than on 
 the northern side. 
 
 Then we enter the rear of the cyclone. The whole of 
 the rear is characterized by a cool, dry air, with a brisk, 
 exhilarating feel, and a bright sky, with hard cumulus 
 cloud. These features are the exact converse of those we 
 found in the cyclone-front. In the cloud-forms especially 
 we see this difference. All over the front, whether high 
 up or low down, whether as delicate cirrus or heavy 
 gloom, the clouds are of a stratified type. Even under 
 the rain, when we get a peep through a break in the 
 clouds, we find them lying like a more or less thick sheet 
 over the earth. All over the rear, on the contrary, clouds 
 take the rocky form known as cumulus ; cirrus is almost 
 unknown in the rear of a cyclone-centre in the temperate 
 zone. 
 
 In the exhilarating quality of the air we find the 
 meaning of the proverb 
 
 " Do business with men when the wind is in the north- 
 west." 
 
38 WEATHER. 
 
 A north-west wind belongs to the rear of a cyclone, 
 and improves men's tempers, as opposed to the neuralgic 
 and rheumatic sensations in front of a cyclone, which 
 make them cross. 
 
 As to the details of the different portions of the rear. 
 Immediately behind the centre small patches of blue sky 
 appear. Further from the centre we find showers or 
 cold squalls ; beyond them, hard detached cumulus or 
 strato-cumulus ; still farther the sky is blue again. 
 
 In the south of the cyclone, near the outskirts, the 
 long wispy clouds known as windy cirrus and "mares' 
 tails " are observed. These indicate wind rather than rain, 
 as they are outside of the rainy portion of the cyclone. 
 
 So far we have only described the different kinds 
 of weather which would be experienced at the same 
 moment in different places. We have not said much 
 about the sequence of weather at any one place. A single 
 chart tells little about this, for it does not indicate which 
 way the cyclone is going. To track a cyclone we 
 want another chart about twelve or twenty-four hours 
 later, from which we can see exactly how the cyclone- 
 centre has moved. Then we can follow the sequence of 
 weather for those twelve or twenty-four hours at any place 
 we choose to select. 
 
 It must specially be borne in mind that the word 
 "front" is a relative term. In our diagram we have 
 pointed it to the north-east, because that is the direction 
 towards which the majority of British cyclones move. 
 In very rare cases we get a cyclone moving from the 
 south-east. The general circulation of wind then remains 
 about the same, but the characteristic qualities of the 
 
WEATHER-PROGNOSTICS. 39 
 
 different portions of a cyclone are shifted to the new 
 position of the front and rear. For instance, if the cyclone 
 in our diagram was moving towards the north-west, we 
 should have muggy weather and dirty sky with a north- 
 west wind, and bright weather and clear sky with south- 
 west wind. This occurs habitually in the northern Tropics, 
 but very rarely in temperate regions. 
 
 But now to take the diagram as it is drawn. We will 
 suppose that the centre has moved along the dotted line, 
 towards the north-east, till it is outside the margin of the 
 figure. What would the sequence of weather be to an 
 observer who was living, say, where the word " halo " is 
 written, just below the word " FRONT." This we may get 
 by taking a line across the diagram, parallel to the line 
 which marks the track of the cyclone. This will take 
 us to the word " detached" just below the word " REAR." 
 Following the words and symbols, we should find that as 
 the barometer began to fall a halo-forming sky would 
 appear, with the wind coming light from the south-east. 
 Soon the sky would grow lower and denser, into what is 
 known as a " watery " sky, and the wind would begin to 
 veer towards the south and to come in uneasy gusts. Then 
 drizzling rain would set in, the barometer still falling, and 
 the muggy, disagreeable feel of the air would be very 
 noticeable. Later, the wind would begin to rise from the 
 south-west, driving the rain before it, and perhaps attain 
 the force of a gale. After a time one of the gusts would 
 be much harder, with heavier rain than any which had 
 been previously experienced, and with a squall the wind 
 would go round with a jump two or three points of the 
 compass to the west or west-north-west. If we looked at 
 
40 WEATHER. 
 
 the barometer, we should find that at that moment the 
 mercury had begun to rise ; this is the passage of the 
 trough of the cyclone. The wind now blows harder than 
 it had done before, and comes in squalls from the north- 
 west, while the whole aspect of the sky and character of 
 weather are changed. The air is cold and dry, the sky is 
 higher and harder, some patches of blue appear in the 
 heavens, hard rocky cumulus appears on the top of the 
 squalls or showers, and the wind moderates. Gradually 
 showers are replaced by masses of cloud from which no 
 rain descends, and after a time the sky becomes bright 
 and cloudless, while the wind falls to a gentle breeze. 
 
 We have endeavoured to show all this in a diagram- 
 matic form in Fig. 3 j but observe that, while we read the 
 
 ^g.o 
 
 \ if'* 
 
 I Trough, 
 
 Halo. - "Watery. Drizzk. Rain. Squall. Showers. Crm* .'Blue. 
 FIG. 3. Weather sequence in a cyclone. 
 
 other diagram from right to left, this one we read in the 
 ordinary manner from left to right. This inversion is 
 obviously necessary, because the cyclone is moving from 
 ri^ht to left. The upper line gives the trace which a 
 
WEATHER-PROGNOSTICS. 41 
 
 self-recording barometer would have marked. In front 
 of the cyclone, where the gradients are moderate, the 
 mercury falls slowly; in rear, where the gradients are 
 steep, it rises rapidly. The arrows below, which are 
 supposed to fly with the wind, marks the shift of wind 
 which an observer would experience ; and the number of 
 barbs denote the varying force of the wind. The sequence 
 of weather, which is written in words, is identical with 
 the sequence of weather as marked on the plan of 
 cyclone-prognostics. 
 
 Wind is said to " veer," or " haul," when it changes 
 in the same direction as the course of the sun ; that is, 
 from east, by south, to west, or from west, by north, 
 round to east again. Wind is said, on the contrary, to 
 " back " when it changes against the sun ; that is, from 
 east, by north, to west, and from west, by south, to east 
 again. We have seen that the wind veers to an observer 
 situated to the southward of a cyclone-centre. An 
 inspection of Fig. 2 will show that the wind would back 
 from east to north-east, and then through north to north- 
 west, if the observer was situated anywhere north of the 
 cyclone's path. If he was exactly on the path of the 
 centre, the wind would jump round from south-west to 
 north-east without either veering or backing; so that by 
 watching the wind any one can tell what part of a cyclone 
 he is in. In Northern Europe cyclones rarely pass so far 
 to the south as to give the backing sequence. When 
 they do they are almost always soon followed by another 
 cyclone, which passes farther north, and brings fresh bad 
 weather, with another nearly complete shift of wind. 
 Hence the meaning of the following prognostic : 
 
42 WEATHER. 
 
 " When the wind veers against the sun, 
 Trust it not, for back 'twill run." 
 
 Here " veering " is used for shifting generally, and not 
 in its more limited sense of shifting in one particular way. 
 
 The explanation which we have just given as to the 
 squall which occurs after the barometer has turned in a 
 cyclone, or at the trough of the cyclone, will show the 
 truth of the following : 
 
 " When rise begins, after low, 
 Squalls expect and clear blow." 
 
 The clear blow refers to the brighter gale which comes 
 from the north-west, as contrasted with the dirty gale 
 which preceded it from the south-west. 
 
 If we take a general view of all the weather in a 
 cyclone, we find that we have a large number of prog- 
 nostics which precede the rain that is associated with 
 wind and a falling barometer. We will now introduce 
 our readers to a kind of rain which is associated with 
 calm and a stationary barometer. 
 
 SECONDARY CYCLONES. 
 
 A secondary cyclone, or, shortly, a " secondary," is so 
 called both because it has some features in common with 
 a primary cyclone, and because its origin and motion are 
 frequently determined by the path of the primary. It is, 
 however, often found at the edges of anticyclones without 
 any primary, and in many parts of the world it is of 
 frequent occurrence where primaries are almost] unknown. 
 The general appearance of that shape of isobars to which 
 we attach the name of secondary will be readily seen by 
 
WEATHER-PROGNOSTICS. 43 
 
 an inspection of Fig. 4. In that diagram the general 
 slope of the isobars is towards the north, but the isobar 
 of 30*1 ins. (765 mm.) is bent into a loop, so as to 
 enclose an area of relatively lower pressure, but not a 
 regular pit of low pressure as in a primary cyclone. In 
 consequence of this the isobaric slope is diminished, as 
 the distance between the two adjacent isobars is increased. 
 
 FIG. 4. Weather in secondary cyclone. 
 
 For the same reason, the wind inside the loop is very 
 light, but round the edges of the loop the gradient is 
 increased, and the wind is stronger. This wind is usually 
 in angry, violent gusts, and not in the steady, heavy blow 
 of cyclone-wind. 
 
 The motion of the secondary is usually parallel to the 
 path of the primary, and very rarely shows any tendency 
 to revolve round the primary. Hence the term " satellite " 
 
44 WEATHEE. 
 
 depressions, which is sometimes applied to secondaries, 
 seems hardly suitable. 
 
 When a secondary is formed at the edge of an 
 anticyclone, the motion is generally very obscure. 
 
 Some very striking weather-changes are grouped 
 round this loop of low pressure. In the extreme front we 
 find the thin nebulous cloud which forms halo. Beyond 
 this is a narrow ring of gloomy cirro-stratus; then we 
 find a ring of heavy rain, with gusts, surrounding that half 
 of the secondary where the gradients are steepest. Inside 
 this, in the heart of the secondary, we find a calm, with a 
 steady, heavy downpour of rain. On the rear side of the 
 ring of gusts there is a narrow belt of irregular cumulus, 
 beyond which the sky is blue. On the low-pressure side 
 of the secondary we find cirrus and cirro-stratus, and 
 outside that the cloud appropriate to the primary cyclone. 
 The general course of the wind follows the universal laws 
 of wind and gradient. The arrows in the diagram show 
 that the direction of the wind would be with the slope of 
 gradients which we have drawn there. We may note 
 that the amount of deflection of the wind is much smaller 
 than in primary cyclones. All over the world secondaries 
 are associated with a peculiar class of thunderstorm, but 
 we are unable to say in what particular portion they are 
 localized. 
 
 If the primary was moving in the direction of the 
 dotted arrow marked "FKONT" on the diagram, the 
 sequence of weather to a single observer would be as 
 follows. The blue sky would become covered either with 
 a thin nebulous haze, and perhaps halos, or else with 
 dirty cirro-stratus, and the wind] would fall light. The 
 
WEATHER-PROGNOSTICS. 45 
 
 clouds would rapidly become black and heavy, and soon, 
 with some angry gusts, heavy rain with big drops would 
 commence suddenly. We saw before that in a primary the 
 rain begins as drizzle. If the barometer is very carefully 
 watched, a very slight rise or fail will occur now ; perhaps 
 only one-hundredth of an inch. This gusty rain only lasts 
 a few minutes, when the wind falls, and the rain pours 
 straight down, not quite as heavily as at first. This stage 
 sometimes lasts four or five hours, and is often very puzzling 
 to Englishmen and others who are accustomed to associate 
 rain with a falling, and not with a steady barometer. 
 When the rear edge of the secondary approaches, the rain 
 suddenly becomes much heavier, with more angry gusts. 
 Just at this moment the barometer moves slightly 
 maybe not more than one-hundredth of an inch. If the 
 general motion of the mercury was upwards when the 
 rain began, this second motion will be upwards also ; if 
 downwards, this will be downwards also. Here is another 
 contrast to a primary cyclone, where a fall of the mercury 
 is always followed by a rise. The heavy rain lasts a very 
 short time, when the clouds break quickly into irregular 
 cumulus, and the sky is soon clear again. 
 
 We have endeavoured to show this in the annexed 
 diagram (Fig. 5). The upper curve shows the barometric 
 changes to a single observer. In the secondary of which 
 we have drawn the plan in Fig. 4, the general motion of 
 the barometer would be downwards ; so in Fig. 5 we find 
 that the small motion of the barometer as the secondary 
 approaches is downwards. Then the mercury remains 
 perfectly steady till the disturbance is just going to pass 
 off, when it takes another small step downwards, and then- 
 
46 WEATHER. 
 
 continues to fall slowly, as before the rain began. Below 
 the barogram, the sequence of weather is shown in a 
 diagrammatic form, so as to show clearly the ring-like 
 character of the rain. The lowest bar, marked overcast, 
 is drawn under the barogram during the whole time that 
 the sky was overcast. The upper shaded bar is drawn 
 double thickness under the portions of the barogram 
 
 o I 2 Hours 
 
 ^-- ___^ Bar- 
 
 _ 
 
 Gusty 
 Steady 
 Overcast 
 
 r 
 r 
 
 >^ 
 
 1 ,-' 3 
 
 t<X\^- v 'v'\ " " ' \ 
 1 v- :v -' ' 'I 
 
 
 FIG. 5. Weather sequence in secondary. 
 
 during which the heaviest gusty rain was experienced, and 
 single thickness during the time that the steady downpour 
 was observed. 
 
 We have already alluded to the idea of intensity in 
 any form of atmospheric circulation ; and as simple cloud, 
 moderate, and heavy rain are, as it were, three successive 
 degrees of intensity, the thickness of the bars in the 
 above diagram is proportional to the intensity of the 
 weather in the different portions of a secondary. 
 
 There are no special prognostics associated with 
 secondaries. Our object in mentioning the subject here 
 was to explain the nature of another kind of rain than 
 that which is found in primary cyclones. When we 
 come to explain various groups of prognostics, we shall 
 find a number of rain-prognostics which depend on a 
 
. WEATHER-PROGNOSTICS. 47 
 
 diminution of barometric pressure. All these indications 
 must obviously be absent before the rain which is pro- 
 duced by a secondary, and we shall now understand why 
 rain often falls without these pressure-prognostics being 
 observed. 
 
 ANTICYCLONE-PROGNOSTICS. 
 
 An anticyclone is an area of high pressure sur- 
 rounded by nearly circular isobars. These are always a 
 considerable distance apart, and extend over a large area. 
 The pressure is highest in the centre, and gradually 
 diminishes outwards. The air is calm and cold in the 
 central portion, while on the outskirts the wind blows 
 round the centre in the direction of the hands of a 
 watch, not exactly parallel to the isobars, but spirally 
 outwards. Unlike a cyclone, which is commonly in rapid 
 motion, an anticyclone is often stationary for many days 
 together. 
 
 Thus in Fig. 6 we see that, while there is a dead calm 
 in the centre, the wind comes from the north on the 
 eastern side, from north-east on the south side, from 
 south-east on the western edge, and from west on the 
 northern edge. 
 
 The broad features of the weather in an anticyclone 
 are blue sky, dry cold air, a hot sun, and hazy horizon, 
 with very little wind in fact, the very antithesis of 
 everything which characterizes a cyclone. As a neces- 
 sary consequence of this, we find in an anticyclone 
 strongly marked radiation weather, and much diurnal 
 variation. These two last ideas are so important that we 
 
48 
 
 WEATHER. 
 
 must devote a few paragraphs to their explanation and 
 consideration. 
 
 Eadiation weather is best explained by an example. 
 On a very fine summer day we generally find the valleys 
 full of mist in the early morning. As the sun gains 
 power the vapour rises and evaporates, so that the sky 
 
 Cloudless 
 
 Stratus 
 Sometimes 
 
 Bitter E Winds 
 Black Sly in Winter 
 
 FIG. 6. Anticyclone prognostics. 
 
 becomes cloudless and the sun very hot. After sunset, 
 the air being still and dry, radiation into the cold space 
 which surrounds the earth proceeds rapidly, and soon 
 mist forms again in the hollows, and dew upon the 
 grass. 
 
WEATHER-PROGNOSTICS. 49 
 
 The sequence of a fine day in winter would be similar 
 in general character, but differ somewhat in details. 
 
 Thus, though we have written "fog," "light cumulus," 
 " cloudless," in certain poitions of our anticyclone "dia- 
 gram," these words only describe the prominent daytime 
 weather which most affects us in the various portions of 
 the anticyclone; but the term "RADIATION WEATHER," 
 written in capital letters, denotes accurately the character 
 by night and by day, in summer or in winter. 
 
 Moreover, radiation is only a secondary product of an 
 anticyclone. The primary feature is calm ; radiation 
 follows as a matter of course, and the weather due to 
 radiation varies enormously in different latitudes, in 
 different seasons, and in -different localities in the same 
 country. 
 
 The theory of radiation is, that when the air is still 
 the heated surface of the earth radiates into the cold 
 surrounding space, and so the former grows cold enough 
 to condense vapour in the air near the ground, or dew on 
 a suitable surface, such as grass. But when there is more 
 or less wind, each successive layer of air which is in 
 contact with the radiating earth gets removed so rapidly 
 that this condensing process cannot take place, and then 
 no dew or fog is formed. Thus the kind and amount of 
 cloud is variable, but always dependent on radiation. 
 
 On the contrary, the particular kind of cloud which 
 forms in any portion of a cyclone is directly and primarily 
 due to the rising current induced by that form of 
 rotating air, and ve shall see in a future chapter that 
 the day character of these clouds is not altered during 
 the night. 
 
 E 
 
50 WEATHER. 
 
 From the same example of a fine day we can readily 
 pass to the idea of diurnal variation. In the kind of day 
 which we have just described we find a regular sequence 
 from fog to blue, and back again to fog, following the 
 course of the sun that is, the time day. 
 
 But besides the amount of mist, every other meteoro- 
 logical element has a complex series of diurnal changes 
 which depend on the time of day. For instance, both 
 the direction and velocity of the wind have a marked 
 diurnal period, and so has the amount of cloud, the 
 amount of vapour, and every other component of weather. 
 These, and many others, will form the subject of a sub- 
 sequent long and rather difficult chapter. All that we 
 have to note here is the important principle of meteorology 
 that the primary character of all weather is given by 
 the shape of the isobars, whether cyclonic, anticyclonic, or 
 otherwise ; that a complex series of diurnal changes are 
 superimposed on this, which modify, but do not alter the 
 intrinsic quality ; and that the resulting weather is the 
 sum of the two together. Thus in a cyclone the changes 
 of weather due to its motion are so marked and so strong 
 that diurnal changes are often entirely obliterated ; while 
 in a calm anticyclone, where there is no motion, the 
 uniform character of the general weather allows full play 
 to radiation, and the diurnal changes are very prominent. 
 
 Throughout this work we shall call the character and 
 changes of weather which are due to the shapes of the 
 isobars, the general character and general changes, 
 because they are caused by alterations in the general 
 distribution of pressure over a large portion of the earth's 
 surface. On the contrary, changes which are due to tho 
 
WEATHER-PROGNOSTICS. 51 
 
 time of day, the season of the year, or to any local 
 peculiarity, we shall call diurnal, seasonal, or local varia- 
 tions of the general character. The first are really 
 changes, the second only variations. The reason why 
 many prognostics which are due to radiation and diurnal 
 causes are signs of settled fine weather, is because in a 
 country like England they can only occur in an anti- 
 cyclone. An anticyclone means settled fine weather, not 
 only because the weather at any moment in it is fine, but 
 because it is usually stationary, and so there is nothing 
 to change the existing conditions. All anticyclone prog- 
 nostics fail when the anticyclone breaks up suddenly, or 
 in the not very common case when it moves onward along 
 a definite path. 
 
 We will now give a few prognostics due to the varia- 
 tions of an anticyclone in some detail. 
 
 The sky being generally clear and the air calm, the 
 temperature is high in the day and low at night. In 
 summer brilliant sunshine prevails during the day, and 
 at night there is a heavy dew, and, in low-lying places 
 mist. 
 
 " Heavy dews in hot weather indicate a continuance of 
 fair weather, and no dew after a hot day foretells rain." 
 
 "If mists rise in low ground and soon vanish, expect 
 fair weather." 
 
 Fine, bright, genial weather raises the spirits and 
 exerts an enlivening influence not only on human beings, 
 but also on animals, birds, insects, etc. Hence the 
 sayings 
 
 "When sea-birds fly out early and far to seaward, 
 moderate winds and fair weather may be expected." 
 
52 WEATHER. 
 
 " If rooks go far abroad, it will be fine." 
 
 "Cranes soaring aloft and quietly in the air fore- 
 shadows fair weather." 
 
 " If kites fly high, fine weather is at hand." 
 
 " Bats or field-mice coming out of their holes quickly 
 after sunset and sporting themselves in the open air, 
 premonstrates fair and calm weather." 
 
 " Chickweed expands its leaves boldly and fully when 
 fine weather is to follow." 
 
 These are merely samples of innumerable similar 
 prognostics in all parts of the world. 
 
 In winter frost is generally prevalent in the central 
 area of an anticyclone, accompanied frequently by fog, 
 which is most dense in the neighbourhood of large towns. 
 This is all due to the radiation of calm weather. 
 
 " White mist in winter indicates frost." 
 
 The wind is usually very light in force. 
 
 "It is said to be a sign of continued good weather 
 when the wind so changes during the day as to follow the 
 sun." 
 
 This "veering with the sun," as it is called, is the 
 ordinary diurnal variation of the wind, which in England 
 is only very obvious with the shallow gradients of an 
 anticyclone. At seaside places in summer very often 
 " the wind is in by day and out by night," which is the 
 equivalent of the land and sea breezes of the tropics. 
 Like the preceding prognostic, it is only in anticyclones 
 that local currents of air, probably due to unequal heating 
 of sea and land, can override the general circulation of 
 the atmosphere in this country. 
 
 Sometimes in winter, on the southern side of the anti- 
 
WEATHER-PROGNOSTICS. 53 
 
 cyclone, bitter east winds with a black-looking sky will 
 prevail for several days together, when it may truly be 
 said 
 
 " When the wind is in the east, 
 It is neither good for man nor beast." 
 
 This class of anticyclone prognostics hold good as 
 long as the anticyclone remains stationary. Occasionally 
 the anticyclone moves on, and is replaced by some other 
 form of isobars ; but far more frequently the anticyclone 
 breaks up that is to say, it disappears without moving 
 on, and is replaced by a cyclone or some other type of 
 isobars. 
 
 WEDGE-SHAPED ISOBARS. 
 
 We have already defined wedge-shaped isobars as a 
 projecting area of high pressure moving along between 
 two cyclones. This wedge may point in any direction, 
 but in practice by far the most frequently to the north. 
 We have therefore selected such a one for the diagram 
 of the wind and weather in an ideal wedge, which we give 
 in Fig. 7. There the highest pressure is at the bottom 
 of the diagram, while the wedge-shaped isobars project 
 towards the north. On the right hand we see the rear of 
 a retreating cyclone ; on the left, the front of an advancing 
 depression. As these two cyclones move forward, the 
 wedge goes on between them, so that there must always 
 be a line of stations where, after the barometer has risen 
 owing to the onward passage of the first cyclone, the 
 mercury has just begun to fall, owing to the advance 
 of the second depression. This line is called the crest 
 
WEATHER. 
 
 of the wedge, and is marked by a dotted line in the 
 diagram. 
 
 The wind blows round the wedge in accordance with 
 the universal law of gradients. Thus on the east side of 
 the wedge the wind is from north-west ; in the centre it 
 is calm ; and on the west side, from south-west to south- 
 
 \CREST 
 
 Refraction 
 
 Rain 
 
 3o- 2 
 
 Blue 
 
 FIG. 7. Wedge-shaped isobar prognostics. 
 
 east, as marked by the symbols on the diagram. In 
 practice the gradients are never steep, so the force of the 
 wind rarely rises to above that of a pleasant breeze. 
 
 The broad features of the cloud and weather in a 
 wedge are written across the diagram (Fig. 7). In front 
 
WEATHER-PROGNOSTICS. 55 
 
 we find blue sky, with beautifully fine weather, refraction, 
 and that unusual clearness of the atmosphere known as 
 "visibility." Nearer the calm under the crest we come to 
 radiation weather, with fog; and then to halo-bearing 
 sky just in front of the crest, and stripes of cirrus-cloud. 
 A thunderstorm or heavy shower is often experienced at 
 the top of the wedge. In rear of the crest the sky 
 becomes covered with cirro-stratus cloud, and further in 
 rear we find the rain of the approaching cyclone. 
 
 Here, as in cyclones, we see the striking fact that the 
 words applied to describe the weather contain the phrase- 
 ology of many familiar prognostics, such as those con- 
 nected with visibility, halos, or the stripes of cirrus which 
 form the cloud popularly known as " Noah's Ark." 
 
 If we remember how we took a sort of section across a 
 cyclone, and so found the sequence of weather at any 
 station, we shall readily understand that we have only to 
 read this diagram (Fig. 7) from right to left to get the 
 sequence of weather during the passage of a wedge. 
 Thus we should have a beautifully fine day, with a north- 
 west wind and a rising barometer, with a hot sun by day, 
 and a cold night with radiation according to the season 
 of the year. Then, while the barometer was still rising, 
 the blue sky would assume that peculiar nebulous white- 
 ness which forms halos, and stripes of cirrus would appear 
 in places. Soon the barometer would begin to fall, the 
 sky to grow denser and overcast, and before long the 
 drizzling rain of the new cyclone would begin to fall, 
 the wind having previously backed to the south-west. 
 
 We now see the meaning of the halo and cirrus-stripes 
 being marked in the diagram partly in front of the crest 
 
56 WEATHER. 
 
 of the wedge, viz. that in a wedge the sky shows the 
 approach of a new cyclone before the barometer at a 
 single station has ceased to rise. This is very interesting, 
 as it is the first opportunity we have had of explaining 
 why the barometer sometimes appears to fail, and rises, as 
 in this case, while bad weather is manifestly approaching. 
 It also shows the great additional power of forecasting 
 which the use of synoptic charts gives to a meteorologist, 
 seated in a central office, with abundant telegraphic com- 
 munication. Suppose any morning that a forecaster found 
 his isobars were wedge-shaped. He could then telegraph 
 to the eastern districts of his territory that the fine 
 weather would not last, though they with their rising 
 mercury might have thought the contrary. 
 
 It may be remarked here that all cyclones are not 
 preceded by a wedge, but only those which roll, as it 
 were, along the northern edge of large stationary anti- 
 cyclones. 
 
 We can now explain in detail the prognostics that are 
 marked on the diagram, and several others for which 
 there was no room. Any appearance of the sky which 
 characterizes the front of a wedge will be a sign of rain, 
 because there is always rain in rear of that shape of 
 isobars. These prognostics of wet which are associated 
 with fine, dry weather are particularly interesting, because 
 they are the very opposite of the rain-prognostics in a 
 cyclone, which are associated with increasing damp and 
 a dirty sky. 
 
 It used to be thought that every prognostic of rain 
 would be explained by showing that the appearance was 
 due to an increase of vapour in the air ; here we find that 
 
WEATHEE-PROGNOSTICS. 57 
 
 the prognostic can only be explained on the supposition 
 that many cyclones develop an area of calm and clear sky 
 in front of, and as a portion of, themselves. It is a crude 
 idea of meteorology to think that all rain-prevision depends 
 on hygrometry. 
 
 Keturning now to Fig. 7, we see that in the rear of 
 the retreating cyclone the air is dry and the weather 
 beautifully fine of the sort of which we would say that it 
 was " too fine to last ; " or, if it lasted a whole day, we 
 should talk of a "pet day." 
 
 During the day the sun is burning hot. 
 
 " When the sun burns more than usual, rain may be 
 expected." 
 
 During the night white frost is formed, owing to 
 calm radiation. 
 
 " A white frost never lasts more than three days ; a 
 long frost is a black frost." 
 
 "Frost suddenly following heavy rain, seldom lasts 
 long." 
 
 As the day advances, after a white frost, the air 
 becomes dull from the influence of the on-coining depres- 
 sion. Whence the saying 
 
 " When the frost gets into the air, it will rain." 
 
 During the very fine weather on the east side of a 
 wedge-shaped area there is often great visibility, with a 
 cloudless sky. 
 
 " The further the sight, the nearer the rain." 
 
 This is one kind of visibility ; there is another class 
 that is associated with a hard, overcast sky, as we shall 
 explain under " straight isobars," and also visibility in 
 the tropics, which depends on causes which cannot be 
 
58 WEATHER. 
 
 explained here. From the same transparency of the 
 atmosphere, the " ashy " light of the dark portion of a 
 new moon is very strong. 
 
 "If the old moon embraces the new moon, stormy 
 weather is foreboded." Great confidence is placed in this 
 old prognostic. 
 
 " I saw the new moon, late yestreen, 
 With the old moon in her arm, 
 And I fear, I fear, my master dear, 
 We shall have a deadly storm." 1 
 
 At the extreme north-west edge of a cyclone there is 
 often a particular kind of "refraction" a well-known 
 sign of rain. This seems to be due to the cold air in the 
 rear of a cyclone being much below the temperature of the 
 sea. If so it is a sign of rain, for the reason that one 
 cyclone is usually soon followed by another. There is 
 another kind of refraction caused by a cool south-east 
 wind in an anticyclone blowing over a heated sea, which 
 is usually a sign of fine weather. This is a good illus- 
 tration of how the same prognostic may portend either 
 good or bad weather, according to its surroundings. 
 
 If the cyclone in front of the wedge has produced a 
 north-west gale, it is not improbable that the on-coming 
 one may begin with a south-west gale. Hence the 
 significance of the well-known nautical saying in the 
 Atlantic 
 
 "A nor'-wester is not long in debt to a sou'-wester." 
 
 In the cyclone and secondary we have found rain of 
 different kinds ; so in the anticyclone and wedge we have 
 found fine weather of different kinds. Anticyclone fine 
 weather is almost always hazy, and is settled weather, 
 
WEATHER-PROGNOSTICS. 59 
 
 because the anticyclone itself is usually stationary. 
 Wedge fine weather is always clear, and is only temporary 
 because the wedge is never stationary. Hence we see 
 that when we talk of rain and fine weather, it is often 
 necessary to say what kind of rain and what kind of fine 
 weather ; and we find, moreover, that a knowledge of the 
 kind of isobars enables us to define the kind of weather. 
 In many discussions on climate and statistical meteor- 
 ology, we find that terrible confusion is caused by mixing 
 up together all kinds of good and bad weather. 
 
 The prognostics which are associated with a wedge are 
 almost less liable to failure than those which accompany 
 other shapes of isobars. When they do fail, it is usually 
 from a sudden break-up of all the existing distribution of 
 pressure. 
 
 STRAIGHT ISOBARS. 
 
 Straight isobars are so called because the isobars have 
 no curvature. The trend of the lines may be in any 
 direction, and so may their slope. For instance, the lines 
 may lie east and west, but the slope may be either towards 
 the north or towards the south. In our general diagram 
 (Fig. 1) of all the fundamental shapes of isobars, we drew 
 some straight isobars sloping to the south. In temperate 
 regions this slope is uncommon, while a slope to the north 
 or north-west is very common. We have therefore selected 
 an instance of a northerly slope for our diagram of straight 
 isobars in Fig. 8, and, as before, have written in words the 
 kind of sky and weather which we find in different parts 
 of the slope. In all the other shapes of isobars which we 
 
60 
 
 WEATHER. 
 
 have hitherto described, the lines enclose an area of high 
 or low pressure, while in straight isobars the lines only 
 mark the position of what may be called a barometric 
 slope. 
 
 On turning to Fig. 8, it will be seen that while the 
 pressure is high to the south, it is generally low to the 
 
 Cloudy- Visibility Audibility 
 
 falling 
 
 Hard 
 
 Feathery 
 
 Cirrus 
 
 300 
 
 Blue Sty 
 
 FIG. 8. Straight isobar prognostics. 
 
 north, without any definite cyclonic system, and that 
 the isobars run straight nearly east and west, with a slope 
 towards the north. The wind is from the south-west or 
 west, and usually strong and gusty, but short of a gale. 
 On the high-pressure side the sky is blue; then as we 
 
WEATHER-PROGNOSTICS. 61 
 
 approach the low-pressure, feathery cirrus, or some form 
 of windy sky, makes its appearance, while a blustery wind 
 whirls the dust or blows the soot down. The falling of 
 soot refers to blacks falling out-of-doors and coming into 
 windows from being blown about. Sometimes, in very 
 damp weather, soot seems to fall from condensation of 
 vapour on itself, and at other times masses of soot fall 
 down a chimney from the action of hail or very heavy 
 rain. 
 
 Getting still nearer the low-pressure, the sky is found 
 to be gathering into hard strato-cumulus,at first with chinks 
 between its masses, through which divergent rays stream 
 down under the sun, which is spoken of as " the sun 
 drawing water." Sometimes, especially in winter, these 
 rays are lurid, and the appearance of the sky is then very 
 striking. This prognostic is common all over Northern 
 Europe, and in Denmark takes the form of "Locke 
 is drawing water." Loki is a well-known demi-god in 
 the Scandinavian Eddas, so that we have here a direct 
 survival of mythic speech. This hard strato-cumulus is 
 especially characteristic of straight isobars in Great 
 Britain. 
 
 At the same time there is often great "visibility," 
 with a hard, overcast sky and moderately dry air, in 
 which the cloud seems to play the part of a sunshade, 
 for as soon as the sun comes out the clearness of distant 
 objects diminishes. This visibility must not be confounded 
 with the visibility already described with a cloudless sky, 
 which occurs with wedge-shaped isobars. j 
 
 Simultaneously we often find " audibility." This dis- 
 tinctness of distant sounds must be carefully distinguished 
 
62 WEATHER. 
 
 from sounds which are not usually heard, being brought 
 up by the wind coming from a rainy quarter. For instance, 
 the whistle of a railway-train to the south of a house will 
 not be usually heard with the normal south-west wind of 
 Great Britain ; but when the wind backs to the south in 
 front of a depression, then the noise will be heard ; and 
 though this will be a good prognostic, still, it is not true 
 audibility. 
 
 "When the gradients are very steep, a little rain some- 
 times falls with straight isobars, generally in light showers, 
 with a hard sky. 
 
 Though, as a matter of convenience, we have described 
 the sequence of weather as we proceed from the high to 
 the low pressure, it must be clearly understood that it 
 does not represent the sequence of weather to a single 
 observer, but rather what the weather will be simultane- 
 ously in different parts of the country ; for instance, that 
 if there is cirrus in London, there may perhaps be a 
 lurid sky in Edinburgh. 
 
 But now audibility, visibility, whirling dust, and lurid 
 chinks with divergent rays are well-known signs of rain 
 almost all over the world, so we have to explain why the 
 appearance of the sky in straight isobars is a sign of rain. 
 It is found by experience that straight isobars are never 
 persistent, and that, practically, the district which they 
 cover one day will be traversed by a cyclone the next 
 day. It does not follow that the cyclone is necessarily in 
 existence when we observe the straight isobars ; but, from 
 the nature of weather-changes, straight isobars seem to 
 be an intermediate form of atmospheric circulation which 
 precedes the formation of a cyclone. 
 
WEATHER-PROGNOSTICS. 63 
 
 We cannot, therefore, draw a section across straight 
 isobars and say that it will give the sequence of weather 
 at any place, for we are not dealing with a moving form 
 of pressure, but with a transitional state of things which 
 cannot last long. The chief interest of these rain prog- 
 nostics lies in the contrast which they present to those 
 associated with a cyclone. While those in a cyclone are 
 accompanied by an almost ominous calm and a dirty, 
 murky sky, these are associated with a hard sky and 
 blustery wind, of which it would be ordinarily remarked 
 " that the wind keeps down the rain," or, " that when the 
 wind falls, it will rain." While, also, the prognostics 
 which precede cyclone-rain hold good for the reason that 
 they are seen in front of the rainy portion of such a 
 depression, those associated with straight isobars hold 
 good because, though there is little rain actually with 
 them, the area which they cover to-day will probably be 
 covered by a cyclone to-morrow the conditions being 
 favourable for the passage of depressions. Another point 
 of contrast lies in the comparative dryness of the air in 
 straight isobars, as compared with the excessive amount 
 of moisture which precedes cyclones. The same remarks 
 apply to these as to the fine-weather prognostics associated 
 with wedge-shaped isobars. 
 
 All the prognostics we have discussed under this 
 heading fail when the straight isobars are formed during 
 a general rearrangement of the whole distribution of 
 pressure over the northern hemisphere, because a cyclone 
 may not then traverse the district where the well-known 
 signs of rain had been observed. 
 
 The other fundamental forms of isobars V-shaped 
 
64 WEATHER. 
 
 depressions and cols are not associated with any dis- 
 tinctive prognostics, so we will defer our consideration 
 of these shapes till a subsequent chapter. 
 
 GENERAL REMARKS. 
 
 We are now in a position to take a general survey of 
 the whole principle of prognostics, and to answer the 
 questions which, we mentioned at the commencement of 
 the chapter, were formerly considered insoluble. What is 
 the place of prognostics in meteorology, and how has 
 modern research developed their utility? Why do prog- 
 nostics sometimes fail? Why are not all prognostics 
 associated with increasing damp ? Why is rain or fine 
 weather not always preceded by the same prognostic ? 
 
 The details which we have already given abundantly 
 show that every portion of every shape of isobars has a 
 characteristic weather and look of sky, and that prog- 
 nostics simply describe these appearances. 
 
 Theoretically, then, when the isobars are well defined, 
 we ought to be able to write down the prognostics which 
 might be visible everywhere, but practically we cannot 
 do so completely; and also, theoretically, all that any 
 prognostic does is to enable a solitary observer to identify 
 his position in any kind of atmospheric circulation. Thus 
 the associates of the front of a cyclone or secondary are 
 signs of bad weather ; while those of the rear of a cyclone, 
 or of any portion of an anticyclone, are signs of fine 
 weather. The word "front" implies not only the idea of 
 motion, but also of the direction of that motion. But 
 here comes in the reason why prognostics can never 
 
WEATHER-PROGNOSTICS. 65 
 
 develop the science of forecasting much further than at 
 present. From the nature of cyclone-motion, as will be 
 abundantly illustrated in a future chapter on Types of 
 Weather, these depressions have a way of advancing so far 
 in a certain direction, and then of either changing their 
 front or else of dying out altogether. No prognostic can 
 give any clue to the probability of either of these 
 changes. On the other hand, a forecaster in a central 
 office, with synoptic charts, can often tell when a cyclone 
 is going to be arrested or deflected from its previous 
 course, and this branch of the science is undoubtedly 
 capable of very great extension. 
 
 Then the question may very naturally be asked, How 
 far the introduction of synoptic charts has developed our 
 knowledge of prognostics ? So far as explaining their 
 true nature, the advance has been very great ; but so far 
 as increasing their practical utility, the progress has been 
 much less. The new explanations have already been 
 given. The principal improvement in reading the in- 
 dications of prognostics is rather that the observer can 
 distinguish between the different kinds of rain and fine 
 weather, and so give greater precision to his previsions, 
 than that he can alter the reputed value of any weather- 
 saying. For instance, if he sees a halo, with a falling 
 barometer and increasing wind, he knows that he is in 
 for the whole sequence of a cyclonic storm ; whereas, if he 
 sees a halo, with a steady barometer and a few angry 
 gusts, he knows that he need only expect heavy rain 
 without any great wind. 
 
 We have already mentioned why prognostics some- 
 times fail, either from the alteration of a cyclone's front, 
 
 F 
 
WEATHER. 
 
 the breaking up of an an ticy clone, and other similar 
 reasons. For those who are unfamiliar with the nature 
 of weather-changes, an actual example will be more 
 acceptable than a general statement. We will select an 
 instance of the failure of a halo-prognostic. Let us look 
 at the chart in Fig. 9 for February 5, 1883, at 8 a.m. 
 At Folkestone, near Dover, in England (marked F in the 
 
 FIGS. 9 and 10. Illustrating the failure of the prognostic that halo 
 indicates wind or rain. 
 
 diagram), a halo was visible off and on from 9.30 a.m. to 
 4 pm., and that was due to the front of the cyclone 
 which is seen lying off the north-west coasts of Great 
 Britain. The whole of that day and night, as well as the 
 succeeding day, was very fine, so that the prognostic 
 might seem to have failed. Before the days of synoptic 
 
WEATHER-PROGNOSTICS. 67 
 
 charts this is all that we could have said about the 
 matter, but now we can explain the reason why. 
 
 In the first chart (Fig. 9) we see an anticyclone 
 marked by the isobar of 30'3 ins. over France, and the 
 extreme edge of another over part of Norway. The col 
 between them covers Denmark and the North Sea. The 
 extreme rear of a cyclone is found near Copenhagen, 
 while on the north-west of Ireland a new cyclone of 
 enormous diameter is approaching. The halo-forming 
 portion of this last has almost reached Dover, while the 
 rainy portion is already causing precipitation over Ireland. 
 If the cyclone continued its course, in due time the rain- 
 area would reach England ; but suppose the cyclone stood 
 still, or some rearrangement of pressure arrested its 
 onward progress, what would happen then ? Why, the 
 halo-prognostic at Dover would fail ; that is, would not 
 be followed by rain and wind, as is usually the case. Now, 
 this is exactly what happened. If we look at Fig. 10, 
 which gives the synoptic conditions after an interval of 
 ten hours at 6 p.m. the same day we see that the two 
 small anticyclones have coalesced into a single large one, 
 which lies over Sweden and North Germany, while the 
 Irish cyclone has disappeared, instead of moving as usual 
 to the north-east; and that a barometric slope, with 
 nearly straight isobars, covers Great Britain and France. 
 In consequence of these changes, the weather remained 
 fine all day near Dover, and so the prognostic appeared 
 to fail. 
 
 We will explain, in another chapter on Forecasting by 
 Synoptic Charts, why a forecaster in a central bureau 
 could not have announced with certainty that there would 
 
68 WEATHER. 
 
 have been no rain in that portion of England ; and also 
 when and how prognostics sometimes assist him in fore- 
 telling rain which he would hardly have expected from 
 the mere inspection of the isobars. 
 
 These charts also give an idea of the extreme rapidity 
 of meteorological changes in a climate like that of 
 Western Europe. The short space of ten hours has been 
 sufficient not only to alter very materially the general 
 distribution of pressure, but also to form new configura- 
 tions of isobars. These charts also show that changes of 
 weather are not only caused by the passage of well-defined 
 shapes of isobars, such as cyclones, in the manner which 
 we have just described, but also by the readjustment of 
 pressure-distribution and the formation of new isobaric 
 shapes over the area under observation. This last con- 
 ception is of fundamental importance. 
 
 Then we come to the question why all rain- prognostics 
 are not associated with increasing or excessive damp. 
 The answer to this is that there are different kinds of 
 rain, such as the rain in front of a primary cyclone, which 
 is associated with great damp ; and the light showers of 
 straight isobars, which are associated with a rather dry 
 air. Also that some rain-prognostics, such as those 
 associated with the much too line weather in front of a 
 wedge, owe their value to the fact that a wedge precedes 
 a cyclone, though the air in itself is tolerably dry. 
 
 A similar train of argument applies to the question 
 why rain and fine weather are not always preceded by the 
 same prognostics. We have just mentioned two different 
 sorts of rain, and, as regards fine weather, we need only 
 mention that in a like manner there are manv kinds of 
 
WEATHEE-PROGNOSTICS. 69 
 
 fine weather. For instance, the formation of small blue 
 patches in an otherwise overcast sky in rear of a cyclone 
 foretells one kind of fine weather, while the radiation 
 phenomena of an anticyclone also indicate fine weather, 
 but of a totally different sort. 
 
 We have shown why the value of the indications 
 which prognostics afford can never be materially im- 
 proved, but at the same time no advance in synoptic 
 meteorology will ever supersede the use of prognostics. 
 Our own researches on hurricanes in the tropics have 
 proved that there, as in Europe, unusual colouration of 
 the sky at sunrise and sunset apparently often precedes the 
 formation of any notable barometric depression ; so that 
 sometimes the indications of prognostics are ahead of 
 those of any other system of forecasting. In isolated 
 stations, and on board ship especially, an observer must 
 always rely to a great extent on his own eyes to gather 
 information from the aspect of the sky as well as from 
 the readings of his own barometer. We shall, in fact, 
 devote a whole chapter at the end of this book to the 
 consideration of the problem of how much weather-fore- 
 casting a solitary observer can do for himself. In the 
 following chapter we shall consider a great many prog- 
 nostics connected with clouds which conld not well be 
 associated with definite shapes of isubars. 
 
70 WEATHER. 
 
 CHAPTER III. 
 
 CLOUDS AND CLOUD-PROGNOSTICS. 
 
 '!N this chapter we propose to discuss the nature of clouds 
 by first explaining their origin and the varying conditions 
 under which they are formed. 
 
 This will lead to a classification of their different 
 shapes and forms, and give us a certain insight into the 
 varying velocity and direction of the upper currents of 
 the atmosphere. But when we come to the more modern 
 developments of cloud-knowledge, we shall have to con- 
 sider the relation of clouds to the great areas of high and 
 low pressure, which we have already described as cyclones 
 and anticyclones. 
 
 Some of this we have already seen in our chapter on 
 prognostics, where we showed that different kinds of 
 cloud are characteristic of different portions of cyclones, 
 etc. But in this chapter we will explain how, from a 
 study of cloud-motion in the upper strata, we are enabled 
 to discover much about the real nature of the circula- 
 tion of the air in the different shapes of isobars. In the 
 course of our remarks, we shall explain incidentally 
 both the meaning and value of the older cloud-lore, and 
 
CLOUDS AND CLOUD-PROGNOSTICS. 71 
 
 also the great development in the science of forecasting 
 by means of clouds which has been made by recent 
 researches. 
 
 NOMENCLATURE OF CLOUDS. 
 
 Unfortunately, in approaching the subject of cloud- 
 nomenclature, we come to one of the most unsatisfactory 
 branches of meteorology. Though the words of Howard's 
 nomenclature are universally employed, the same word 
 is by no means always employed for the same kind of 
 cloud ; and for this reason, though the words we shall 
 employ to designate clouds are those wbi'-h are used by 
 many, we shall be very careful to describe exactly the 
 kind of cloud we mean by any particular name. 
 
 For practical purposes clouds are divided into four 
 different classes, according to their most obvious differences 
 of shape ; but these classes are only as a matter of con- 
 venience, for in nature they all run into each other by 
 imperceptible gradations. The forms are 
 
 1. Cumulus. All cloud which has a rot ky or lumpy 
 look is either pure cumulus or must contain the word 
 cumulo in combination with some other name. 
 
 2. Stratus. All cloud which lies as a thin flat sheet 
 must either be pure stratus or contain the word strata in 
 combination. 
 
 3. Cirrus. All cloud which has a wispy, feathery, or 
 curly look must either be pure cirrus or must contain 
 the word cirro in combination. 
 
 4. Nimbus. Any cloud from which rain is falling is 
 nimbus in some form. 
 
72 WEATHER. 
 
 It must be fully understood that these names and 
 their derivatives, which we shall give presently, do not 
 by any means exhaust all the varieties of clouds which 
 very experienced observers can detect and classify. All 
 that we propose to give here are the broad distinctions 
 which anybody can understand, for all meteorologists 
 who have to deal with corps of observers are agreed that 
 eight or ten names are as many as can practically be 
 employed. 
 
 We will first explain the simple forms of these 
 clouds, and then the more complicated combinations, such 
 as cirro-stratus, cirro-cumulus, cumulo-stratus, etc. But 
 besides giving a broad classification to the leading kinds 
 of cloud, these terms also give a rough relative scale of 
 altitude. Thus in practice stratus and cumulus are usually 
 the lowest, the composites the middle, and cirrus the 
 highest layer of cloud ; but no absolute level can be 
 assigned to each stratum at any season, or in any country. 
 For instance, cumulus may be as low down as 2000 feet, 
 and cirrus as about 12,000 feet ; and, on the other hand, 
 cumulus may be formed up to at least 25,000 feet, and 
 cirrus probably up to at least 50,000 feet ; but true cirrus 
 can never be formed under cumulus, whatever the relative 
 latitudes may be. 
 
 These relative heights also partially determine the 
 nomenclature. If a cloud is very high up, we have to add 
 the word cirro, to indicate altitude, to the word which 
 denotes the form only; while cumulo would suggest a 
 lower level. The first word before a compound name 
 gives the idea of relative altitude : thus cirro-cumulus is 
 higher than cnmulo-cirrus. 
 
CUMULUS. 
 
 Pure cumulus may be described as convex or conical 
 heaps increasing upwards from a flat horizontal base, as 
 in Fig. 11, a. It is undoubtedly formed by the condensa- 
 tion of the summit of an ascensional column of vapour- 
 laden air, as shown by the dotted lines. When this is 
 cooled, either by rising into a colder stratum than that 
 from which it started, or by expansion, the water-vapow 
 
 t 
 
 t 
 
 <E^<^ 
 
 FIG. 11. Cumulus and cirrus, a. Cumulus, surface rapid. I. Cirrus, 
 surface rapid, c. Cumulus, upper rapid, d. Cirrus, upper rapid. 
 
 condenses into cloud, like the condensed steam from an 
 engine. 
 
 The flat base marks the level where condensation 
 temperature is reached, and the upper rocky summit 
 represents the heads of the air-columns protruding into 
 a cold space. A cumulus is, in fact, the visible capital of 
 an ascensional column of air. 
 
74 WEATHER. 
 
 There is one very remarkable feature of /*11 cumulus : 
 it is never seen as such overhead, but only ou ^he horizon, 
 or at a moderate height above it. 
 
 The reason is obvious, that as the flat-based mass 
 drifts overhead, the flat under-surface hides the character- 
 istic rocky top, so that we no longer see the typical 
 features of this kind of cloud. 
 
 In Northern Europe, and the interior of many conti- 
 nents, cumulus usually only forms during the summer 
 months ; for the absolute amount of vapour in the air 
 during the winter months is rarely sufficient to develop 
 a lump of cloud. 
 
 EELATION TO CIRRUS. 
 
 If the ascensional column is stationary, we get a very 
 curious appearance ; the top of the cloud seems to be, 
 and is, in a state of commotion, but still the cloud as a 
 whole does not move in any direction. This is very 
 puzzling at first, and is not uncommon before thunder- 
 storms ; but we can readily understand its origin by 
 watching the stationary cloud on a hilltop. Then we 
 see the same contradictory appearance a cloud in rapid 
 motion, but never moving forwards. The reason is that, 
 as each fresh portion of cloud is projected upwards and 
 blown away by the wind, it is immediately evaporated, 
 but a new column of vapour instantly takes its place. 
 But suppose that, whether stationary or moving, the 
 rising column, after depositing a certain amount of its 
 vapour at one level, continues to rise, it will at length 
 reach a second level, at which the condensation-point of 
 
CLOUDS AND CLOUD-PROGNOSTICS. 75 
 
 the diminished amount of vapour which it now contains 
 will be again reached, and a second layer of cloud will be 
 formed at a higher level. Sometimes this will ba deposited 
 as another cumulus, but more frequently the rising column 
 has so much diminished both in column and upward 
 velocity that the vapour is condensed in a thin, hairy, or 
 curly form, as in Fig. 11, 6. This is pure cirrus, or curl- 
 cloud, and may often be seen floating above a cumulus. 
 If the upper and lower strata of air are moving at an 
 equal speed, the cumulus, when once formed, sails on 
 without any change of shape from the action of the wind, 
 however much it may alter from any difference in the 
 supply of uprising vapour. But if, as in Fig. 11, a, we sup- 
 pose an ascensional column of air to start from near the 
 earth's surface, and, when it has risen nearly to its con- 
 densation-level, to meet an upper current in the same 
 direction as itself, but moving more slowly, then we would 
 get a flat-based cumulus, headed back, as it were, like 
 the cloud, marked a in the figure, while the whole mass 
 would move end on with the wind. 
 
 In Europe this is practically only found in the rear of 
 cyclones, and we may therefore deduce, from the shape 
 of this cloud, that there the surface is quicker than the 
 upper current. 
 
 Suppose, now, that the column of air was attenuated 
 into a thread, then under the same conditions we should 
 get a feathery cirrus marked b in the figure, also moving 
 end on. If, now, under similar conditions, the upper 
 current is moving faster than the lower one, we should 
 get in the lower strata a cumulus heading forwards as 
 marked c ; and if at a higher level the ascensional column 
 
76 WEATHER. 
 
 was attenuated into a thread, then we should have a light, 
 hairy cirrus marked d, moving long-ways with the wind. 
 In this case, sketched from nature, we may note that the 
 foremost curl of the cloud is lumpy like a small cumulus, 
 instead of hairy like the other tufts. 
 
 It is from simple illustrations of this sort that we 
 seem to find the connecting link between some forms of 
 cirrus and cumulus. If cumulus is the visible capital of 
 an ascensional column of air, cirrus is the visible form 
 of the condensation of a column attenuated or wire-drawn 
 into a thread. Of the three threads which have formed 
 the cloud d t two have died out as hairs, but the third, a 
 little more intense, had had enough volume or energy to 
 form a tiny nubecule. 
 
 A common case in which we can trace the gradual 
 development of fibrous cloud into cumulus occurs on any 
 bright morning with a blue sky and heavy dew. When 
 the sun gains a little power, the first threads of cloud 
 formed by the moisture rising into the air are usually 
 condensed into cirrus; these quickly get big and swell, 
 till in a very short time the sky is nearly covered with 
 true cumulus. 
 
 The reason why the stripes a and b are slanting is 
 that in them we have supposed, as is often the case, that 
 the cloud continues to rise after it has begun to condense ; 
 in c and d, on the contrary, the cloud has ceased to rise, 
 and the stripe lies straight. 
 
 Ley has shown that descending threadlets of icy 
 particles can be drawn into wispy cirrus, if they fall into 
 a layer of air which moves more or less quickly than 
 themselves. The conditions for such threadlets would be 
 
CLOUDS AND CLOUD-PROGNOSTICS. 77 
 
 found if the upper outflow from a cyclonic vortex injected 
 vapour into the cold still air of an adjacent anticyclone. 
 
 Now for the weather-portent of pure cumulus. In 
 our chapter on prognostics we have shown that cumulus is 
 the specially characteristic cloud of the rear of a cyclone, 
 and it is then often associated with showers. When this 
 occurs we have cumulo-nimbus ; that is, rain falling from 
 cumulus to distinguish it from strato-nimbus, where rain 
 falls from stratus cloud. At the edge of anticyclones 
 we often have a fine-weather light cumulus, which is 
 apparently formed by the simple rising of the vapour 
 evaporated from the ground by the heat of the sun, in 
 contradistinction to the cyclone-cumulus, which we may 
 suppose to be produced by the ascensional currents which 
 are generated by the whirling motion of an atmospheric 
 eddy. The form alone does not enable us to say whether 
 a cumulus indicates good or bad weather. This cloud, 
 like every other, must be judged by its antecedents and 
 surroundings. 
 
 Cumulus is the almost universal cloud of the tropics, 
 and may indicate either fine weather or non-isobaric rain, 
 according to circumstances. 
 
 FESTOONED CUMULUS. 
 
 There are two or three modifications of cumulus which 
 it is important to notice, as they throw much light on the 
 nature of cloud-formation. In looking at them, as at all 
 others, we must consider the life-history of a cloud from 
 what it originates, and to what it develops. Sometimes 
 the lower base of a cumulus assumes a festooned appear- 
 
78 WEATHER. 
 
 ance, as in Fig. 12, 1. In Orkney, this is known as the 
 " pocky cloud," and is there usually followed by a severe 
 gale of wind. In Lancashire, the festoons are called 
 " rain-balls," and are only considered a sign of rain. We 
 have frequently seen them in all parts of the Tropics ; but 
 all festooned forms of cloud are unknown in Scandinavia. 
 For a technical international name, Poey has suggested 
 " globo-cumulus," while Ley has proposed the term 
 
 a. i. a . 2. 
 
 FIG. 12. Festooned cumulus. 
 
 " mammato-cumulus," and this latter seems to be very 
 suitable. 
 
 The origin of this form will be readily understood by 
 the following example. One summer evening in London, 
 towards sunset, the author saw a flat-based cumulus, like 
 that marked a 1 in Fig. 12, suddenly become festooned 
 at the base, and diminished at the top, as marked a 2 in 
 the diagram. A few minutes later the whole cloud 
 evaporated, and the succeeding night was fine. 
 
 The explanation which immediately suggested itself 
 was, that the ascensional current which formed the flat- 
 based cumulus had suddenly failed, and that the festoons 
 were simply the masses of vapour falling downwards for 
 want of support. 
 
 Another very striking case is marked b in the figure, 
 
CLOUDS AND CLOUD-PROGNOSTICS. 79 
 
 and was observed before a shower. Here a detached 
 cumulus was observed to form, first, festoons, and then 
 they in turn degraded into raggy cloud ; the whole dis- 
 appeared very shortly, but were quickly followed by fresh 
 rain-bearing clouds. The impression was that the festoons 
 were formed by a sudden drop of the cloud, and that the 
 rag was produced when the drop was less sudden. The 
 appearance is, unfortunately, not well rendered in the 
 diagram. From many similar instances, we are led to 
 the conclusion that the constant condition necessary to 
 form festoons is the sudden failure of an upward current 
 of air, and then we can readily see why they should prog- 
 nosticate a storm in some cases, and only rain in others. 
 
 Before many squalls or showei^, we are all familiar 
 with the short abortive gusts which so often precede them. 
 Now, we have only to assume that the ascensional uptake 
 in front of the main body of the shower is as unsteady as 
 the surface-wind, and we have at once all the conditions 
 necessary to form festoons. All observers are agreed 
 that they are usually formed at the edges of cloud-masses. 
 In the case of rain or thunder, they ordinarily appear just 
 before or after the rain ; but when a gale follows some 
 time afterwards, the festoon must have been formed l>y 
 some local squall or shower, that bore some relation to 
 the disturbed weather which produced the gale. In 
 Orkney the festoons are usually seen with the squalls of 
 a north-west wind in rear of a cyclone ; the storm they 
 prognosticate belongs to another cyclone, which there 
 usually follows quickly behind the first. In the tropics, 
 of course, festoons are always associated with non-isobaric 
 rain or thunderstorms. 
 
80 WEATHER. 
 
 DEGRADED CUMULUS. 
 
 A very similar line of argument applies to another 
 well-known sign of rain the appearance of cloud shown 
 in Fig. 13, a, where a thin stripe of cloud seems to cross 
 a well-formed cumulus. 
 
 This is a foreshortened view of a cumulus (&), and a 
 degraded patch of cloud (o). Sometimes, but most 
 unfortunately, this cloud is called cumulo-stratus, as by 
 Howard and others. We shall presently see, however, 
 
 -as=G5^ 
 
 FIG. 13. Cumulus, degraded cumulus, and line cumulus, a. Cumulus 
 crossed by another cloud, b, c. The same from another side. 
 d. Line cumulus, or high cumulus, e. Degraded cumulus, lens- 
 shaped. 
 
 that it has nothing in common with true stratus, but is a 
 mixture of pure cumulus with a degraded patch of cloud. 
 The origin of this cloud is very simple. We have shown 
 in the preceding diagram that a detached cumulus 
 (Fig. 12, a 1) can become degraded into a flat, thin mass, 
 with festooned base ; but in certain conditions the failure 
 of the rising current takes place more gradually, and 
 then the base of the mass remains flat instead of 
 becoming festooned. 
 
CLOUDS AND CLOUD-PROGNOSTICS. 81 
 
 This, too, is a sign of rain for the same reason as in 
 the former instance. The existence of failing or abortive 
 rising currents is of itself a sign of disturbed weather, 
 and is really more of an accompaniment than a prognostic 
 of rain. This cloud is common in the equatorial doldrums, 
 and in any other part of the world where showers fall 
 from cumulus. Thus we see that festooned, raggy, and 
 this streaked cumulus are all associates of rain, and for 
 similar reasons. A very similar degraded cumulus-patch 
 is very common duriug the finest weather in the trade- 
 wind districts. The small isolated patches of cumulus, 
 which are so common there, often seem to lose so much of 
 their rising impulse that a rocky top cannot form, but at 
 the same time the stoppage is not so sudden, or the cloud 
 so heavy, as to develop festoons. Then we get a cloud 
 nearly flat below, with a smooth round surface above, like 
 a plano-convex lens, as in Fig. 13, e. But, as Ley finds 
 an almost identical form as the embryo of a cumulus 
 whose rising force is very weak, we must judge the import 
 of this, as of every other cloud, by its surroundings. 
 
 MINOR VARIETIES. 
 
 Another form of cumulus is developed almost at the 
 level of cirrus, in long thin lines made up of little 
 heads of condensed vapour, sometimes called thunder- 
 heads. This is shown at the right-hand top corner of 
 Fig. 13, d. It is only noticed heie to guard against its 
 being called cirro-cumulus. In practice this cloud is 
 almost invariably produced in front of thunderstorms, and 
 it is difficult to see how it can be foimed otherwise than 
 
82 WEATHER 
 
 by assuming the air to rise in a thin vertical curtain. In 
 our chapter on line-thunderstorms, we shall find from other 
 reasons that the air really does sometimes rise in long 
 narrow sheets. This is the cumulus simplex of Weilbach 
 and rain-cumulus of Howard ; it has also been called high 
 cumulus, line-cumulus, and turretecl cumulus. 
 
 There is one other variety of cumulus, which need 
 only be mentioned here. 
 
 Sometimes the top of the cumulus becomes hairy,^as 
 if it had been combed out ; this cannot be explained, but 
 is usually seen over heavy rain. But occasionally this 
 peculiar process on the top of a rainy cumulus develops 
 a sort of flat sheet of cloud, apparently touching the 
 summit, and the cloud may conveniently be called 
 cumulo-stratus. 
 
 STRATUS. 
 
 We now come to the second variety of clouds, to 
 which the name of stratus is applied, because it always 
 lies in a thin horizontal layer, like a stratum of rock or 
 clay. Pure stratus has no sign of any hairy or thread- 
 like structure except at the edges, for a stratum which 
 shows much marking would be cirro-stratus, and has 
 quite a different origin. Pure stratus is essentially a 
 fine- weather cloud, and is especially characteristic of 
 anticyclones. One very beautiful variety is often seen 
 during a fine night, when the cloud forms thin broken 
 flakes, something like mackerel sky, from which, however 
 it is really quite distinct. 
 
 In Howard's original work on clouds, "stratus" was 
 
CLOUDS AND CLOUD-PKOGNOSTICS. 83 
 
 applied to ground-mist, but that idea is now entirely dis- 
 carded by all meteorologists. What we call pure stratus 
 is the " strato-pallium " of Weilbach, and the " stratus " of 
 Hildebrandson. The origin of this cloud seems to be 
 that when the air is tolerably still, and radiation is going 
 on, the general mass of the air gets gradually cooler, 
 till at last the temperature is reached at which some 
 stratum touches the dew-point, and therefore condenses 
 its moisture into cloud. Sometimes the cloud is formed 
 by rising fog. 
 
 This at once explains both why the stratum of clouds 
 should be flat and thin, and why this form of cloud 
 should be characteristic of anticyclones. We can also 
 understand why, under these conditions, the sky some- 
 times becomes overcast almost instantaneously. Yery 
 often a mass of fine-weather stratus is uniform in the 
 centre, but hairy or striated at the edges, and we get a 
 cloud indistinguishable by form alone from some kinds 
 of strato-cirrus, though very different in origin and sur- 
 roundings. Sometimes the lower surface of a sheet of 
 cloud is festooned for a short time like the flat base of a 
 cumulus. The cloud is probably not then pure radiation 
 stratus, but a smooth form of strato-cumulus, which, by 
 sudden failure of the generating current, begins to fall in 
 lumps just like the festooned cumulus before described. 
 
 CIRRUS. 
 
 The third primary form of cloud is cirrus, a word taken 
 from the Latin, and meaning literally " a curl of hair." We 
 have already explained the origin of pure cirrus and its 
 
84 WEATHER. 
 
 relation to pure cumulus, together with the rudimentary 
 idea of the formation of a stripe of cloud from a current 
 of vapour-laden air, which rises in currents of different 
 velocities, but in the same direction. 
 
 CIRRUS-STRIPES. 
 
 When cirrus rises irregularly, and appears not to be 
 all at the same level, we have seen that it is then pure 
 cirrus ; but there is a modified form, in which more or less 
 of the sky is covered with long thin stripes of cirrus, all 
 apparently at the same level. Technically this is known 
 as cirro-filum (literally " hair-thread "), a name first sug- 
 gested by Mr. Ley, and the term is suitable for inter- 
 national use ; but we shall call them cirrus-stripes. As 
 these are by far the most important form of cirrus for 
 forecasting purposes, we shall devote several paragraphs 
 to their consideration. 
 
 First, as to their origin. We have already explained 
 how a stripe can be formed which moves end on to the 
 wind that is propelling it, but most frequently we see the 
 curious spectacle of a long stripe of cloud moving either 
 broadside on or obliquely to its length. As we must 
 suppose that a stripe always sails with the wind in which 
 it floats, we have to find out how a stripe can be formed 
 which moves across its length. At first sight this is one of 
 the most puzzling phases of cloud-motion. These forma- 
 tions of cloud are, however, exactly analogous to the 
 smoke left by a steamer running before the wind. If she 
 runs faster than the wind, her smoke trails behind ; but if 
 the wind blows faster than she steams, then the smoke is 
 
CLOUDS AND CLOUD-PROGNOSTICS. 85 
 
 blown forwards in front of her. But now, suppose her 
 to be heading to the east with a south-west wind ; it is 
 obvious, from Fig. 14, that her smoke would lie in a stripe 
 bearing somewhere between north-west and south-east, 
 and would drift towards the north-east, that is nearly 
 at right angles to its length. The smoke that left the 
 funnel when the steamer was at A would have been blown 
 to C by the time she had reached B, while that at B would 
 
 Course from W. 
 FIG. 14. Formation of cloud.stripes. 
 
 be just leaving the smoke-stack ; so that the whole line 
 of smoke would lie from B to C, but drift from south-west 
 with the wind. The angle the smoke forms with the 
 course of the ship obviously depends on the speed of the 
 ship and the velocity of the wind, so much so that we 
 have used measurements of the angle A B C to determine 
 the velocity of the wind at sea. 
 
 Now, this is exactly what happens in nature. The 
 ascensional column of moist air, which will eventually 
 form a cumulus, starts from near the earth's surface, 
 drifting with the wind which blows there ; when it arrives 
 at a certain height, it meets an upper current moving in 
 
86 WEATHER. 
 
 a different direction to that on the surface, and probably 
 begins to condense there. The stripe which would be 
 formed under these circumstances would behave exactly 
 like the smoke of a steamer ;. that is to say, it would lie 
 obliquely to the wind which was driving it. 
 
 The direction of a stripe is sometimes called the 
 direction of its filature, but we shall employ the less 
 technical term of " the lie of the stripe." The triangle 
 ABC, Fig. 14, is called the triangle of filature, and it 
 is evident, from the nature of the composition of velocities, 
 that the precise direction of the lie of the stripe depends 
 on the relation of the velocities of the upper and lower 
 currents. 
 
 If the stripes were caused by descending threadlets of 
 ice or snow, the above principles of stripe- formation would 
 equally hold good. 
 
 LIE AND MOTION OF STRIPES. 
 
 Before explaining the nature of the upper currents in 
 cyclones and anticyclones, we must first explain how to 
 find out both the lie of a stripe and the direction in 
 which it is moving, as both these points are important, 
 and both rather difficult to observe. If the sky is well 
 covered with stripes, we find that when we look at them 
 lengthways they appear to converge towards a point on 
 the horizon, while, if viewed transversely or in profile, 
 they appear arched. The convergence, of course, is a 
 matter of perspective. 
 
 One great peculiarity of stripes is that, while in their 
 simplest form the threads of which they are composed 
 
CLOUDS AND CLOUD-PROGNOSTICS. 87 
 
 lie in the direction of its length, as marked a, c, in 
 Fig. 15, sometimes the whole stripe is made up of a series 
 of cross-bars, and the stripe is then said to be " striated " 
 (Fig. 15, b and d). Most frequently these bars, or striae, 
 are at right angles to the length of the stripe, but they 
 are also sometimes oblique to the lie of the stripe. 
 
 Whenever the cloud is observed, the points to be 
 noted are (1) the direction or lie of the stripe ; (2) the 
 direction of the striae ; and (3) the direction in which the 
 
 FIG. 15. Diagram illustrating cloud-perspective. 
 
 stripe as a whole is moving. Of these the first and third 
 are the most important, the direction of the striae being 
 only secondary. 
 
 Cases, however, occur in which the whole sky is 
 covered with a cloud reticulated like a chess-board, and it 
 is then difficult to say which is the primary filature of 
 the cloud. We will first consider how to determine the 
 direction of the stripes and striae, as that is far easier 
 
88 WEATHER. 
 
 than to discover their motion. The method of doing so 
 is based on the fundamental principle of perspective 
 that a line drawn from the observer to the point on the 
 horizon towards which parallel lines converge, gives the 
 lie of the parallel lines. Thus suppose, as in Fig. 15, 
 that an observer looking north saw stripes a, fc, c, and d 
 converging on the north point of the horizon, he would 
 conclude that what he saw was the perspective view of 
 four stripes lying in a direction given by a line drawn 
 from himself northwards that is, north and south. On 
 the one hand, he would know that the striae of stripe b 
 were lying east and west that is, at right angles with the 
 filature ; for these striae converge nowhere, but are parallel 
 to the horizon-line when looking north. On the other 
 hand, he would see that the striae of stripe d converge 
 on the north-east point of the horizon, and that, there- 
 fore, the striae lie north-east and south-west, while the 
 stripe as a whole points north and south. For a similar 
 reason, he would know that the single stripe e lay also 
 from north-east to south-west. 
 
 The above are very striking instances of the deceptive 
 nature of perspective, for in stripe I, where the striae are 
 really at right angles to the strips, they appear oblique ; 
 while in stripe d, where they are really oblique, they 
 look as if they were nearly at right angles to the lie of 
 the cirrus. The point on the horizon towards which 
 stripes or striae converge, is called their vanishing-point, 
 or, more shortly, their V-point. Some observers, how- 
 ever, prefer the term "radiation-point," and talk of the 
 radiation of cirrus. Had the stripes been viewed looking 
 straight either east or west, they would have presented 
 
CLOUDS AND CLOUD-PEOGNOSTICS. 89 
 
 the appearance of an arch, whose flat top bore due east 
 or west, and whose ends pointed north and south. 
 
 A very simple way of learning cloud-perspective is to 
 stand at the end of a long room and assume that you are 
 looking north ; then you see at once that the lines of the 
 two cornices, on either side of the room, converge towards 
 the north ; and, if you can suppose striated lines like those 
 of Fig. 15 to be painted on the ceiling lengthways to the 
 room, you will see that the striaB would converge in the 
 manner shown in that diagram. 
 
 Now for the more difficult question of finding the 
 motion of the stripe. 
 
 Usually, the motion of the stripe is not in the direction 
 of its length. Frequently it moves broadside on that is, 
 at right angles to its length, but more frequently at an 
 oblique angle to its length. 
 
 The most accurate observations can be made when 
 clouds are exactly overhead. Then, of course, there is 
 no delusive perspective, and the direction from which the 
 cloud comes is the direction wanted. A stripe moving 
 obliquely to its length is, however, always a difficult 
 subject. 
 
 In most cases, however, it is impossible to catch a 
 cloud just overhead, and even then it is most incon- 
 venient to observe ; so that as a rule we must use clouds 
 at a moderate elevation, and allow for perspective. The 
 perspective of motion is, of course, the same as the per- 
 spective of shape; that is, the point from which the 
 motion appears to diverge is the point from which the 
 cloud is really moving, and the point to which the motion 
 converges is the point to which the clouds are travelling. 
 
90 WEATHER. 
 
 For instance, take an easy case, when fine detached 
 cumulus is moving rapidly from the north. If we look 
 anywhere towards the north-west, a glance at Fig. 15 will 
 show that a cloud moving along one of the lines diverging 
 from the V-point would appear to have some motion from 
 the east as well as north ; in fact, would look like a north- 
 east motion. Conversely, when looking towards the 
 north-east at a cloud like d, the motion would appear to 
 be somewhere from the north-west ; but if looking due 
 north, as at e, the cloud would appear to rise straight out 
 of the horizon. The rule therefore is, watch the point 
 from which the motion of the clouds seems to diverge, 
 and that is the direction from which they are really 
 moving. Sometimes it is more convenient to determine 
 the point towards which the clouds converge, the V-point 
 giving then the direction towards which the motion is. 
 If possible, however, the point of divergence should be 
 selected as being the easier to observe. 
 
 The reason why the motion of cirrus- stripes is so 
 much more difficult to determine than the direction of their 
 filature is that, being very narrow, you get only a very short 
 line from which to estimate the V-point, if the motion is 
 not in the direction of the stripe's length. For instance, 
 in Fig. 15, where the dotted lines denote the divergence 
 motion of the different stripes, suppose that we had been 
 able to watch the motion of the stripe a as it drifted past 
 a star or any fixed point, we should find that the line of 
 motion was coincident with the length. The motion of 
 the stripe is, therefore, from the V-point that is, from 
 the north. On the other hand, if we watched the stripe 
 e, which is really moving only partially sideways, though 
 
CLOUDS AND CLOUD-PKOGNOSTICS. 91 
 
 apparently it moves at right angles to its length, we 
 should only have the short length of the dotted line 
 which passes through the stripe to produce by eye till it 
 cuts the horizon from which to estimate the V-point. 
 
 Then consider the increased complication when the 
 stripe is striated. In stripe &, though the motion of the 
 stripe coincides with its length, each bar, or stria, would 
 appear to lie obliquely to the line of its motion, though 
 really moving at right angles to its length. On the other 
 hand, the striae of stripe d appear to move at right angles 
 to their length, when they really move obliquely. 
 
 Another and sometimes^ even greater difficulty arises 
 from the changes which are going on in the cloud itself. 
 If we take two photographs of a cloud at an interval of 
 only three minutes^ it is sometimes impossible to identify 
 the same portion of the cloud in the two pictures. When, 
 therefore, we get the still more complicated case of an 
 obliquely striated stripe, which moves very slowly at a 
 considerable angle to its filature, we can readily under- 
 stand that the true character of its motion can only be 
 determined under favourable circumstances and by a 
 skilful observer. 
 
 Such are the fundamental principles on which all 
 observations on cloud-motion depend. The observer 
 must not be deterred by difficulties at first starting. If 
 he begin by taking simple cases of fast-moving clouds, 
 and then, after he has fully realized the meaning and 
 importance of the V-points, tries more difficult cases, he 
 will soon attain such proficiency as will enable him to 
 make valuable observations in the most recent branches 
 of modern cloud-science. 
 
92 WEATHER, 
 
 If possible, the velocity of the upper clouds should be 
 noted, for quickly moving upper clouds are a sign of 
 much worse weather than slow-going ones. 
 
 KELATION TO CYCLONES AND ANTICYCLONES. 
 
 It has been found, as a matter of observation, by Ley 
 and others, that the lie of cirrus-stripes bears a tolerably 
 constant relation to the shape of isobars over the locality 
 where they are seen. It is from this circumstance that 
 cirrus derives its great forecasting value ; also from the 
 fact that it is the first cloud which appears in a sky 
 which had been previously blue. Hildebrandson finds 
 the following deviation of the stripe to the isobar out of 
 171 observations. The stripes whose angles of deviation 
 are greater than 45 are, of course, less nearly parallel 
 to the isobar than those whose angle is less than 45. 
 
 CYCLONES 
 (minima). 
 
 ' ANTICYCLONES ^ * ^ WEDGE 
 
 DEVIATION. (maxima). Front. Rear. ISOBARS. TOTALS. 
 
 Angle greater than 45 ... 58 5 8 3 74 
 
 Angle less than 45 .., 12 18 29 38 97 
 
 Total 70 23 37 41 171 
 
 Consequently, cirrus-stripes lie in regions of maximum 
 pressure most often nearly perpendicular to the isobar, 
 while round minima and along wedges they are more 
 nearly parallel to the isobars. 
 
 To explain the reason of this, we must now show the 
 relation of the upper to the lower currents in cyclones 
 and anticyclones. Our knowledge of the upper currents 
 has been deduced entirely from cirrus observations. 
 
CLOUDS AND CLOUD-PROGNOSTICS. 
 
 93 
 
 VERTICAL SUCCESSION OF AIR-CURRENTS. 
 
 In Fig. 16 we give a diagram of the surface and 
 highest currents in both a cyclone and an anticyclone, as 
 deduced by Ley, Loomis, and Hildebrandson of Upsala. 
 
 The solid arrows denote the surface-winds, while the 
 highest currents are given by the dotted arrows ; and the 
 two arrows are supposed to diverge from the point of 
 observation. In a few places, where the velocity of the 
 two currents is usually different, we have drawn the 
 respective arrow of different lengths, otherwise the arrows 
 
 d , 
 
 Cyclone. A n ti cyclone. 
 
 FIG. 16. Surface and highest currents over cyclones and anticyclones. 
 
 must be supposed to give only the relative directions of 
 the winds. 
 
 First for the cyclone. Let us consider the nature of 
 the surface-winds, as shown by the solid arrows. We see 
 at once that on the whole the direction of the surface- 
 wind may be described as an ingoing spiral, more incurved 
 
94? WEATHER. 
 
 in the right front than in any other portion, and that in 
 all parts the wind is a little less incurved the nearer we 
 approach the centre. In the diagram we have assumed 
 that the cyclone is moving due west, and that it is also 
 truly circular. The dotted arrows, on the contrary, show 
 that the wind in the upper strata blows in an irregular 
 spiral outwards ; and that, while in front of the cyclone 
 the upper winds are very much inclined outwards, in 
 rear they are very nearly parallel to the surface-currents. 
 
 If we had put in arrows to show the direction of the 
 lower cloud which floats from 6000 to 8000 feet above 
 the earth, we should have found that intermediate layer 
 moving almost exactly parallel to the isobars that is, 
 nearly in a circle. We also know, from other observations, 
 that the upper cirrus-current in front of a cyclone is much 
 nearer the surface than the same current in rear of the 
 centre. Now, when we come to look at the direction of 
 cirrus-stripes, as given approximately by an imaginary 
 line drawn through the points of each pair of arrows, we 
 see at once that in the outer circle especially, as at &, c, d, 
 the cirrus-stripes will lie at less than 45 to the isobars, at 
 the point from which the arrows diverge in the diagram 
 if stripes are formed as shown in Fig. 14. 
 
 In practise the stripes are more nearly parallel to the 
 isobars than would appear from the generalized diagrams, 
 as the majority of cyclones are not circular, but oval ; and 
 that the effect of an intermediate current of air less 
 incurved towards the centre, would be to make the stripe 
 less at right angles to the isobar than would appear from 
 the diagram. Note, also, that in front the surface-winds 
 are slower than the upper ones, while in rear the surface 
 
CLOUDS AND CLOUD-PROGNOSTICS. 95 
 
 are the quicker ; also especially that in every portion 
 with one exception, as at a, the upper current is always 
 more veered than the lower one that is to say, that if 
 the surface is east, the upper will be more south of east, 
 and if the surface is west, the upper will be more north of 
 west. Or we may put it thus : stand with your back to 
 the wind, and the upper currents always come more from 
 the left, and the higher the currents the greater the 
 amount of veering. This is the almost universal law of 
 upper winds in the northern hemisphere for clouds at all 
 levels. 
 
 If the surface is east, and the low clouds south, the 
 higher cirrus will be from some point west of south ; but 
 if, with the same surface east wind, we saw cirrus driving 
 from south, we should know that the intermediate currents 
 of wind cannot be veered so much, but must come from 
 some point of south-east. This sequence, which we will 
 call the law of vertical succession of upper currents, is 
 another of the fundamental principles of meteorology. 
 
 In certain well-defined cases we find an apparently 
 anomalous sequence, but these need not be described in 
 an elementary work. 
 
 Then for the anticyclone. Referring to Fig. 16 again, 
 we see that, as a general rule, the surface and highest 
 winds are much more opposed to one another than in 
 cyclones, and therefore in most cases the cirrus-stripes 
 will be more nearly perpendicular to the isobars, as at e, /, 
 g. Taking a general view of the surface-winds, we may 
 say that on the whole they blow spirally outwards, in 
 the direction of the motion of the hands of a watch ; that 
 they are less square to the isobars the further they are 
 
96 WEATHER. 
 
 from the centre, but that they are always more nearly 
 perpendicular to the isobar than the wind in any portion 
 of a cyclone. 
 
 The upper currents, on the contrary, blow spirally 
 inwards, also in the direction of the watch-hands, and also 
 much inclined to the isobars. 
 
 Now, taking a general view of the relation of stripes to 
 isobars, we must not expect the lie of the stripe to be more 
 than a moderately good guide to the lie of the isobars. 
 Independently of the fact that there is no hard line 
 of demarcation between a cyclone and an anticyclone, 
 across which the stripes should suddenly alter from 
 parallel to transverse to the isobars, it is manifest that 
 when so much depends on the relative velocities of the 
 upper and lower currents, much variation in the lie of the 
 stripes must be expected. 
 
 But besides this, the author has sometimes found that 
 the intermediate current between the surface and the 
 cirrus materially affects the lie of the clouds, and that 
 the lie of some stripes cannot be explained on this 
 principle at all; but, in spite of all this, the above 
 generalizations on the lie of stripes are very valuable. 
 
 The following example, as observed by M. Phillipe 
 Weilbach, Copenhagen, illustrates our general principles. 
 Fig. 17 represents a portion of the cirrus-stripes as seen 
 at 1 p.m. on October 27, 1880, at Copenhagen, foretelling 
 the bad weather which occurred on the following days. 
 The sky generally would be said to be covered with 
 striated cirrus-stripes, really forming a thin layer of cirro- 
 stratus, which appeared in great quantity and covered the 
 whole of the eastern sky from the zenith to the horizou, 
 
CLOUDS AND CLOUD-PROGNOSTICS. 97 
 
 forming long bands, which diverged from the noith-north- 
 west tangentially to the isobars. 
 
 The barometer was at 29'6 ins. (752 mm.) ; the wind 
 blew lightly from the north, while the relative humidity was 
 only sixty- seven per cent. ; the rest of the sky was sparsely 
 covered with fibrous clouds of divers forms. For several 
 hours the bands of cirrus, striated by fine lines, moved 
 
 FIG. 17. Converging striated cirrus-stripes. 
 
 from the north-west ; then the sky at Copenhagen became 
 clear for some time. 
 
 In the afternoon the preceding appearance was re- 
 placed by a del pommele (dappled sky), clearly marked, 
 but of a rather heavy aspect, which also moved slowly 
 from the north-west. 
 
 At the same time the telegraph gave notice of a 
 storm from the south-east over Ireland, and the following 
 day the tempest raged with heavy snow over Denmark, 
 
 H 
 
98 WEATHER. 
 
 while the depression moved to the south of that country. 
 The landscape in the figure is looking up the Sound. 
 
 All this is very easily explained. When the cirrus 
 was first observed, Denmark was under the influence of 
 the rear of a cyclone, rather than of the wedge, which lay 
 a little farther to the west, while a new cyclone was 
 forming behind the wedge. The isobars would, of course, 
 lie from north-west to south-east, nearly the same as the 
 cirrus. The episode of the sky becoming perfectly clear 
 after the first indications of dangerous cirrus is very 
 common, and seems due to the first cat's-paws, as it were, 
 of the oncoming cyclone developing cirrus, and then 
 failing, so that the sky clears again. 
 
 FINE WEATHER AND DANGEROUS CIRRUS. 
 
 There are many forms of pure, hairy cirrus that 
 indicate fine weather all over the world ; while others, 
 such as " mare's tails," " cat's tails," " goat's hair," " sea- 
 grass," and " gashes " (balafres), etc., are forerunners of 
 bad weather in every country. 
 
 In England " mare's tails " usually portend wind, and 
 'goat's hair" only rain; while "mare's tails," "cat's 
 tails," and lalafres precede every hurricane in the 
 tropics. " Mare's tails " are long, straight fibres of grey 
 cirrus ; " cat's tails " are a denser bundle, often slightly 
 striated, so as to look like brind lings on the tail ; " goat's 
 hair " is a short bundle of white cirrus-hairs ; while 
 " sea-grass " and lalafres are both somewhat similar to 
 the above allied forms. 
 
 These all represent forms of cirrus at the outskirts of 
 
CLOUDS AND CLOUD-PROGNOSTICS. 99 
 
 cyclones, intermediate between the pure wisp of fine 
 weather and true cirrus-stripes, with the exception of 
 " goat's hair," which is a form of cirrifi cation on the top 
 of rain-bearing cumulus ; but in every country there are 
 sometimes illusory forms, which it is very difficult at 
 first to connect with good or bad weather. In England, 
 on a fine summer day, detached cumulus, which has 
 formed during the afternoon, will become very small and 
 disappear towards sunset; and straight fibres of cirrus 
 will gradually appear in the sky, which by form alone 
 are indistinguishable from "mare's tails" and similar 
 forms of cirrus which presage wind. All over the tropics 
 the typical sky by day is lumps of cumulus floating below 
 wisps of cirrus, and, without considering their surround- 
 ings, the latter might be thought to be indicative of 
 coming danger. Clouds must always be judged by their 
 antecedents and surroundings. The gradual growth of 
 cirrus-fibres after cumulus in England, and the general 
 appearance of the weather and the diurnal fall of the 
 wind, will usually prevent any mistake from being made 
 between the " mare's tails " of a summer evening and the 
 similar cloud which streaks the sky on a windy, gusty 
 day. 
 
 The "cat's tails" which precede a tropical hurricane 
 do not disappear shortly after sunset like the ordinary 
 wisps, and when combined with a slow, steady fall of the 
 barometer, a dangerous storm is certainly indicated. 
 
 This is a simple case of what we find throughout all 
 cloud-lore that the same cloud does not always indicate 
 the same weather, even in the same country. Here the 
 explanation is doubtful. Some think that the essential 
 
100 WEATHER. 
 
 to form a fibre of cirrus is a thin thread of damp air 
 rising slowly into a current different in speed or direction 
 from that in which the air started. 
 
 If the rising impulse is merely the effect of the sun 
 heating air near the ground, the resulting wisp is a fine- 
 weather cirrus ; but if, on the contrary, the ascent of air 
 is due to the upward impulse of a cyclone, then the 
 bundle of cirrus-fibres indicates wind and rain. 
 
 Others consider that the advent of damp upper currents 
 in front of a cyclone induce the condensation of vapour 
 at a high level into icy particles, which latter are drawn 
 into wisps of cirrus as they descend into lower strata. 
 
 We believe that cirrus may be formed by both 
 methods, but it is impossible to pronounce definitely on 
 the subject, till we know more of the mechanism of a 
 cyclone. 
 
 CIRRO-STRATUS. 
 
 We now eome to the composite forms of clouds, and 
 here, unfortunately, we find the utmost confusion in the 
 words applied by different meteorologists to the same 
 clouds. We will first begin with cirro-stratus. By this 
 we mean a thin stratum of cloud which, instead of being 
 uniform like pure stratus, is composed of fibres of cirrus 
 in any complexity, but not of streaked, fretted, or speckled 
 nubecules. 
 
 Sometimes the fibres of cirrus interlace, and give this 
 cloud a reticulated appearance like a woven cloth, and 
 the variety of forms is unlimited. The cloud we call 
 cirro-stratus is practically identical with what Howard 
 
CLOUDS AND CLOUD-PROGNOSTICS. 101 
 
 and Hildebrandson call "cirro-stratus," and almost co- 
 extensive with the cirro-velum of Ley. As to the origin 
 of cirro-stratus, we can say little with certainty. As a 
 matter of observation, it is usually formed in front of 
 cyclones or secondaries. When the sun or moon shine 
 through it, we generally find that a halo is formed, and 
 then we may conclude with certainty that it is composed 
 of frozen particles of vapour. The difficulty is to explain 
 the innumerable forms which it assumes, and the rapid 
 changes which it undergoes. Though we are obliged to 
 employ the word strata to describe this cloud, because it 
 forms a thin layer, it is extremely doubtful whether its 
 formation has much in common with that of pure stratus, 
 which we have seen is due to the radiation of anticyclones. 
 It does, however, seem to have something in common 
 with pure cirrus, and still more with cirrus-stripes ; but 
 we cannot say why the front of a cyclone should develop 
 stratiform, and the rear cumulo-form, clouds. 
 
 Sometimes cirro-stratus is formed lower down, and 
 more compact in structure, when it should be called 
 strato-cirrus. This is unknown in Scandinavia, but quite 
 common in some parts of the tropics. 
 
 ORIGIN OF STKIJE. 
 
 With regard to the striae which we find both in cirrus- 
 stripes and in cirro-stratus, the only reasonable suggestion 
 which has been proposed to account for their formation 
 is, that a stripe, or a thin stratum of ice-dust, may some- 
 times be supposed to be relatively at rest to a wind more 
 rapid than itself, which may strike it suddenly. Then 
 
102 WEATHER. 
 
 we can conceive that a smooth layer of cloud might be 
 farrowed into small waves at right angles to the wind ; but 
 this, of course, would only account for striae square to the 
 stripe, and not for oblique markings. We often see an 
 apparently structureless patch of cirro-stratus suddenly 
 become striated, as if a cat's paw of wind had blown on it 
 like a gust on a pond. 
 
 But, as a matter of fact, striae are as often as not 
 oblique to the lie of a stripe, and to the direction of 
 the motion of cirro-stratus. Here also the only rational 
 suggestion is that the oblique striations are in some way 
 the effect of an upper current, which moves in a different 
 direction to that on the surface, and forms cloud rolls. 
 
 There is certainly something to the eye about the 
 sideways motion of some cirrus-stripes that is not the 
 same as the drive of a detached cumulus before the wind. 
 If one is really propagated by a dynamical disturbance, 
 while the other merely floats in an air-current, the 
 difference would probably be explained. If this can ever 
 be satisfactorily worked out, we should get the motion of 
 higher currents more accurately than at present, for now 
 we always assume the motion of a cloud is the same as 
 that of the wind which drives it along. 
 
 Sometimes a succession of rising threads of air, one 
 behind the other, form nearly vertical parallel fibres of 
 cirrus, which must not be mistaken for horizontal striae. 
 All observers are agreed that the fact of striation, or 
 reticulation, is of no practical importance in forecasting 
 weather from clouds, so that we do not make definite 
 varieties of these forms. 
 
CLOUDS AND CLOUD-PROGNOSTICS. 103 
 
 ClRBO-CUMULUS. 
 
 The next great class of compounds is cirro-cumulus. 
 By this we mean a broken layer of cloud, at a high or 
 middle level, of which the component masses are not 
 fibrous like cirro-stratus, but more or less rounded or rolled, 
 though without any of the rocky look of pure cumulus. 
 
 For this reason the term cirro-cumulus is to a 
 certain extent unfortunate; but we are almost obliged 
 to use the word, so as not to introduce new expressions, 
 and, so long as it is conventionally recognized what kind 
 of cloud is meant by cirro-cumulus, it does not so much 
 matter if the word is not quite logical. The misfortune 
 of the word " cirro-cumulus " is that, even excluding the 
 small high cumulus that sometimes grows out of hairy 
 cirrus, and which we have described as linear, or high, 
 cumulus, there are still two rather distinct forms, to 
 either of which the definition we have given of cirro- 
 cumulus applies. 
 
 The first kind, and far the commoner all over the 
 world, is composed of rolled masses of cloud, with a fleecy 
 appearance, that are universally known in different 
 languages as '* wool-pack," "sheep," "lambs," or by 
 similar terms. This is the cirro-cumulus of Fitzroy, 
 Weilbach, Hildebrandson, and of Howard. The clouds 
 called nubes hiemales by Weilbach, are a variety of this 
 type that is formed with great persistency over Scandinavia 
 and Northern Europe during the cold season. The thin 
 layer of cloud is then at a moderate altitude, and tends 
 to arrange itself in long parallel bands of quickly moving, 
 fleec) masses. 
 
104 
 
 WEATHER. 
 
 It is extremely difficult to render that kind of cloud 
 in an engraving. Fig. 18 is, however, a moderately 
 successful attempt to reproduce a photograph of a fleecy 
 sky. There, as always, the cloud has a more or less 
 pronounced tendency to arrange itself aloDg two lines 
 
 FIG. 18. Fleecy cirro-cumulus. 
 
 one for the length of the bands ; the other for the lie of 
 the striae. Sometimes the effect of these two crossing 
 lines is to give the individual nubicules which compose 
 the whole a square or lozenge shape, and the whole sky 
 the appearance of a gigantic chess-board. 
 
CLOUDS AND CLOUD-PROGNOSTICS. 105 
 
 We can say little with certainty as to the formation 
 of this fleecy sky, though, in a general way, there seems 
 to be little doubt that both the woolly look and the 
 striation are due to the contact and rolling friction of two 
 layers of air moving in different directions. Fleecy 
 clouds, though apparently so different in form, are really 
 not very far removed from wispy cirro-stratus. We often 
 see in England wispy clouds develop rapidly into fleecy 
 ones for a few minutes, and then back again into wisps 
 and curls; but, as a rule, cirro-stratus develops into 
 strato-cumulus, and is practically a sign of worse weather 
 than fleecy cirro-cumulus. 
 
 We know by observation that fleecy cirro-cumulus is 
 chiefly formed in the temperate zone on the edges of 
 anticyclones, and also before thunderstorms and some 
 forms of non-isobaric rain. These are both cases in which 
 there would be upper currents varying much in direction 
 from the surf ace- winds, while the rapidity of motion 
 would depend upon circumstances. This enables us to 
 explain the following set of widely reputed prognostics. 
 
 " If woolly fleeces spread the heavenly way, 
 Be sure no rain disturbs the summer's day." 
 
 Or the provincial French saying, "El ciel pecoun pro- 
 mete un bel matin." But, on the other hand, Virgi 
 ("Georg." i. 397) considers it a sign of rain if it should 
 happen that 
 
 " Tenuia . . . lanse per cselum vellera ferri." 
 
 And so in the neighbourhood of Pisa they say, "Cielo a 
 pecorelle, Acqua a catinelle ; " and in the Tyrol, " Sind 
 Morgens Himmelschaflein, wird's Nachinittags hageln 
 
106 WEATHER. 
 
 oder schnei'n ; " and in France they have a proverb con- 
 trary to the one we have first quoted : 
 
 " Temps pommele, fille fardee, 
 ]S T e sont pas de loiigue duree." 
 
 The term " dappled sky " (del pommele) is a little 
 equivocal, and might refer to the other form of cirro- 
 cumulus, known in Northern Europe as " mackerel sky." 
 
 Anyhow, we have to reconcile an apparently contra- 
 dictory set of prognostics. The reason appears to be 
 that in Northern Europe rain is chiefly cyclonic, and 
 therefore rarely preceded by fleecy cirro-cumulus, so that 
 the appearance of that cloud denotes the edge of an 
 anticyclone, and fine weather for a day at least. In 
 Central and Southern Europe, on the contrary, fleecy 
 clouds are usually formed in front of secondaries, thunder- 
 storms, and non-isobaric rains, so that their cirro-cumulus 
 is a sign of approaching rain. We can readily imagine 
 that, both at the edges of anticyclones and in front of 
 secondaries, thunderstorms, etc., we have upper currents 
 moving in very different directions to those on the surface, 
 with a layer of cloud between them, though the origin of 
 the condensed vapour is not the same. In anticyclones 
 the vapour probably rises from evaporation, till it reaches 
 an altitude where the temperature falls to the dew-point ; 
 in secondary cyclones, etc., the upward impulse is due to 
 the dynamical properties of cyclonic or other motion. 
 
 This is exactly analogous to the difference between 
 the wispy cirrus formed of an evening at the edges of 
 anticyclones in fine weather, and the same cloud which 
 precedes a dangerous storm. 
 
 In practice the surroundings are so different that the 
 
CLOUDS AND CLOUD-PROGNOSTICS. 107 
 
 apparent similarity of names rarely misleads the rno4 
 ordinary observer. 
 
 The second chief variety of cirro-cumulus is composed of 
 rounded and isolated nubicules without any fleecy texture. 
 
 This is the well-known " mackerel sky " of Northern 
 Europe; and when the cloudlets are a little angular, 
 we get a form called " mackerel-scales." We may 
 call this hard cirro-cumulus, to distinguish it from the 
 fleecy form of the same generic name. While fleecy 
 cloud is one of the commonest, mackerel is one of the 
 rarest skies, so that we have not got a sufficient number 
 of observations to correlate these isolated cloudlets with 
 any particular form of isobars or kind of rain. 
 
 However, all weather-lore connects mackerel with fine 
 weather, for even in rainy Ireland we find the saying, 
 " Mackerel sky, twelve hours dry." Why this should be 
 the case we are unable to say, but there is no doubt about 
 the fact. 
 
 In a still rarer form of cirro-cumulus, the lower 
 surface of the general cloud-stratum exhibits very small 
 pendulous protuberances, resembling sacks or bags, by 
 which a part or even the whole sky is festooned. Ley 
 calls this eirro-velum mammatum, but we may call it 
 festooned cirro-cumulus. When, near sunset in the 
 tropics, these festoons take up a rosy tint, and hang like 
 pink grapes in a serene sky, these clouds can scarcely 
 be surpassed for beauty. 
 
 Sometimes a more compact form of fleecy cirro- 
 cumulus is found at a lower level, when the cloud may 
 be more appropriately reported as cumulo-cirrup, so as 
 to indicate its lower level. This apparent multiplication 
 
108 WEATHER. 
 
 of cloud-names is forced on us by the necessity of giving 
 some idea of height in reports of the motion of the upper 
 currents. For instance, on the west edge of an anticyclone 
 low cumulo-cirrus might be moving from south, whilst 
 the higher cirro-cumulus would come from the south-west ; 
 so that observations which reported cirro-cumulus and 
 cumulo-cirrus indiscriminately would lead to a discordant 
 or erroneous view of the general circulation of the air in 
 an anticyclone. 
 
 STRATO-CUMULUS. 
 
 Another of the great series of compounds is strato- 
 cumulus. By this we mean a large mass of cloud, forming 
 a layer, which is not sufficiently uniform to be called 
 stratus, and not sufficiently rocky to be called cumulus. 
 This is the cumulo-stratus of Fitzroy. Howard's cumulo- 
 stratus is not a true variety of cloud at all, but a 
 compound of a thin patch of cirro-stratus, resting either 
 on the top of a cumulus or crossing an isolated lump of 
 cumulus, as in Fig. 13, a. The origin of the name is 
 obvious. The general mass of the cloud is a layer, and 
 therefore the name must contain the word strata, while 
 the components are lumpy, and it must therefore con- 
 tain the word cumulo. 
 
 This form of cloud is typical of a cyclone-front in 
 Great Britain. We can trace its gradual development in 
 all stages. Cirrus-stripes first get thicker and lower, so 
 as to form cirro-stratus. As we get nearer the rainy 
 portion of the cyclone, the cirro-stratus loses its fibrous 
 texture, becomes still denser and nearer the earth's 
 
CLOUDS AND CLOUD-PROGNOSTICS. 109 
 
 surface, till at last all trace of structure is lost in the 
 irregular, shapeless masses of cloud which cover the 
 whole sky. Still later, the cloud gets even lower and 
 blacker, till rain ultimately begins to fall. Then the 
 cloud would be called nimbus, because it forms a layer 
 and precipitates rain. Sometimes, when the sky breaks 
 ior a moment, we get a glimpse at the composition of 
 this cloud ; we then see that it differs much from pure 
 rocky cumulus, by reason of its flatness and comparative 
 thinness. We must, in fact, look at strato-cumulus as a 
 development of cirro-stratus, and not as an ally or hybrid 
 of cumulus, though we have to use the word " cumulus " 
 in composition. 
 
 The point which we cannot altogether explain is, why 
 in front of the cyclone's trough the clouds should have 
 such a marked tendency to form stratus, while in rear 
 the rising currents take the form of well-defined columns, 
 and produce rocky cumulus. This points to some 
 difference of symmetry between these two portions of a 
 cyclone, and the only suggestion which we can make is, 
 that perhaps it may be partly due to the upper currents 
 in front of the trough being much more opposed to those 
 on the surface than those in rear of the centre, which are 
 nearly parallel to the lower winds; and partly to the 
 forward motion of the cyclone, as a whole, meeting the 
 incurving winds in front, and running away from them in 
 rear of the disturbance. 
 
 Another form of strato-cumulns is very common in 
 the tropics. The component masses of cloud are more 
 isolated than in Great Britain, and so thin that when 
 seen in perspective each only looks like a dark thin bar, 
 
110 
 
 WEATHER. 
 
 and, with the brighter intervening spaces, the whole sky 
 near the horizon is striped like a Venetian blind. 
 
 Nearer overhead we see only the irregular flat base of 
 scattered clouds, without any trace of arrangement or of 
 bars. The difference between these apparent long bars and 
 real stripes of cirrus can be detected in a moment by 
 turning in any direction. The bars of strato-cumulus 
 follow you by remaining parallel to the horizon which- 
 ever way you look, for the linear arrangement is only an 
 effect of perspective ; while cirrus-stripes always converge 
 to the same point on the horizon. Fig. 19, which is a 
 
 FIG. 19. Strato-cumulus j roll cumulus. 
 
 fair specimen of this kind of cloud, is engraved from a 
 photograph by the author in lat. 18 S., long. 4 E. ; that 
 is, in the south-east trade between Goree and Cape Town. 
 We see at once that the sky is too irregular for pure stratus, 
 but that the masses into which the cloud is gathered 
 
CLOUDS AND CLOUD-PROGNOSTICS. Ill 
 
 have nothing in common with pure cumulus ; and also 
 very clearly that the linear arrangement increases towards 
 the horizon. This is the cloud to which the term " roll- 
 cumulus " has been unfortunately applied in England. 
 
 Though true strato-cumulus is not really allied to 
 cumulus at all, we sometimes see a cloud of this type 
 with a distinct but irregular cumulus form in places. 
 This must also be called strato-cumulus, as it merges by 
 indistinguishable gradations into the purer form of the 
 same name. 
 
 Sometimes also we get strato-cumulus from a develop- 
 ment of cumulo-cirrus with fine weather in the temperate 
 zone ; so that the name and form alone of this cloud tell 
 us little either of its origin or portent. 
 
 NIMBUS. 
 
 The term nimbus need not detain us long, and then 
 principally to explain the unfortunate confusion which 
 has arisen from the uncertain use of this word. 
 
 Every kind of cloud from which rain falls is a nimbus, 
 and there are practically two sorts cumulo-nimbus, the 
 rocky cumulus-cloud from which rain falls in squalls or 
 showers ; and pure nimbus, a flatter cloud, more like 
 heavy strato-cumulus, that forms from or under cirro- 
 stratus in front of extra tropical cyclones. Howard calls 
 nimbus " a cloud, or system of clouds, from which rain 
 is falling. It is a horizontal sheet, above which the 
 cirrus spreads, while the cumulus enters it laterally and 
 from beneath." 
 
 Hildebrandson uses the word in a more contracted 
 
112 WEATHER. 
 
 signification, and reserves the name of nimbus for the 
 lower layers of dark torn clouds from which rain falls. 
 Poey calls the same broken clouds fracto-cumulus. 
 
 Weilbach designates by nimbus the property which a 
 cloud manifests to be or to become a source of rain in 
 particular circumstances, and then gives three varieties 
 nimbo-pallium, the rain-cloud in front of cyclones, which 
 we have called pure nimbus ; nubeculse, or scud ; and 
 nimbo-stratus, the rain-cloud, in rear of cyclones, which 
 we have designated cumulo-nimbus. He also gives a 
 plate marked cumulo-nimbus, which is identical with 
 our application of the same name. 
 
 The reason for making nimbus a class of its own 
 comes from the fact that a sudden striking change comes 
 over the look of the upper surface of a cloud the moment 
 rain begins to fall, the precise nature of which we cannot 
 at present explain. 
 
 The following remarkable description of the changes 
 which often take place in the appearance of the summit 
 of a cumulus when it commences to discharge rain, is 
 given by Mr. Ley : 
 
 " Under a summer sky a massive cumulus begins to 
 form a few miles distant from the observer. The atmo- 
 sphere being nearly calm up to the height of twelve or 
 fourteen thousand feet, the cumulus preserves its hemi- 
 spherical form, and an enormous aggregate of cloud- 
 inatter is produced, the contents of which may occupy 
 a space of upwards of a hundred cubic miles, while the 
 extreme opacity of the cloud shows that the water- 
 spherules which compose it are somewhat closely packed. 
 No rain falls from such a cloud while it preserves the 
 
CLOUDS AND CLOUD-PROGNOSTICS. 113 
 
 hard outline of its upper portions and its general hemi- 
 spherical figure. Suddenly the summit of this cloud 
 becomes soft-looking, and spreads out laterally in cirri- 
 form fibres, this change being always simultaneous with 
 the fall of a sheet of rain out of the cloud. 
 
 " The electrical charge which prevented the collision 
 of the particles composing the cloud while these particles 
 remained spherical, has been suddenly diminished in the 
 upper portions of the cloud as soon as these particles are 
 congealed into ice-needles, from the edges and extremities 
 of which the electricity immediately escapes. 
 
 " The particles, now only moderately electrified, unite,, 
 and in their rapid descent absorb the smaller spherules 
 with which they come in contact. 
 
 " The falling rain, and perhaps still more rapidly the 
 disruptive discharges, if such occur, further tend to ' tap ' 
 the electricity of the cloud i.e. to lower the potential of 
 the cloud-mass and the shower-making process continues 
 till all, or nearly all, of the lower portion of the cloud has 
 disappeared. Now, in those instances in which there is 
 not only very little motion in the lower layers of the 
 atmosphere, but also in the higher, the ice-cloud left in 
 the upper regions of the atmosphere is a true cirrus, the 
 curls and twisted forms of which are probably due to 
 slight lateral inequalities of pressure produced by the- 
 processes of condensation and congelation. A cirrus so 
 produced may hang nearly motionless for upwards of 
 twenty-four hours in the sky, or may more commonly 
 drift very slowly over districts from which the shower 
 which produced it was invisible." 
 
 Mr. Ley further says that he can always tell by the 
 
114 WEATHER. 
 
 appearance of the top of a cloud whether it is discharging 
 rain or not ; but the cirrification of rain-cumulus is cer- 
 tainly not necessary to precipitation. 
 
 The allusion to the discharge of electricity at the 
 moment of precipitation refers to an idea which has 
 much to support it that free statical electricity tends to 
 keep small condensed globules of vapour apart. 
 
 Lord Eayleigh has proved, experimentally, that 
 moderately electrified water-drops tend to coalesce, but 
 that strongly electrified drops repel one another. The 
 precise bearing of this on the formation of rain cannot be 
 given, but it shows unmistakably that there is a real con- 
 nection between rain and electrical manifestations. We 
 may, however, remark that it is almost certain that the 
 presence of electricity is quite secondary to the other 
 influences which develop rain. 
 
 Electricity may determine the precipitation of a cloud, 
 but it cannot give rise to the ascensional current, which 
 is the primary cause. Connected with the appearance of 
 the top of cumuli there is a well-known saying, that 
 " When clouds look woolly, snow may be expected." 
 This refers to the tops of cumuli, and not to the ordinary 
 woolly cirro-cumulus which is so so often seen in summer. 
 There is reason to believe that this woolly look is really 
 due to the cloud being composed of frozen, and not of 
 liquid, globules of water. The author has made some 
 observations on the sudden splash of rain or hail, which 
 often comes directly after a flash of lightning, and the 
 thunder-clap which accompanies them. By measuring 
 the time which elapses between the flash of lightning and 
 both the thunderclap and the splash of rain that follows 
 
CLOUDS AND CLOUD-PKOGNOSTICS. 115 
 
 to one-fifth of a second, he has found that the flash, the 
 clap, and the splash of rain may be supposed really to 
 occur simultaneously, but that the three impressions 
 reach the earth's surface at different times, because light, 
 sound, and a falling body all travel at various rates. Thus 
 light travels practically instantaneously; sound at the 
 rate of about 1100 feet a second ; while rain-drops fall a 
 definite distance in any given time, under the influence 
 of gravitation. This would be proved if we found that 
 the distance of the origin of lightning, as measured by 
 the velocity of the sound of the thunder, was the same as 
 that measured by the velocity of falling rain. For 
 instance, on one occasion the interval between the 
 lightning and the thunder was five seconds, while the rain 
 did not arrive for nineteen seconds. Now, calculating 
 the distance of the origin of the lightning from the 
 velocity of sound, we find the altitude to be 5500 feet ; 
 while the distance through which a drop would fall in 
 nineteen seconds would have been 5800 feet. The 
 difference is only 300 feet, which is very little considering 
 the nature of the observations, and the unknown retarda- 
 tion of a falling drop from the resistance of the air. In 
 practice the thunder always arrives before the rain; in 
 fact, we may consider that the same disruptive discharge 
 of electricity sends three messages to the earth at different 
 rates, and to different senses the light to the eye, the 
 sound to the ear, and the rain to the touch 
 
116 WEATHER. 
 
 UNCLASSIFIED CLOUDS. 
 
 So far for the great subdivisions of cloud-forms, but 
 we must now mention a few minor forms, because they 
 have some importance in judging weather. 
 
 ClRRO-NEBULA, OR ClRRUS-HAZE. 
 
 Sometimes, as a cyclone approaches, in any part of 
 the world, and we are very nearly on the line of its path, 
 we see a blue sky first get white, then grey, and then 
 work up to drizzling rain, without the formation of any 
 true cloud-form. When this happens, the sky is said 
 popularly to sicken, and this is an almost infallible sign of 
 rain, and probably of wind. Mr. Ley has proposed the 
 name " cirro-nebula," or " cirrus-haze," for this appear- 
 ance, and the term seems most appropriate. We may, 
 however, observe here the necessity for our caution about 
 the words cirro, cumulo, etc., conveying a rough idea of the 
 height of clouds. 
 
 This cloud has no fibrous or hairy structure to which 
 the name of cirro could be strictly applied ; but if we also 
 lay down that the word cirro is to convey the idea of a 
 high level cloud, then the word cirro-nebula is quite 
 correct. It is invariably formed at a great height, and as 
 it nearly always shows a halo when the sun or moon shines 
 through it, we may assume that it is composed of frozen 
 particles, or ice-dust. After it has formed, we can often 
 watch a layer of cirro-stratus being formed underneath 
 the haze. From all this we may draw the important 
 
CLOUDS AND CLOUD-PKOGNOSTICS. 117 
 
 inference that, though the front of a cyclone is charac- 
 terized by excessive warmth on the surface, the upper 
 strata are then very cold. 
 
 SCUD, WEACK. 
 
 Under any mass of cloud which is verging on the 
 precipitation of rain, we have just mentioned that small 
 detached clouds are frequently seen in rapid motion. In 
 England they are called " scud ; " in France, fuyards, or 
 diabletons ; while Poey suggests the name of fracto- 
 cumulus. If, instead of being shapeless, they are raggy, 
 they are then known in England as " wrack," from their 
 drawn-out appearance. In all cases it is obvious, from 
 the above description, that they are rather associates of 
 heavy rain-clouds than true prognostics. What we have 
 to explain is their origin. This seems to be simply, that 
 in very disturbed weather small masses of cloud form like 
 ordinary ragged clouds, from the irregular nature of the 
 rising currents, while the apparently very rapid motion 
 <eomes from their being nearer the surface than ordinary 
 clouds. 
 
 CLOUD- WBEATHS. 
 
 Sometimes, in front of certain kinds of squalls and 
 thunderstorms, we see a long, narrow roll of black cloud 
 moving rapidly, broadside on, and a very well-developed 
 example will be found in Fig. 56, under the heading of 
 "Pamperos." Dark cloud-wreaths in a very much less 
 pronounced form are very common in England before 
 
118 WEATHEE. 
 
 certain classes of squalls and showers, and in front of a 
 curious light grey vault of rain-producing cloud, as illus- 
 trated in Fig. 54, in our chapter on Squalls. 
 
 There are, of course, many other minute differences of 
 the various classes of cloud, to which it is impossible even 
 to allude in an elementary work like the present. Our 
 object will have been attained if we have succeeded in 
 explaining the general principles of cloud-formation, and 
 the method of making prognostications from their vary- 
 ing appearance. A few hours spent in watching the 
 changing and degrading forms of a sky which is covered 
 by detached cumulus, or the very different modifications 
 almost from minute to minute of cirro-stratus, will better 
 assist any one to understand the nature of cloud-forms 
 than reading pages of the best printed matter. 
 
 We may conveniently summarize here the various 
 varieties of cloud-forms which we have already described. 
 The general idea of our classification has been that, 
 though for large bodies of observers all practical men are 
 agreed that eight or ten principal varieties are all that 
 can safely be used, still more advanced cloud-observers 
 will not be satisfied with so coarse a subdivision, and 
 that therefore more minute varieties are necessary. 
 
 The ten principal varieties are therefore printed in 
 capitals, while the minor varieties are denoted by smaller 
 letters. 
 
 But there is another point in our subdivision of 
 varieties. Almost all the smaller varieties are so rare 
 or transient that for practical purposes they may be neg- 
 lected; but if, on the contrary, the ten main words are 
 restricted to the forms of clouds we have described under 
 
CLOUDS AND CLOUD-PROGNOSTICS. 119 
 
 them that is, cumulus, pure rocky cloud; stratus, pure 
 sheet cloud ; cirrus, pure wispy cloud ; cirro-stratus, thin, 
 high, wispy, or striated sheet cloud of all sorts ; strato- 
 cirrus, a similar low cloud ; cirro-cumulus, fleecy cloud at 
 high level ; cumulo-cirrus, the same, lower down ; strata- 
 cumulus, extended lumpy cloud ; nimbus, low rain-cloud ; 
 cumulo-nimbus, rocky rain-cloud then the author can 
 say, from an experience of cloud-observation in all longi- 
 tudes, and in latitudes ranging from 72 north to 55 
 south, that ninety per cent, of skies in every part of the 
 world can be sufficiently accurately defined by these ten 
 words. 
 
 VARIETIES OF CLOUDS. 
 
 With the mean height of the principal varieties at Upsala 
 in summer: 
 
 FEET. 
 
 /CIRRUS ... 27,000 
 
 Cirrus stripes. 
 
 HIGH / Cirrus haze. 
 
 ) CIRRO-STRATUS 27,600 
 
 \ClRRO-CUMULUS ... 20,000 
 
 f STRATO-CIRRUS ' 15,OCO 
 
 MIDDLE / CuMUL - CIRRUS 12,000 
 
 " j Festooned cumulo-cirrus 
 Mackerel sky 
 
 / STRATO-CUMULUS 6,000 
 
 CUMULUS (base) 4,000 
 
 1 Turretted or line cumulus 
 Festooned cumulus 
 
 L OW I CUMULO-NIMBUS (base) 4,000 
 
 " \ Cumulo-stratus 
 
 NIMBUS 4,500 
 
 STRATUS 1,900 
 
 Scud, wrack 
 \ Wreaths. 
 
120 WEATHER. 
 
 Of course it will be understood that the levels given 
 here for Upsala do not apply to all the world, but vary 
 with the season and latitude. They are introduced here 
 to illustrate a great principle, which holds from the 
 equator to the pole, that clouds tend to form at a few 
 definite levels, rather widely separated. Thus at Upsala 
 we find the high, middle, and low clouds at about 25,000, 
 14,000, and 6000 feet respectively. 
 
 If we wish to simplify still further cloud names for 
 observers who cannot see the difference between the ten 
 principal varieties, we can put cirro-stratus and strato-cirrus 
 collectively into cirro-stratus ; cirro-cumulus, and cumulo- 
 cirrus into cirro-cumulus ; and treat cumulo-nimbus simply 
 as cumulus. Then we get only seven terms : cirrus, 
 cumulus, stratus, nimbus, cirro-stratus, cirro-cumulus, and 
 strato-cumulus ; and still ninety per cent, of all skies 
 could be defined by these words, only not with the same 
 precision as by the ten varieties before mentioned. 
 
 MODERN IMPROVEMENTS. 
 
 We will conclude this chapter with a few remarks on 
 how far the increased knowledge of clouds improves our 
 capabilities of forecasting, both for a solitary observer, like 
 a man on board ship, or for a central meteorological 
 bureau, which can construct synoptic charts by telegraphic 
 observations from distant points. 
 
 No advance is more important than what we have 
 insisted on so much in this chapter that no sky can be 
 read mechanically, without reference to its surroundings. 
 We have seen that there is fine-weather cumulus as well 
 
CLOUDS AND CLOUD-PEOGNOSTICS. 121 
 
 ^as cumulo-nimbus, and a fine-weather as well as a 
 dangerous cirrus, while fleecy clouds have not the same 
 import in London as on the equator. In practice the 
 good and bad forms can rarely be mistaken, but some- 
 times very difficult cases arise. Clouds, in fact, tell us 
 by their appearance, what might be written in words, that 
 more or less damp air is rising or falling under certain 
 conditions of upper and lower wind-currents. The signifi- 
 cance must be judged by the surroundings and ante- 
 cedents, just as the sense of many words can only be 
 judged by the context. 
 
 Then the cloud-observer who has added the modern 
 knowledge of the motion of cirrus to the older lore, which 
 is only concerned with the kind of cirrus, would some- 
 times be able to indicate weather better than his neighbours. 
 And even if both would agree as to the approach of rain, 
 the former would sometimes be able to give much greater 
 precision to his prognostications than the latter. 
 
 Another most important advance has been made by 
 Mr. Ley. He finds that, like every other phenomenon 
 of a cyclone, the relation of the upper to surface winds 
 is relative to the direction in which the depression is 
 moving, and that, to a certain extent, the direction of the 
 highest clouds coincides with that of the path of the 
 cyclone. For instance, if the cirri in front of a cyclone 
 come from the south, the depression will probably also 
 advance from that direction at some distance to the west 
 of the observer ; while if they come from the west or 
 north-west, the depression will then most likely move 
 from the westwards also at some distance to the north. 
 Unfortunately, the details of these relations are too com- 
 plicated and too local for an elementary work. 
 
122 WEATHER. 
 
 But when we coine to think how far cloud-observations 
 may assist a central office, the case is different. The 
 foundation of all modern cloud-knowledge turns round the 
 relation of cloud-forms to the shapes of iso baric lines, so 
 that though, as Mr. Ley says, an isolated observer in the 
 English Midlands can plot out on a map the general dis- 
 tribution of atmospheric pressure, and of weather existing 
 over the whole of the British Isles at the time of his 
 observations, with very considerable accuracy, still an 
 observer at the central bureau could do so with absolute 
 accuracy. He could also often telegraph to an observer 
 in the Midlands, while the sky was still cloudless there, 
 that cirrus of a particular type would form after a certain 
 time. 
 
 Where cloud-observers can assist a central office is in 
 forecasting that kind of rain which is associated with the 
 small secondaries and non-isobaric rains that hardly 
 show on synoptic charts. These will be abundantly dis- 
 cussed in our chapter on the subject. For instance, 
 suppose the morning reports give rather ill-defined 
 isobars, with no rain, but a good deal of cloud at various 
 stations, the central forecaster would only say generally 
 fine weather, with, perhaps, local showers. But now, if, 
 instead of telegraphing the vague word "cloud," the 
 observers could not only define the kind of cloud accu- 
 rately, but also give information as to the direction and 
 velocity of its motion, relative to the surface-wind, then 
 in many cases the central office would see the incipient 
 formation of small rainy secondaries, and the forecasts 
 sent to the different districts would gain much in accuracy 
 and definiteness. 
 
PART H. 
 ADVANCED. 
 
( 125 ) 
 
 CHAPTEE IV. 
 
 ISOBARS. 
 
 IN our introduction to prognostics, we have already ex- 
 plained the leading features of the science of isobars, and 
 of their relation to the changes in the readings of the 
 meteorological instruments which are most usually 
 observed. In our chapter on Clouds, we have also intro- 
 duced the reader to the idea that over the surface-circula- 
 tion of the air round cyclones and anticyclones there is an 
 upper circulation of a very different character. But we 
 must now go more deeply into the subject, so as to explain 
 many details which could not then be conveniently given, 
 and we shall not only complete our description of the 
 nature of cyclones, anticyclones, etc., but also describe 
 the two remaining forms of isobars Y-shaped depressions 
 and cols which were omitted in the previous chapters. 
 
 CYCLONES. 
 
 We have already sufficiently explained the broad 
 features of a cyclone, and the wind and weather which are 
 associated with it. The reader will now comprehend 
 what is meant by the centre, the trough, the front or rear. 
 
126 WEATHEE. 
 
 the intensity, the path, and the velocity of the cyclone ; 
 &nd he will also understand that it is generally associated 
 with bad weather, and rapid shifts of wind according 
 to very definite laws. 
 
 What we want to consider now is the kind of circula- 
 tion which constitutes a cyclone, and some points connected 
 with the propagation and motion of this particular kind 
 of low pressure. Conventionally, we shall not call a low 
 pressure a cyclone, unless the isobars form a well-defined 
 closed-curve. With the exception of V-shaped depres- 
 sions, any other irregular area will be called by the 
 generic name of a " depression." 
 
 GENERAL CIRCULATION. 
 
 As the surface-wind in a cyclone is always a little 
 incurved and the upper wind always more or less out- 
 curved, the inference is irresistible that the main body of 
 the air near the centre of a cyclone must be rising; 
 otherwise, as the wind is always blowing in, the cyclone 
 would soon fill up if there was no escape upwards. To 
 this ascensional movement undoubtedly must be attri- 
 buted the rain and cloud which we find there rain near 
 the centre, where the ascensional impulse is strongest ; 
 cloud round the outside, where the uptake is less strong. 
 From this we can readily understand the effect of what 
 we have called intensity in a cyclone. It is not difficult to 
 conceive a cyclone which possessed so little intensity 
 that it could only 'develop cloud in the centre. Then, 
 if from any cause the intensity of the ascensional current 
 could be increased, rain would be developed where only 
 
ISOBARS. 127 
 
 cloud had been formed previously. Thus we get hold 
 of the idea, which we shall work out in some detail in 
 future chapters, of the influences which can modify any 
 existing cyclone. 
 
 If, for instance, any cause, such as the heating of the 
 ground by the sun, increased the velocity of the wind, 
 and so poured more vapour-laden air into the centre in a 
 given time, then the uptake would be greater, and the 
 tendency to form rain would be increased. Similarly, if 
 the wind was unchanged, but local causes, such as a 
 range of hills, gave the inpouring currents an increased 
 ascensional impulse, then, too, the precipitation of rain 
 would be still further developed. 
 
 Axis. 
 
 Returning now to our conception of the cyclone as a 
 circulatory system, it is manifest that we may consider 
 the whole as constituting an extremely complicated 
 vortex, something analogous to an eddy of water. There 
 is, however, this difference that a water-eddy sucks 
 down, while an aerial cyclone draws upwards. 
 
 The line along which any particle of air may be 
 supposed to move must not only curve irregularly inwards, 
 but also upward, and finally outwards. As the whole is 
 treated as a whirling system, there must be a line, "more 
 or less perpendicular to the earth's surface, round which 
 the air rotates in this complicated manner. This imaginary 
 line is called the axis of the cyclone. 
 
 There is much uncertainty as to the nature of the cir- 
 culation round this axis. Some writers have thought of 
 
128 WEATHER. 
 
 the axis of a top, and believed that the axis of a cyclone 
 can nutate, always keeping, a revolving disc of air per- 
 pendicular to itself, so that the cyclone would be pressed 
 down on the ground in the direction towards which the 
 axis inclined, and be lifted off, as it were, on the opposite 
 side. This, they say, would explain the anomalies that are 
 sometimes found both in the position of steepest gradients 
 relative to the centre, and in the variable .destructiveness 
 of the wind. 
 
 With the same velocity, wind will sometimes unroof 
 houses, at other times do little damage ; and they consider 
 that in the first case the direction of the wind is a little 
 upwards, in the latter a little downwards. They believe 
 that this conception of an inclined axis is confirmed by 
 the fact that, in tropical cyclones, the small, clear patch 
 of blue sky in the centre of a cyclone is not always 
 exactly over the point of lowest barometer. They would 
 then consider that the axis of the cyclone is like a 
 telescope pointed upwards, at some angle from the ground, 
 instead of truly vertical. 
 
 The insuperable difficulty in the way of all this lies in 
 the fact that a cyclone is often one or two thousand miles 
 across, and certainly not more than ten miles deep ; so 
 that the amount of tilt required to give the observed 
 deflection of isobars would be sometimes 20 or more, 
 and that, with a disc of two thousand miles, would be 
 impossible under the conditions of our earth. 
 
 If, for instance, we glance back for a moment to our 
 typical cyclone (Fig. 2), we see that the isobar of 29 ins. 
 is not concentric with that of 30 ins. The idea would be 
 that a vertical-axis cyclone would have concentric circular 
 
ISOBARS. 129 
 
 isobars, but that both the oval form and setting back 
 of the isobars at one side are due to the axis being tilted 
 forward towards the word Front. 
 
 Another theory to account for all these facts supposes 
 that a cyclone is made of a series of flat oval horizontal 
 sections, but that these are not superimposed concentrically 
 one on the top of the other, but pressed successively more 
 or less to one side by surrounding influences. In Fig. 2 
 this shunt would have been towards the rear, instead of 
 forwards as by the preceding hypothesis. 
 
 This view is probably partially correct, though it is 
 impossible to suppose that the air does not get more or 
 less inclined upwards at times, for no cyclone is ever 
 absolutely symmetrical. 
 
 We often see the conical revolving cloud of a whirl- 
 wind or tornado bending about like the trunk of an 
 elephant, with both a true axial inclination and a certain 
 amount of sideways shunt. Here, however, the vertical 
 height is enormously greater than the diameter, which is 
 just the opposite to the proportions of a cyclone. Anyhow, 
 if we suppose that upper winds follow the same laws as 
 surface-currents with relation to isobars, observations on 
 cirrus-clouds tend to the belief that the axis of a cyclone 
 is very often inclined backwards from the direction in 
 which the cyclone is moving, as if the surface portion was 
 going faster than the upper. 
 
 This is just the converse of what might have been 
 expected a priori, that surface friction would retard the 
 lower portions, so that the axis of the cyclone would have 
 been inclined forwards. The whole question is, however, 
 still very obscure. 
 
130 WEATHER. 
 
 PEOPAGATION. 
 
 When we come to consider the nature of the propaga- 
 tion of a cyclone, we are met with many difficulties. At 
 first it might be thought that a cyclone could be treated 
 as a rotating disc, which was impelled along the earth's 
 surface by some force ; but there are conclusive reasons 
 against such a supposition. If this was really the case, 
 we ought to be able to compound the rotation and trans- 
 lation motions of any particle of air in the usual manner ; 
 but when we do so, we find that we get winds very 
 different from what are actually observed. Take that 
 portion of the front of a cyclone where the wind from 
 rotation would be south, and suppose it to be compounded 
 with even a slow motion of translation towards the west, 
 then the resulting wind must have a slant from tlie west 
 that is to say, it would blow outwards in front of the 
 centre. Now, this is exactly what it does not do. Observa- 
 tion shows that the wind is more incurved in front of a 
 cyclone than in any other portion, and therefore the idea 
 of a rotating disc cannot be maintained. We are almost 
 compelled to believe that a cyclone-vortex is propagated 
 in a manner somewhat analogous to a wave of water. 
 When a wave approaches the shore, the first impulse is 
 always an indraught, though, of course, the motion of the 
 wave is forwards ; and when a cyclone approaches, the 
 first impulse is likewise inwards. Here, however, the 
 analogy probably ceases. 
 
 Another analogy to vortex-motion is found in the 
 manner in which a cyclone, as a whole, is deflected by 
 areas of high pressure. 
 
ISOBARS. 131 
 
 We know by experiment that a vortex-ring of smoke 
 has great stability, and can be twisted and deflected like 
 an elastic body. Similarly we see a cyclone move up 
 against an area of high pressure and be rebuffed from it, 
 even though the cyclone may be one thousand or more 
 miles in diameter. Of course, it must be understood that 
 in a vortex-ring each particle of air revolves in a complete 
 circle, while in a cyclone any particle hardly describes a 
 semicircle, so that the analogy is only very partial. 
 
 STABILITY. 
 
 However, from this conception of a stable vortex we 
 can understand what has long been a puzzle to meteor- 
 ologists why great changes of temperature between 
 night and day can be associated with fine weather. 
 
 Differences in temperature between two adjacent areas 
 have always been supposed to set up currents of air, and 
 from this it has been thought that weather-changes could 
 be deduced. In practice, however, it is far otherwise. 
 In some of the most settled climates in the world, such as 
 Persia or Northern Africa, the difference between the day 
 and night temperatures is often from 30 to 40 Fahr. ; 
 while in unsettled climates, like Great Britain, 10 Fahr. 
 (5 C.) is a large amount. 
 
 Then consider that a difference of 16 Fahr. (9 C.) 
 makes as much change in the density or specific gravity 
 of a cubic foot of air as a change of one inch of mercurial 
 pressure, and we may well wonder why such great changes 
 have so little effect on weather. 
 
 But when we know that a cyclone and, as we shall 
 
132 WEATHER. 
 
 afterwards see, an anticyclone are both to a certain extent 
 vortices, and that all the world is generally covered by 
 one or the other, then we can readily understand that as 
 such they have great stability, and that, though changes 
 of temperature might affect the velocity or direction of 
 the general circulation, they could not break up or destroy 
 any existing system. In fact, the atmosphere is not an 
 inert mass, ready to be swayed by any trifling disturbance, 
 but is always broken up into circulatory systems, which 
 possess a very considerable amount of stability. When 
 we come to discuss diurnal variations of meteorological 
 elements, we shall find that the difference of day and 
 night temperatures only impose a very small modifica- 
 tion on the general character of the weather. 
 
 INFLUENCE OF KAINFALL AND TEMPERATURE. 
 
 There are two other points connected with the origin 
 and motion of cyclones which need only be alluded to 
 in an elementary work like the present the influence of 
 rainfall, and the surrounding distribution of temperature. 
 In most cases the greater portion of the rain falls in front 
 of the cyclone's centre, or trough, and it used to be thought 
 that, the rain being produced by some unknown cause, 
 the cyclone ran after the vacuum which was left by the 
 condensation of vapour. It was, moreover, believed by 
 some that this condensation of vapour was also the origin 
 of the cyclone. Air was supposed to rush into the 
 vacuum, to pick up circulation through the influence of 
 the earth's rotation, and thus to form an eddy. 
 
 Later observations have, however, completely disproved 
 
ISOBARS. 133 
 
 both these views. Under certain conditions of surround- 
 ing pressure, the greatest rainfall in a cyclone occurs in 
 rear of the centre ; and the commencement of a cyclone 
 is neither always preceded by rain, nor does the depth of 
 a cyclone bear any relation whatever to the amount of 
 precipitation. 
 
 In practice the heaviest rain is with the slight 
 gradients of a secondary, and in very heavy showers the 
 barometer generally rises. 
 
 The general view we wish to present is that of the air 
 rising over a hot equator, and pouring down towards the 
 pole by its own weight. Mere irregularities of this down- 
 flow would be sufficient to form eddies without any con- 
 densation of vapour to cause vacuum, and develop heat, 
 as we can readily prove by experimenting with water or 
 smoke. 
 
 If the atmosphere was absolutely vapourless, and the 
 sun moved round a motionless earth, the irregular over- 
 flow of air would form eddies of some sort, very different 
 from those we now know. If the same vapourless atmo- 
 sphere rotated with the earth, round a stationary sun, as 
 at present, cloudless cyclones and anticyclones would 
 undoubtedly form, perhaps not so very different from 
 those we know now ; while the addition of water- vapour 
 would make the rotational systems precisely such as we 
 now observe. 
 
 Though heat and vapour do play a considerable part 
 in the mechanism of a cyclone, we shall abundantly show, 
 in the course of this work, that cyclones are only incidents, 
 as it were, in greater movements of the atmosphere, and 
 that most temperature-changes are due to differences of 
 
WEATHER. 
 
 radiation, caused by the wind, calm, cloud, or blue sky 
 associated with different kinds of aerial eddies. 
 
 An enormous amount has been written in Germany, 
 India, and the United States on the influence of surround- 
 ing temperature, and of the latent heat released by the 
 condensation of vapour into rain in determining the course 
 of a cyclone. Most of this is, however, too much mixed up 
 with theory, and too much encumbered with mathematical 
 formulae, to find a place in this work. There is, unfor- 
 tunately, no consecutive account of those investigations in 
 the English language, except certain papers of Professor 
 Ferrel, published by the Signal Office in Washington, 
 which cannot be purchased; but they are admirably 
 given in Sprung, " Lehrbuck der Meteorologie," and 
 J. von Bebber, " Handbuch der ausubenden Witterungs- 
 kunde." 
 
 We shall recur to this subject in our chapter on Fore- 
 casting by Synoptic Charts, as by that time we shall be 
 better able to understand the nature and surroundings of 
 a cyclone. 
 
 Our knowledge of the nature of cyclones would be 
 much more complete if we knew how high in the atmo- 
 sphere they extended. In thunderstorms we often get a 
 complete circulation of the wind on the surface, while the 
 upper currents retain their direction unchanged, and the 
 drift of the storm is usually with this higher wind. Un- 
 fortunately, we cannot trace a continuous development of 
 these small circulations into regular cyclones. 
 
ISOBARS. 135 
 
 TROPICAL AND EXTRA-TROPICAL CYCLONES. 
 
 It has been too much the custom in meteorological 
 books to treat tropical cyclones apart from similar dis- 
 turbances in extra-tropical or temperate regions. We 
 have made numerous researches on the subject in India, 
 the China Seas, Japan, and Mauritius, and found that, 
 though the general character of all cyclones is the same, 
 there are differences of detail which throw an immense 
 amount of light on the cause of the great variety in the 
 appearance of the sky in different parts of the same 
 cyclone. 
 
 All cyclones agree in the great features of the wind 
 rotating round the centre with a variable indraught, and 
 of an upward and outward circulation of the higher 
 currents. 
 
 No more conclusive proof of this can be found than 
 the fact that cyclones often pass out of the tropics, and 
 then join or coalesce with others which have been formed 
 without the tropics. Two similar eddies can easily unite, 
 but two that rotated on different systems would infallibly 
 break each other up. 
 
 The typical cyclone in all parts of the world is certainly 
 oval, with the inner isobars usually closer to the rear than 
 to the front ; and the rain extends further before than 
 behind the trough. But the tropical cyclone has a striking 
 feature which is absent in our latitudes. There is a 
 patch of blue sky over the calm centre, which is well 
 known in hurricane countries as the " eye of the storm," 
 or as a "bull's-eye." Then cirrus and halo appear all 
 round a tropical cyclone, while they are never seen in 
 
136 WEATHER. 
 
 rear of a European storm ; and though the way in which 
 the rain seems to grow out of the air in front of a cyclone 
 is the same everywhere, the sky and clouds in rear of a 
 hurricane are much softer and dirtier than in temperate 
 cyclones. There is not that sharp difference between the 
 quality of clouds in front and rear which is so striking 
 in higher latitudes. Still greater is the absence of any 
 marked squall or change of weather during the passage of 
 the trough in the tropics that is, at the moment when 
 the barometer begins to turn upwards. Some who study 
 hurricanes have scarcely noticed any change then ; and 
 all are agreed that the trough-phenomena are very slight. 
 
 We have already shown, in our chapter on Prog- 
 nostics, that a cyclone has, as it were, a double symmetry. 
 One set of phenomena, such as wind, cloud, and rain, are 
 grouped round the centre ; while the second set, such as 
 the different character of the heat and clouds in front 
 and rear, and the line of squalls along the line where the 
 barometer begins to rise, are related to the trough of 
 the cyclone. If we call the first set the rotational, and 
 the second set the translational, phenomena of a cyclone, 
 we find that the former are all more marked in the 
 tropical, and the latter in extra-tropical, cyclones. Then, 
 if we examine the charts of cyclones, we see that, while 
 tropical hurricanes are much smaller, and have much 
 stronger winds than any others, they only move from two 
 to ten miles an hour ; while extra-tropical cyclones rotate 
 much more slowly, but are propagated at a rate of from 
 twenty to seventy miles an hour. 
 
 Thus we might readily suppose that what we call 
 rotational phenomena are really due to the circulation, 
 
ISOBARS. 137 
 
 and the translational phenomena to the forward motion, of 
 a cyclone ; and we are confirmed in this view by an 
 examination of Japanese typhons. That semi-tropical 
 country is traversed by cyclones of two different types at 
 different seasons of the year, that move with different 
 velocities, and they find that all the trough-phenomena 
 are more marked in the quickly moving cyclones than in 
 those whose progress is slower. 
 
 These researches also lead to another most important 
 conclusion that the character of cloud and weather 
 depends on the position relative to the front of a cyclone, 
 and not on the direction of the wind. Cyclones in 
 Europe move towards the east, and the dirty sky comes 
 with a south-east wind ; while in the northern tropics 
 hurricanes move towards the west, and the same sky 
 comes with a north-west wind. People sometimes say 
 that of course the rear of a cyclone must be clear, because 
 of a cold, dry north-west wind ; but when a cyclone moves 
 west, even in Europe, that wind becomes close and 
 dirty. 
 
 We shall defer our consideration of cyclones in the 
 southern hemisphere till our chapter on Winds, because 
 it is only the direction of the wind, and not the sequence 
 of weather, which is altered in comparison with northern 
 storm-systems. 
 
 ANTICYCLONES. 
 
 If we turn to the diagram (Fig. 16) of surface and 
 upper winds in an anticyclone which we gave in our 
 chapter on Clouds, we shall see at once that it presents 
 
138 WEATHER. 
 
 some analogies, as well as some very striking contrasts, to 
 the cyclone figure. The anticyclone blows round and 
 out below, round and in above, and therefore the con- 
 clusion is obvious that the air in the centre of an anti- 
 cyclone must be descending. It must, then, necessarily 
 be unusually dry, and this is just what observation 
 shows it is. Then exactly the same argument holds as in 
 a cyclone, that, as a whole, an anticyclone is a complex 
 vortical system which possesses so much stability that 
 great diurnal changes of temperature do not affect it as a 
 whole. 
 
 It may be well to note the higher character of the 
 explanation of weather which we can give now, as com- 
 pared to what we said when treating of prognostics. 
 Then we merely said that, as a matter of blind obser- 
 vation, the centre of a cyclone was rainy, and that of an 
 anticyclone bright. Now we show that these two varieties 
 of weather are the necessary product of different kinds of 
 atmospheric eddies. 
 
 PRESSURE OVER CYCLONES AND ANTICYCLONES. 
 
 Simultaneous observations at the top and bottom of 
 high mountains have demonstrated that the difference of 
 pressure for a given height is always less in cyclones than 
 in anticyclones; also that the fall of the barometer is 
 always less pronounced on the summit than at the base 
 of a mountain. For instance, if the difference of pressure 
 between a high and low level station was four inches in 
 a cyclone, it might be four and a quarter in an anti- 
 cyclone ; and if the barometer fell an inch at the sea-level, 
 
ISOBARS. 
 
 139 
 
 the fall might only be about eight-tenths of an inch on 
 the top of a mountain five thousand feet high. 
 
 The inference which is drawn from this is that, as we 
 ascend, the gradients between a cyclone and its adjacent 
 anticyclone must diminish, and it is by no means im- 
 probable that if we went up high enough we should find 
 them inverted ; that is to say, that the higher pressure 
 would be over the cyclone. 
 
 In Fiof. 20 we have drawn an ideal sketch of the 
 
 *3 
 
 Spoo. _ 
 
 3 1 
 
 Cyclone, Anticyclone. 
 
 FIG. 20. Probable vertical gradients over cyclone and anticyclone. 
 
 probable so-called vertical gradients over a cyclone and 
 its adjacent anticyclone. The line at the bottom repre- 
 sents the level of the earth's surface. If the pressure is 
 31 ins. over the anticyclone, and only 27 ins. over the 
 cyclone, the vertical isobar of 27 ins. must be as we have 
 drawn it. But, as pressure decreases more rapidly over an 
 anticyclone, it seems probable that at a certain level 
 
140 WEATHER. 
 
 which we have assumed here as ten thousand feet there 
 would be no gradient either way ; and that still higher 
 up the pressure would be actually greater over the cyclone, 
 and the gradient inverted as compared to that on the 
 surface. It is evident, therefore, that, though we may 
 use the analogy of cyclones to whirlpools, we must not 
 picture the former to ourselves as saucer-shaped depres- 
 sions of the whole envelope of our atmosphere, like the 
 little eddies that pit the surface of a flowing river. The 
 important bearing of these vertical gradients on the pro- 
 blem of measuring heights by the barometer will be 
 very obvious. 
 
 This brings us to a curious question, as to what 
 gradients give direction to the upper winds. Some have 
 maintained that the uprushing currents of a cyclone have 
 so much momentum that they can override moderate 
 gradients. Others, on the contrary, hold that it is only 
 by a complete or partial inversion at high levels of the 
 gradients which are found on the surface, that the observed 
 phenomena of upper currents can be produced; but 
 materials do not at present exist which can decide the 
 question, and some of the published sketches of the 
 higher isobars over a cyclone are more than problematical. 
 
 Another point on which considerable uncertainty 
 exists is the relation of anticyclones to cyclones. There 
 is no doubt of their partial dependence on one another, 
 for cyclones always tend to travel round the anticyclone 
 on whose edge they have been formed. But, on the other 
 hand, we sometimes find cyclones which have detached 
 themselves from their generating anticyclones, and this 
 seems to prove that they can exist independently. 
 
ISOBARS. 141 
 
 ANTITHESIS OF CYCLONIC AND ANTICYCLONIC WEATHER. 
 
 Perhaps the best method of showing the antithesis 
 between cyclone and anticyclone weather will be the 
 method we have adopted in Figs. 21 and 22. We all 
 know how much weather is affected by the time of day, 
 as well as by the season of the year, and by local pecu- 
 liarities. We have, therefore, selected two charts for the 
 same day of different years, at the same hour of the 
 morning, and for the same portion of Western Europe. 
 Every diurnal, seasonal, or local influence is therefore 
 identical in both cases, and the whole of the difference of 
 wind and weather which we find between the two days is 
 entirely due to cyclonic or anticyclonic influences. 
 
 In Fig. 21 we give the synoptic conditions of pressure, 
 temperature, wind, and weather over Western Europe at 
 8 a.m., May 17, 1877. There we see a small oval cyclone 
 of very moderate intensity lying over the south-west of 
 England. Bound this the wind circulates in the usual 
 manner, but, as the gradients are not steep, the force 
 nowhere exceeds a fresh breeze, as at Brest. Near the 
 centre, and some distance in front, we find, by looking at 
 the weather-symbols, nothing but rain reported ; outside 
 the rain, an overcast sky or detached clouds ; and beyond 
 them, blue sky in a few places. The path of the cyclone 
 is marked by the letters &, x, so as to give the position of 
 the front. Lastly, the isotherm of 60 Fahr. (16 C.) runs 
 just north of the Pyrenees, while that of 50 Fahr. (10 C.) 
 stretches from the north of Scotland to Denmark. 
 
 Now turn to Fig. 22, where we give the same data for 
 May 17, 1874, at the same hour. Then an anticyclone 
 
142 
 
 WEATHER. 
 
 lay over the British Islands ; the gradients were much 
 less steep, and the wind, therefore, was everywhere light 
 and variable. For this reason the general circulation is 
 not so marked as in the preceding chart, but still it is 
 very evident that on the whole the wind blew round and 
 out in the direction of the watch-hands. Then the weather- 
 symbols are very interesting. Almost every station which 
 
 x 7-5-77 
 8. a.m. 
 
 50 
 
 17-5-74 
 8. a.m. 
 
 FIG. 21. Cyclone weather. 
 
 FIG. 22. Anticyclone weather. 
 
 reported rain in the previous chart is now marked with 
 b for blue sky, or with m for radiation mist in several 
 places. Then, though, as at Fano, in Denmark, we find 
 the same symbol, c, for detached cloud in both maps, we 
 know that it does not refer to the same kind of cloud. 
 The temperature also shows a marked contrast. The 
 isotherm of 60 has not much altered its position, but 
 
ISOBARS. 143 
 
 that of 50 Fahr. (10 C.) bends abruptly south from the 
 north of Scotland, across England, and the west of France, 
 while temperatures below 40 (5 C.) are reported from 
 Hanover and the Netherlands. This is partly due to the 
 prevailing set of the wind being from the north, whereas 
 in the preceding chart it was from the south or south- 
 west. 
 
 From these examples, we see that the whole of the 
 difference of weather on the two days was produced by 
 the difference of isobars, which we may put thus : 
 
 The weather was wet on May 17, 1877, over England, 
 because then the atmosphere was eddying in that manner 
 which we call cyclonic ; while it was fine on May 17, 1874, 
 because then the eddy-circulation took the form which we 
 call anticyclonic, and drew down dry air from the upper 
 regions of the atmosphere. 
 
 V-SHAPED DEPRESSIONS. 
 
 We must now describe a very interesting shape of 
 isobars, to which the name of V-depressions is applied in 
 England, but which the German writers call "tongue- 
 formed" depressions. In these the isobars are shaped 
 like the letter V, and enclose an area of low pressure. In 
 the northern hemisphere the point of the V is usually 
 directed towards the south, as in Fig. 23. The wind 
 follows the universal law of gradients ; being from south 
 to south-west in front, and from west to north-west in 
 rear of the trough. This latter line is given at once by 
 joining the southern points of each successive isobar, and 
 in practice is nearly always curved, the convexity being 
 
144 
 
 WEATHER. 
 
 turned towards the east, as in the diagram. As the V is 
 usually moving towards the east, this line marks out the 
 position of all the places at which the barometer, having 
 fallen more or less, has just turned to rise, and is called 
 the " trough " of the V. 
 
 These features are common to all V's, but the position 
 of rain divides these depressions into two distinct types. 
 
 In the first, and by far the 
 commoner kind in Great 
 Britain, a narrow strip of cloud 
 precedes an area of rain, 
 shaped like a portion of a 
 crescent. This is shown in 
 Fig. 23, where the single 
 shading marks the position 
 and shape of the cloud-area, 
 and the double shading that 
 of the rain. The rear of the 
 rain-area is very sharply de- 
 fined by the line of the trough, 
 which also marks the position 
 of a line of squalls. Beyond 
 this we find detached clouds, 
 and then blue sky. 
 
 The sequence of weather, 
 as a V of this type drifts 
 over an observer, is obviously from blue sky to halo, 
 cloud, and, later on, rain, with a falling barometer and 
 south-west wind ; then a heavy squall, during which the 
 wind jumps (does not veer) to north-west, and the sky 
 rapidly clears as the barometer rises. 
 
 Blue 
 
 Sky 
 
 FIG. 23. Weather in Y-de- 
 pression. 
 
ISOBARS. 145 
 
 In the other kind, which is less common, the front of 
 the V is cloudy, but half a crescent-shaped area of rain is 
 formed in the rear. The front of this area is sharply 
 defined by the trough, while the rear tails off through 
 cloud to blue sky. An illustration of this type will be 
 found in Figs. 49, 50, and 51, in our chapter on Squalls. 
 
 The sequence of weather to a solitary observer will 
 be a falling barometer, with a cloudy sky and wind from 
 the south-west. A heavy bank of cloud approaches from, 
 the north-west ; this passes over him with a heavy squall, 
 the wind jumps to north-west, and the mercury turns 
 upwards. After the first violence of the squall is over, 
 driving rain continues for some time, and dies away 
 gently as the sky becomes clear again. 
 
 This class of V is usually followed by a second depres- 
 sion of some sort, as will be seen by Fig. 49 in our chapter 
 on Squalls. There seems to be some connection between 
 the two depressions and the area of high pressure between 
 them, but the subject has not yet been worked out. 
 
 V's are generally formed either along the southern 
 prolongation of the trough of a cyclone, or else in the col 
 between two adjacent anticyclones ; and a V of the fii st 
 type bears the same relation to a wedge that a cyclone 
 does to an anticyclone ; that is to say, one is as nearly as 
 possible the converse of the other. The most interesting 
 feature about V's is their relation to cyclones ; for, while 
 a cyclone has, as it were, a double symmetry namely, one 
 set of phenomena, such as temperature and the kind of 
 cloud symmetrically disposed in front and rear of the 
 trough, and another set, such as wind and rain, symmetri- 
 cally arranged round the centre a V-shaped depression 
 
146 WEATHER. 
 
 has only one line of symmetry the trough to the front 
 and rear of which alone both wind and weather are related. 
 When we consider that in extra-tropical cyclones, though 
 the isobars are circular, the wind makes a sudden shift as 
 the trough passes, and that V's have nothing cyclonic at 
 all about them, we can readily understand the difficulties 
 which many felt in accepting the theory of cyclones, and 
 how at first sight it appears much simpler to assume the 
 conflict of two opposing currents, rather than the circula- 
 tion of a great eddy. 
 
 Though a cyclone is a edcly, the opposite properties 
 of its front and rear" do not suggest true circulation, while 
 a V is something of a totally different nature. The error 
 consisted in assuming that the bad weather of the V was 
 of the same nature as that in a true cyclone. There is 
 Very great difficulty in forming a rational conception of 
 the general circulation in a V. The currents in front and 
 rear of the trough are not exactly opposite, and we know 
 nothing of the motion of the upper currents, so that we 
 must await the results of future research. 
 
 SOUTHERLY BURSTERS. 
 
 V-depressions are not at all common in the tropics, 
 though we have observed a squall of this type near New 
 Caledonia, in about lat. 22 south ; but we have discovered 
 that a great many of the so-called " southerly bursters " 
 off Australia are due to the class of Y's in which rain 
 falls in rear of the trough. 
 
 The point of a V in the southern hemisphere is 
 pointed towards the north, while the wind is north-east 
 
ISOBARS. 147 
 
 or north in front, and south-west or south in rear of the 
 trough (see Fig. 39, p. 199). 
 
 The first blows in Australia across a burning continent 
 of hot sand, and as the gusts increase near the trough, 
 clouds of suffocating dust are added to the furnace-like 
 heat till existence is almost unbearable. Then suddenly 
 the wind jumps round to south-west or south, and heavy 
 rain is driven before the chilly blasts from the ice-bound 
 southern pole. Temperature occasionally falls 30 or 40 
 in a single hour. Sometimes the " burster " is associated 
 with the very similar sequence of weather during the 
 passage of the trough of a cyclone. 
 
 COLS. 
 
 The last shape of isobars which we have to describe is 
 the " col," or neck of low pressure, which lies between two 
 adjacent anticyclones. We will describe a specimen of 
 the most common case of European col. We find in 
 Eig. 24 that a portion of one anticyclone lies over the 
 Bay of Biscay, while another lies to the north-east of this 
 over the Scandinavian peninsula. Then, while one area 
 of comparatively low pressure lies to the north-west of 
 Iceland, another covers Central Europe and the north of 
 Italy, so that a saddle-back area, or neck, of low pressure 
 is found over England. In the middle of the col there is 
 no gradient, and therefore a calm, while all round the 
 winds and weather conform to the usual law of isobars. 
 The weather is dull, gloomy, and stagnant, while in 
 summer violent thunderstorms are frequently found in 
 different portions of a col. 
 
148 
 
 WEATHEE. 
 
 20.4.83. 
 
 As a whole no sequence of weather can be assigned to 
 a col. It does not move itself, but no law can be laid down 
 to say whether the col will remain stationary, or whether 
 
 the area which it covers to-day 
 will be occupied by some 
 other type of pressure to- 
 morrow. 
 
 The importance of this 
 shape of isobars in forecasting 
 arises from the fact that, as 
 both the anticyclones are 
 usually stationary, the col 
 represents, as it were, a line 
 of weakness, along which dis- 
 turbances will be propagated. 
 Unfortunately, though a 
 col can be safely forecast in 
 general terms as for unsettled 
 weather without much wind, 
 the motion of cyclones as 
 they meet a col is most uncertain. Sometimes they 
 pass in a south-eastern direction across Europe between 
 the two anticyclones, while more frequently the main 
 body of the cyclone is deflected or dies out, while an 
 irregular secondary pushes its way more or less across 
 the col. 
 
 FIG. 24. Wind in a " col." 
 
 OEIGIN OF ISOBAES. 
 
 Secondary cyclones, wedges, and straight isobars have 
 already been sufficiently described, so that we may now 
 
ISOBAES. 149 
 
 conclude with a few general remarks on the whole question 
 of the shapes or forms of isobars. In the first place, what 
 is the meaning of the seven fundamental shapes ? From 
 the analogy of water, to which we have so often referred, 
 there can scarcely be any doubt that just as circulating 
 water can only take a certain limited number of forms, 
 eddies, backwaters, ripples, etc., so air can only move in 
 a limited number of ways, of which these seven are the 
 most important. 
 
 A cyclone may be anything from two thousand to 
 fifty miles across ; a wedge may fill the whole North 
 Atlantic, or it may also be measured by single miles, but 
 their respective characters are in no way changed. Then, 
 though we are only at present able to give the general 
 nature of the upper and lower circulation in cyclones and 
 anticyclones, there is no doubt that the other five shapes 
 are capable of being worked out in a similar manner. 
 The conclusion is irresistible. Just as we proved that not 
 only the broad features of the weather, but also the 
 minute characteristics of every cloud in cyclones and 
 anticyclones are the product of the nature of their respec- 
 tive circulations, so must the weather and cloud in the 
 remaining five shapes be also the product of some form 
 of atmospheric motion. 
 
 In an eddying river the general cause of all motion is 
 the downward current from the source to the mouth ; but 
 water cannot slide like a weight on an inclined plane, 
 without forming horizontal or vertical whirls. In the 
 atmosphere the prime source of all motion is the general 
 circulation of air set up between the hot equator and the 
 cold poles. This, like water, must form vortices, or 
 
150 WEATHER. 
 
 ripples, and isobars map out the varying pressure on the 
 earth's surface induced by the uneven flow of air. If we 
 had a series of barometers at the bottom of a river, and 
 observed them all simultaneously, we should find a defect 
 of weight under the eddies, with an excess of pressure 
 under the backwater, and we might draw isobaric lines 
 which would map out the position of these vortices 
 exactly as we do in air. 
 
 But it must be specially noted that the depression of 
 a cyclonic eddy is not entirely due to centrifugal force 
 like that of a water eddy. The depression of an ordinary 
 cyclone could not be produced by the centrifugal force of 
 the fiercest hurricane that ever blew ; and the cause of 
 the diminution of pressure in a cyclone is at present 
 unknown. 
 
 When we discuss the changes shown on two different 
 synoptic charts, we shall talk of changes in the shapes and 
 positions of isobars as if those lines were graphical ab- 
 stractions. What we really do is to trace by their means 
 the ever-changing eddies of the atmosphere the death 
 of old ones, the birth of new ones, or the fusion of several 
 existing systems into a new disturbance of a different type. 
 
 To avoid all danger of theoretical errors, we simply 
 define the kinds of eddies by certain abstract shapes of 
 isobars, and then, entirely from observation, we collate 
 certain definite kinds of weather and sky, with different 
 portions of each isobaric configuration. 
 
CHAPTER V. 
 
 BAEOGEAMS, THEEMOGBAMS, METEOGEAMS. 
 
 WE have already seen that a series of synoptic charts 
 are, as it were, a series of bird's-eye views, not only of the 
 appearance of the sky, but also of instrumental readings 
 over a considerable area. Owing to the expense of 
 telegraphy, these can rarely be taken oftener than every 
 eight hours, but we all know by experience that very 
 great changes of weather may occur within that time. 
 Every well-equipped observatory is therefore supplied 
 with instruments which record automatically the height 
 of the barometer, of the thermometer, the direction and 
 velocity of the wind, the occurrence and quantity of 
 rain, while a wet-bulb thermometer for moisture and an 
 electrometer are sometimes added. 
 
 The trace marked on paper by a barograph is called a 
 barogram ; that by a thermograph, a thermogram ; both 
 or either of the records left by the wind-instruments are 
 called anemograms; and if all or several of these are 
 combined in one diagram, the whole is called a meteogram, 
 because it is a writing of meteorological instruments. 
 Fig. 25 is a copy of one of the meteograms which are 
 
152 
 
 WEATHEK. 
 
 published by the British Meteorological Office, and we 
 propose to devote several paragraphs to its consideration, 
 so as to explain how, by means of these continuous records, 
 we can fill up the gaps in the history of weather-changes 
 
 JO.O 
 
 29.0 4.0 
 
 28.0- 
 
 W- 
 S - 
 E - 
 
 7 Dec. , 1874. 
 
 i i I i i i | i 
 
 i i 
 
 i r 
 
 -20 
 JO 
 
 Raiv. 
 
 Direct. 
 
 . 
 -E 
 
 -N 
 
 Noon . Noon . 
 
 FIG. 25. A meteogram. 
 
 Noon . 
 
 which are shown at considerable intervals only, on synoptic 
 charts. 
 
 It should be remarked that in treating of these we 
 no longer deal with generalities, but with the actual 
 changes and variations of each day. Also that the aim 
 and object of all meteorology is to explain the details of 
 the sequence of weather as it occurs at any one place, if 
 possible with great minuteness, and that synoptic charts 
 are only a means to that end. Ever since the discovery 
 of the barometer and thermometer, innumerable attempts 
 have been made to discover the nature of weather-changes 
 by calculations, which were based on the fluctuations of 
 these instruments as observed at any one place, but with 
 
fftiNr 
 \,ou 
 
 BAROGRAMS, THERMOGRAMS, METEOGRAMS. 153 
 
 Tery moderate success. We shall very soon see why this 
 was so; and, in fact, why it could not be otherwise, from 
 the nature of things. 
 
 METEOGRAMS. 
 
 Now for our actual meteogram in Fig. 25. The 
 barometer-trace is marked "bar," with an appropriate 
 scale at the edge. Temperature is marked " therm ; " 
 while the upper curve of all, marked " vel," gives the 
 varying velocity of the wind, only we must note that for 
 convenience its base is taken from the top of the diagram, 
 and that, therefore, it is measured downwards. Eain is 
 marked near the bottom of the diagram by oblique lines, 
 which are proportional to the amount of rain measured at 
 the end of each hour, so that we can see at a glance when 
 rain fell and how much of it. Below all is the trace 
 marked " direct," which refers to the direction of the 
 wind. The notation requires a word of explanation, as we 
 have to represent an angular scale on a rectangular 
 diagram. It will be seen that the bottom of the diagram 
 is marked N for north ; then as we rise upwards we get 
 successively east, south, west, and back to north again at 
 the top of the direction-figure. By this means we can see 
 at a glance whether the wind is veering or backing. 
 When the wind veers, the trace moves upwards ; while, 
 when the wind backs, the curve descends. A little 
 practice is necessary to read this easily, but when once 
 learnt it is very convenient. 
 
 But to understand this meteogram fully, we must look 
 at the charts given in Figs. 26 and 27, for the 8th and 9th 
 
154 
 
 WEATHER. 
 
 of December, 1874, respectively. The broad features are 
 very simple. The cyclone which we see approaching in 
 the first chart passes across England to Holland, where 
 we find it in the second chart ; and now let us see how 
 
 9-12-74 
 8. a.m. 
 
 FIGS. 26 and 27. Charts to illustrate meteograrn. 
 
 this affected the sequence of weather at Stonyhurst, near 
 Manchester, marked s on the charts. 
 
 The meteogram, Fig. 25, refers to the three days, 
 December 7 to 9, 1374, while the charts refer to the 
 two latter days only. The barometer rose till nearly mid- 
 night on the 7th, under the action of the wedge which we 
 find over the North Sea on the chart for the morning of 
 the 8th. Then we see by the trace that the mercury was 
 falling fast, owing to the approach of a cyclone. This fall 
 continued till about midnight, when a rapid rise com- 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 155 
 
 menced, which lasted all the next day. The charts alone 
 would scarcely enable us to mark the exact line of the 
 passage of the cyclone, but from the wind-traces we know 
 that the centre passed very nearly exactly over Stony- 
 hurst. 
 
 The temperature-curve is much more complicated. 
 On the first day we see a most irregular curve, with little 
 trace of the ordinary diurnal range of temperature, for 
 the highest and lowest points were reached within about 
 an hour of each other, and in the middle of the day. This 
 can be easily explained. The station was the whole 
 day under the influence of the rear of a cyclone, with a 
 north-west wind and cold showers. These and driving 
 clouds gave rise to the sudden changes which are marked 
 on the curve. Next day there are more signs of diurnal 
 variation, but the highest temperature was at 6 p.m., and 
 the second midnight is much hotter than the first. Also, 
 though rain was collected almost every hour, we do 
 not see the sudden changes of the preceding day. The 
 interpretation of all this is as follows : 
 
 Commencing from the early morning, the wind began 
 to back from west-north-west to south and south-south- 
 east. This increased the general warmth at the second 
 midnight, and the diurnal range took its usual course on 
 the top of the more general change. Why the rain did 
 not send down the thermometer will be obvious when we 
 know that this all happened in the front of an intense 
 cyclone, and we remember that the soft, warm, drizzling 
 rain which falls there is not like the chilly showers of the 
 rear of a depression. 
 
 On the third day the thermogram is still more curious. 
 
156 WEATJIEE. 
 
 Immediately after midnight the thermometer fell 5 Fahr., 
 just as the trough of the cyclone passed, and the wind 
 jumped from south-west to north. During the whole day 
 the temperature fell, and while the greatest heat was at 
 the first, the greatest cold was at the second, midnight. 
 No rain was reported, so the small sudden changes must 
 be the effect of passing clouds. Here it may be well 
 to note that a passing cloud by day will send down the 
 thermometer by hiding the sun, while by night the same 
 cloud will raise temperature by cutting off the radiation 
 of the cold sky. The general explanation is that cold 
 winds set in when the barometer turned, and that their 
 influence apparently completely overrode diurnal varia- 
 tions. 
 
 Though the thermometer fell all day that is to say, 
 there was no hour at which the temperature was not lower 
 than at the preceding one it does not follow that there 
 was no diurnal range with a regular maximum and 
 minimum ; but, as the explanation of the superposition 
 of curves requires some collateral details, we will defer 
 our remarks till we have considered the wind-traces of 
 our meteogram. 
 
 First, then, for the velocity-trace. Presuming that 
 the normal course of the wind is to increase regularly in 
 velocity from 4 a.m. till 2 p.m., and then fall gradually 
 again, the most obvious feature in our wind-diagram is 
 the increase of the wind towards the middle of the day ; 
 that is to say, the regularity of this diurnal variation in 
 spite of the great cyclonic changes which were going on. 
 The influence of these latter is seen in the comparatively 
 high velocities from thirty to forty miles an hour which 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 157 
 
 the wind attained a little after noon on the first two days ; 
 but the trace for the last day, December 9, is very different. 
 On that day the strongest winds were before 6 a.m., and 
 during the hottest time of the day, when the wind is 
 usually strongest, the velocities decreased steadily. The 
 reason of all this was that, in the early morning, the steep 
 gradients in rear of the cyclone were passing over the 
 station, while in the middle of the day the gradients were 
 becoming so much less steep that the diminishing 
 velocity due to them entirely overrode any increase which 
 would naturally have occurred from diurnal influences. 
 The calm just before midnight on the 8th, and the rapid 
 rise of the wind to thirty miles an hour directly afterwards, 
 are very interesting ; for the calm is that in the centre of 
 an intense cyclone, and the high wind is associated with 
 the steep gradients which we see in rear of the depression 
 in Fig. 27. 
 
 The direction-changes for these three days are tolerably 
 simple. The natural diurnal variation of the wind is to 
 veer a little in the forenoon, and back again as the sun 
 goes down ; but almost all trace of this is lost in the stormy 
 weather to which our diagram refers. On the 7th the 
 wind kept pretty steady between west and north-west, 
 and there is little sign of diurnal variation. Next day, 
 the 8th, as the cyclone approached, the wind backed 
 rapidly as far as south-south-east ; then veered rapidly to 
 south-west ; and just at midnight, when the barometer 
 turned, jumped up to the north and north-east ; and then 
 backed during the day to about north-west. From these 
 changes, and the sudden jump of the wind from west to 
 north, it is evident that the cyclone's centre passed very 
 
158 WEATHEE. 
 
 nearly over the station. All traces of diurnal variation 
 are, of course, entirely marked by these greater changes. 
 
 SUPEKIMPOSITION OF VARIATIONS ON CURVES. 
 
 The simple conception that the actual weather is the 
 balance or sum of various influences will now be sufficiently 
 obvious, but we must go a little more into detail of all 
 that can be learnt from an inspection of instrumental 
 curves. As the idea of superimposing curves of different 
 kinds of variation one on the top of another, and of so 
 deducing a resulting curve, may not be familiar to some 
 of our readers, we will commence by an easy example. 
 
 Here, and all through this work, we shall use con- 
 ventionally the word " changes " to denote alterations in 
 weather due to cyclones, etc. ; and " variations " to denote 
 alterations due to the time of day. The passage of a 
 cyclone, or its replacement by an anticyclone, really 
 changes the weather; diurnal influences only impose a 
 certain variation on these greater changes. 
 
 For instance, let us try and find out what would be 
 the nature of the curve left by a thermograph if a regular 
 diurnal variation of temperature, which was highest at 
 2 p.m. and lowest at 4 p.m., was superimposed on a 
 steady general fall of temperature, due to other than 
 diurnal influences say, the setting in of cold northerly 
 winds. The line on which the diurnal curve is super- 
 imposed is called the level of variation. In the familiar 
 curve of mean diurnal variation, such as B or c in 
 Fig. 28, the straight, horizontal line A represents the 
 mean temperature of the place, and the curves B or c 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 159 
 
 are the resulting traces. If they are unaltered, the only 
 effect of any change in the level of A would be to bring 
 the whole nearer 
 or further from 
 the base of the 
 figure. For the 
 level of A might 
 either be 40 or 
 70, but the 
 shape and mag- 
 nitudes of the 
 diurnal curves B 
 or c would be 
 the same. 
 
 When we come 
 to deal with the 
 significance of a 
 curve for any 
 particular day, 
 the horizontal 
 line A no longer 
 represents the 
 mean tempera- 
 ture of the sta- 
 tion, but the 
 level of tempera- 
 ture from gene- 
 ral causes inde- 
 pendent of the 
 time of day. If 
 
 the thermograph FIG. 28. Superlinposition of curves. 
 
150 WEATHER. 
 
 gave such a trace as B or c, it is possible, from geometrical 
 considerations, to draw the line A, which denotes the level 
 of general temperature on which the diurnal curves are 
 superimposed. The method is as follows. As B and c 
 are both diurnal curves, which only differ in magnitude, 
 but not in character, we will confine our attention to c 
 only. Then, if the instrumental trace gave c, find from 
 it the mean temperature of the day by taking the 
 mean of the readings at every hour ; mark this value m 
 on the vertical hour-line for noon; join n and t, the 
 points where the trace cuts the first and second mid- 
 night hour-lines ; then a line A drawn through m parallel 
 to nt is the level of variation required. The line nt is 
 omitted in the diagram for the sake of greater clear- 
 ness. 
 
 But now suppose that the diurnal variation B or c 
 remained the same, but that from other causes the general 
 temperature fell so uniformly that it may be represented 
 by a straight line, such as D, what would the resulting 
 trace be like ? This we can easily find graphically, by 
 drawing the line D and taking it as the variable level on 
 which to add B and c. To do this we have only to 
 measure the values of B and c at different hours from 
 the level of the line D at the same hour. For instance, 
 the minima at 4 a.m. of E and F are the same distance 
 from D that B and c are from A at the same hour ; also 
 at 8 a.m. and 8 p.m. the diurnal curves equally cross the 
 line of general level, and the maxima at 2 p.m. are also 
 at equal distances from that level. Now look at the 
 resulting curves E and F. The latter, which is the 
 stronger, is able, as it were, to reverse the general fall of 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 161 
 
 the line D, and every one would recognize that there was 
 a diurnal range. But then turn to curve E. Here the 
 diurnal variation is so small that it can only deflect, but 
 not reverse, the general line D, and thus we get the 
 apparently impossible result that the thermometer may 
 fall all day, and yet that there may be a very distinct 
 diurnal maximum and minimum, which only modify the 
 rate of fall of the general sweep of the curve. Now, this 
 is exactly what happened on December 9, 1874, at Stony- 
 hurst, as shown in our meteogram (Fig. 28). There the 
 thermometer fell all day, but by joining mentally the 
 points where the trace cuts the first and second midnights 
 of that day, we see at once that there is a diurnal maximum 
 about 3 p.m. in the general sweep of the curve. In this 
 case the curve is so irregular that, though we can detect 
 the fact of a diurnal maximum, we cannot measure the 
 amount even approximately. When, however, the trace 
 is more regular, it is obvious that from a curve like E we 
 could infer D, and the maximum and minimum diurnal 
 values of E by a method exactly similar to that which we 
 employed to find A from B. In fact, given B, c, and D, 
 we can draw E and F ; while, given E and F, we can 
 separate them into a general line D and diurnal variations 
 B and c respectively. 
 
 If the line of general level is so irregular that it 
 cannot be represented by a straight line for twenty-four 
 consecutive hours, then we can no longer separate the 
 general and diurnal changes, for we are unable to draw 
 the general level, which is then a curved line. 
 
 We have taken our illustration from temperature- 
 curves, as their diurnal changes are the simplest and most 
 
 M 
 
162 WEATHER. 
 
 obvious ; but the same general principles apply to every 
 meteorological element. 
 
 In barograms the diurnal variations in Great Britain 
 are so small compared to the general changes that the 
 former can usually be neglected ; but in the tropics the 
 mercury often falls regularly O12 in. in six hours, while 
 the general changes of an approaching distant hurricane 
 reach half that amount. Then the discrimination between 
 general changes and diurnal variation of pressure is of vital 
 importance. 
 
 The question of sorting out various sources of 
 barometric movement is so important in every branch of 
 meteorology that we must give the subject in greater 
 detail. 
 
 BAROMETRIC KATE. 
 
 The measure of the rapidity with which the mercury 
 rises or falls is called the " barometric rate." This is 
 usually expressed by saying how many hundredths of an 
 inch, or tenths of a millimetre, the mercury changes in an 
 hour. 
 
 In a variable climate like Great Britain, anything 
 under 0*02 in. per hour is a low rate, while anything 
 over O05 may be considered high. On only a few occa- 
 sions in any year will O10 be exceeded, though as high as 
 O20 has been recorded in exceptional cases. In the 
 tropics diurnal variation alone may give a rate of nearly 
 O02, and anything higher than this would be a warning 
 of danger. 
 
 As many people have a very vague and inaccurate 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 163 
 
 idea of the relation of cyclone-motion and gradients to 
 barometric rate, it may be expedient to give some 
 further explanations. If we look at the diagram of a 
 cyclone given in Fig. 29, it is obvious that the rate of fall 
 of A'S barometer will depend on 
 three things the velocity of the 
 cyclone ; the steepness of the 
 gradients ; and the position of the 
 observer relative to the cyclone- 
 centre. If we assume that a dis- 
 tance equal to A B would be 
 traversed by the cyclone-centre 
 in four hours, A'S rate would be 
 
 IUL = 0*025, as the distance be- FIG. 29. Barograms, baro. 
 ,1-1 A i i metric rate, and filling up 
 
 tween the isobars is O'l in. ; and of cyc i ones . 
 it is evident that this rate might 
 
 be doubled by doubling either the velocity of the cyclone 
 or the barometric difference between the isobars. Then, 
 in the same cyclone, with the same velocity and gradients, 
 the rate to E, who was in the line of the cyclone's path, 
 would be much greater than that to A, who was more 
 remote. This is manifest, because, as a cyclone is generally 
 nearly round, the shortest line between two concentric 
 isobars is along a radius of the circle. In this diagram 
 the distance E D is ab >ut half A B, so that E'S barometric 
 rate would be double that of A. 
 
 From these considerations we can see that. a rapid fall 
 of the barometer is dangerous, because, in a general way, 
 it shows that the observer is nearly in a line with the 
 path of the cyclone, that the gradients are steep, and that 
 the disturbance is moving rapidly. The first gives an 
 
104 WEATHER. 
 
 almost complete reversal of the wind, which is most 
 dangerous to ships ; the second, high wind ; while the 
 third increases the intensity of the weather in every 
 way. 
 
 When we come to discuss squalls and thunderstorms 
 we shall find that the barometer often jumps up O'l in. 
 in a few minutes, just as the heavy rain begins. The 
 cause of this is uncertain, but we must notice that the rise 
 is of a totally distinct nature from that produced by the 
 passage of a cyclone. We must not, therefore, be led 
 into the error of treating all barometric changes as 
 identical, and of comparing the barometric rate of a squall 
 with that of a cyclone. A difficult case arises sometimes 
 with the squall in the trough of a cyclone. As the wind 
 goes round with gusts and he ivy rain, the mercury turns 
 upwards a id rises with a sudden jump. Sometimes a 
 slight fall then occurs, but directly afterwards the baro- 
 meter rises quickly and steadily. It is almost impossible 
 to say how much of the first jump is due to the squall, 
 and how much to the general increase of pressure due to 
 the passing on of the cyclone. 
 
 We have already explained that a barogram is a 
 section, as it were, of a cyclone which is seen in plan on 
 a synoptic chart. This, however, evidently only holds 
 good on the supposition that the cyclone changes neither 
 in depth nor shape during the time whicli elapses between 
 the beginning and the end of the trace. Now, in practice 
 a cyclone is perpetually changing both its depth and 
 shape; and, consequently, it is often extremely difficult 
 to see how the changes of pressure which are seen in two 
 synoptic charts at even a short interval would have 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 165 
 
 influenced the trace of the barometer at any one place. 
 Two or more sets of general changes are going on simul- 
 taneously, and we have to work out the result of their 
 combination. But the importance of investigating the 
 question will be evident when we remark that on this 
 depends all the apparently anomalous movements of the 
 barometer. If the mercury always fell before rain, and 
 rose when the weather began to mend, meteorology would 
 be the simplest of sciences ; but, unfortunately, we often 
 see rain while the barometer is rising, or the sky begin 
 to clear while pressure is still on the decrease. These 
 exceptions have not been hitherto explained, but in this 
 work we propose to do so. 
 
 Suppose that, as in Fig. 29, a cyclone, with centre at 
 C, was moving along the crossed lane marked " path," at 
 such a rate that it would traverse a distance equal to 
 A B in four hours. .Then noting that the isobars at A 
 and B are Ol inch of pressure apart ; that the line A B 
 is parallel to the path of the cyclone ; and that B is on 
 the line of the trough, where the barometer naturally 
 begins to rise if the cyclone moved onwards without 
 any change in the depth of the centre, which is 29'0 in., 
 the barometer at station A would fall 0*10 in. in these four 
 hours, and then commence to mount. 
 
 But now, suppose that, while the centre moved 
 onwards, the cyclone began to fill up at the rate of O'lb* 
 in. in the four hours (and this is quite within practical 
 limits), then the barometer at A would rise on balance, 
 0*06 in. in that time. In fact, it niight be supposed to 
 fall (HO in. from the approach of the cyclone's trough 
 but to rise 0'16 in. from filling up, so that a gain of 
 
166 WEATHER. 
 
 0*0(5 in. would remain. Thus we explain the apparent 
 anomaly of the barometer rising while a cyclone ap- 
 proaches ; and here we see the enormous gain to know- 
 ledge which synoptic charts have effected. Formerly 
 these barometric anomalies were inexplicable ; now we 
 can interpret them readily, for we know that rain and 
 wind depend on the shape, and not the level, of the isobars. 
 So that, though the cyclone is filling up and the barometer 
 rising, the wind and weather at station A remain charac- 
 teristic of the front of a cyclone. We shall refer to this 
 subject aoain, and give a striking example in our chapter 
 on Forecasting for Solitary Observers. 
 
 SURGE. 
 
 Any change of barometric level which is not due to 
 the passage of some sort of depression or diurnal varia- 
 tion is called a " surge " of pressure. The word " wave " 
 has often been applied to barometric changes, but in such 
 an uncertain way that it seems best to coin a new word 
 for a very definite and important phenomenon. 
 
 We have just explained the idea of a moving cyclone 
 filling up, and of the resulting balance of a gain of pressure. 
 It would have been just as easy for the cyclone to grow 
 deeper in the same time, when we should have had the 
 barometer falling in rear of the cyclone, with clearing 
 weather. Sometimes filling up of a cyclone is tolerably 
 local ; other times surging is on an enormous scale. 
 Nothing is more commou in winter than to find a 
 moderate-sized cyclone in mid-Atlantic one day, and 
 that, though by next morning the shape of the isobars has 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 167 
 
 hardly changed, the whole level of the cyclone and sur- 
 roundings has perhaps decreased half an inch. 
 
 This hardly shows at first on a synoptic chart, for you 
 see no change in the configuration of the lines ; but, on 
 looking at the figures attached to the isobars which 
 denote the level, you see that what was 29*5 in. the first 
 day is only 29*0 in. on the following morning. In like 
 manner, a persistent anticyclone will often rise and fall one 
 or two-tenths of an inch without any motion or material 
 change of shape on the chart, while the barometer at any 
 station will have appeared to rise or fall without any 
 reason or apparent change of weather. 
 
 When we look at a series of these surges we find a 
 decided tendency of the motion to travel from west to 
 east, or from south-west to north-east. For instance, 
 suppose that one day there was a deep depression with 
 one or more cyclones in the United States, an anti- 
 cyclone in mid- Atlantic, and a shallow set of depressions 
 over Europe. We might find by next morning that the 
 American cyclones were filling up, but that the Atlantic 
 high pressure was lower in level, but unchanged in posi- 
 tion, while the European system was practically un- 
 altered. The third day might see that, with little change 
 in America, the Atlantic anticyclone had regained its 
 former level, while a great decrease of pressure had 
 occurred over the whole of Europe. 
 
 This is the "barometric wave" of Birt and other 
 writers, to which little importance is now attached, but 
 which, the author believes, contains the germ of great 
 development. The insuperable difficulty of tracing waves 
 at present arises from the impossibility in most cases of 
 
168 WEATHER. 
 
 separating the total barometer-change into its two com- 
 ponents of surge and cyclone. Nothing is easier than to 
 record the hour at which any barometer has touched its 
 lowest point, but we cannot tell how much is due to the 
 depression of a cyclone, or to the depression of a surge. 
 It is manifest, from our last diagram, that we only observe 
 when the balance is lowest. 
 
 A surge of itself has no characteristic weather, but the 
 passage of a surge exercises a moderate influence on the 
 characteristic weather of any isobaric shape, and a very 
 powerful one on the formation of new systems. 
 
 Let us define the front of a surge as all the part where 
 pressure is decreasing ; the rear as all the part where 
 pressure is increasing ; the trough as the line of change 
 from fall to rise ; and the crest as the line of change from 
 rise to fall. Then we find that the front of a moving 
 surge, or the mere deepening of a cyclone, does not alter 
 the typical character of the front and rear of the cyclone, 
 but increases the general intensity ; while the rising part 
 of a surge decreases the intensity, and so improves the 
 weather. The lowering of an anticyclone decreases the 
 dryness and increases the tendency to form cloud, while 
 gain of pressure has the opposite effect. But the most 
 striking and by far the most important effect of surge is 
 the influence on the development of new systems of dis- 
 turbance. The tendency of all reduction of barometric 
 level all over the world is to induce cyclonic systems, 
 while that of gain of pressure is to dissipate existing 
 cyclones. 
 
 We shall find abundant examples of this great prin- 
 ciple in our illustrations of types of temperate weather. 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 169 
 
 There, as we have just mentioned, surge and cyclone are 
 so mixed up together that we can only partially dis- 
 entangle them ; but in the tropics we find the same law 
 under simpler conditions. For instance, in the South 
 Indian Ocean, during the period of the north-west monsoon 
 from about December to March there is a long furrow 
 of low pressure about 10 south latitude, where the north- 
 west monsoon meets the south-east trade. During the 
 whole of that season this general depression goes through 
 a series of small surges, gradually lowering, perhaps one- 
 tenth of an inch for six or seven days, and then rising 
 about the same amount in another week. Now, as a 
 matter of observation, hurricanes almost invariably form 
 during the downward period of the surge, and practical 
 forecasters, like Meldrum at the Mauritius, are always 
 specially on the look-out for signs of serious bad weather 
 whenever there is the slightest symptom of a non-diurnal 
 diminution of pressure. We believe that the same prin- 
 ciple of watching surges might be used for forecasting 
 with great advantage in temperate rf gions, in spite of the 
 difficulties in the way of practical application, to which 
 we have already alluded. 
 
 It is absolutely necessary, in dealing with such com- 
 plicated problems as occur in meteorology, to have short 
 simple terms to denote certain sets of phenomena. Here, 
 and throughout this work, we shall talk of all changes of 
 pressure due to the passage of an unchanging area of low 
 pressure as cyclonic-changes, and all due to surging or 
 re-arrangement of existing systems as surging changes ; 
 and we shall talk of surge overriding the cyclone, or 
 cyclone overriding the surge, when, in our chapter on 
 
170 WEATHER. 
 
 Forecasting for Solitary Observers, we have to explain 
 more fully many apparent anomalies in the behaviour of 
 the barometer. 
 
 INTERPRETATION ON METEOGRAMS. 
 
 We must now examine still further the relation of 
 charts to ineteograms, and explain their respective values 
 and interpretations. 
 
 Synoptic charts in practice can rarely be constructed 
 more than three times a day, and it is obvious that, 
 though general changes, such as the formation or motion 
 of cyclones, can be shown on them with the greatest clear- 
 ness, the nature of diurnal variations could not be properly 
 discovered by their means. We shall illustrate this more 
 fully in our chapter on Diurnal Weather, but here we 
 must consider how to collate .the variations due to the 
 time of day with the general changes. 
 
 Almost all over the world the velocity of wind in- 
 creases with the day and falls during the night, as we saw 
 in our meteogram, Fig, 25, and this occurs both in 
 cyclones and anticyclones. How cnn we collate this with 
 the fact that, from charts constructed at the same hour on 
 different days, the velocity is proportional to the different 
 isobaric gradients ? ' The answer obviously is, that if we 
 suppose the gradient to remain unchanged for twenty-four 
 hours, the mean velocity of the wind may be considered 
 the speed due to that gradient, and that a diurnal varia- 
 tion of velocity for gradient is superimposed on this. For 
 instance, suppose the wind to vary diurnally between ten 
 and twenty miles per hour during any day, so that the 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 171 
 
 mean velocity was fifteen miles, and the variation due t 
 diurnal influences ten miles an hour ; also that the 
 gradient remained unchanged : then a synoptic chart 
 constructed at the hour of minimum wind about 4 a.m. 
 would give a velocity of ten miles an hour for the 
 given gradient, while another constructed at the hour of 
 maximum wind say at 2 p.m. would give a velocity of 
 twenty miles an hour for the same gradient, and so on fur 
 every other hour. 
 
 The diurnal variation of direction introduces some 
 other considerations. In tbe temperate regions of the 
 northern hemisphere, the wind usually veers a little 
 during the day and backs again at night, from whatever 
 direction it may come. If, then, we consider the angle 
 between the wind and the isobar, the above means that 
 the angle is less by day than during the night. When 
 we stand with our backs to the wind, we are generally at 
 an angle of about 35 to the left of the isobar ; so that 
 if the wind veers, say, from south to south-south-west 
 during the day, while the lie of the isobar remains the 
 dame, the angle between the wind and isobar would bd 
 diminished. 
 
 It has already been noticed that the wind in a cyclone 
 is always incurved, while in an anticyclone it is out" 
 curved. We therefore infer, from the fact of the mean, 
 diurnal veering of wind, that in cyclones the wind is a 
 little less incurved, and in anticyclones a little less out- 
 curved, by day than by night. 
 
 The following will illustrate the above principles. 
 In Figs. 30, 31, and 32 we give a reduction of the 
 United States daily charts for January 20, 1873, at 
 
172 
 
 WEATHER. 
 
 11 pm., together with that at 4.35 p.m. and 11 p.m. on 
 January 21. 
 
 These charts may be taken as representing a freely 
 moving cyclone, the intensity of which, as measured by 
 the gradients, is pretty constant; bat when we look at 
 the wind-arrows it will be seen that, while in the two 
 1 1 p.m. charts there is only one station in the first where 
 
 FIG. 30. Dinrnal variation of wind in a cyclone. 
 
 the wind exceeds twenty miles an hour, and none in the 
 second, the 435 p.m. chart not only has three stations 
 with that velocity, and one over thirty miles, but contains 
 a far larger number of arrows indicating more than ten 
 miles an hour, as shown by the feathers on the arrows. 
 The original records show that the total miles of wind at 
 all the seventy-five reporting stations in the first chart is 
 449 miles, with eight calms; i.i the second, 681 miles, 
 
BAROGRAMS, THERMOQRAMS, METEOGRAMS. 173 
 
 wilh only five calm stations; while in the third chart, the 
 wind has fallen to 420 miles, and the calms have increased 
 to twelve, though the gradients remain pretty constant. 
 
 Next as regards the diurnal variation in the wind's 
 direction. Though not very obvious, still, on the whole 
 the arrows in the 4.35 p.m. chart will be found rather 
 less incurved than in either of those at 11 p.m. relative 
 
 FIG. 31. Diurnal variation of wind in a cyclone. 
 
 to the cyclone-centre ; so that at every station the wind, 
 from whatever direction it may blow, appears to veer a 
 little with the sun during the day. and to back towards 
 night, unless overridden by the greater changes due to 
 the cyclone's motion. Similarly, if the charts had been 
 constructed at the same hours for an anticyclone, the 
 wind would have been found a little less outcurved at 
 4.35 p.m., and at every station the wind would also have 
 
174 WEA.THER. 
 
 veered a little during the day, and then backed towards 
 evening. 
 
 The relation of the weather in isobars to the diurnal 
 variation in the frequency of rain and cloud at different 
 hours is rather more complicated. Suppose that all rain 
 was cyclonic, arid that the curve of mean diurnal fre- 
 quency of rain showed a maximum at 2 p.m., and a 
 
 FIG. 32. Diurnal variation of wind in a cyclone. 
 
 minimum at 4 a.m., what difference should we expect to 
 see in a synoptic chart for any particular cyclone if it 
 was constructed for those two hours ? The inference 
 undoubtedly is that the general position of rain and 
 cloud relative to the lowest isobars would be unchanged, 
 but that the rain and cloud would extend further from 
 the centre at 2 p.m. than at 4 a.m. Thus, if we could 
 conceive a stationary and unchanging cyclone, it would 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 
 
 175 
 
 rain the whole twenty-four hours to an observer inside 
 the diminished rain-area which the 4 a.m. chart would 
 show, while it would begin to rain in the morning and 
 cease towards evening to an observer situate anywhere 
 between this and the outside of the 2 p.m. extension of 
 the rain-area. As an illustration, we give in Figs. 33, 
 34, and 35, in a diagrammatic form, the position of cloud 
 
 FIG. 33. Diurnal variation of rain and cloud in a cyclone. 
 
 and rain in a typical cyclone of pretty constant shape 
 and gradients in the United States on January 20, 1873, 
 at 11 p.m., and on the 21st at 4.35 p.m., being the same 
 cyclone whose winds have already been discussed. 
 
 From these it will be readily seen that the area of 
 rain and cloud round the centre in the first 11 p.m. chart 
 is considerably increased in the 4.35 p.m. chart, and again 
 diminished in size in the second 11 p.m. chart. The 
 
176 
 
 WEATHER. 
 
 portion of the outside bounding line of the cloud-area 
 which is dotted shows where observation gave the end of 
 cloud and appearance of blue sky. The portion where 
 the single shading ends without a dotted line merely 
 shows where observations ceased, and that the cloud 
 extended to some unknown distance beyond these limits. 
 So far for the interpretation of the relations of the 
 
 FIG, 34. Diurnal variation of rain and cloud in a cyclone. 
 
 diurnal variations which we find in meteograms to the 
 facts concerning the nature of weather which we derive 
 from synoptic charts; but we must now consider some of 
 the more minute phenomena of cyclones, etc., which can 
 only be learnt from meteograms and from verbal descrip- 
 tions of the sequence of weather on each day. 
 
 We have already explained the broad features of the 
 sequence of blue sky, clouds, rain, back to blue sky again, 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 177 
 
 in a cyclone ; but it is obvious that, as the stations from 
 which the materials for making synoptic charts are 
 derived are rarely less than from eighty to a hundred miles 
 apart, many of the details of a cyclone are lost. For 
 instance, in practice, we rarely get more than one or two 
 stations to report halo at the same time, and from that 
 we could never deduce the shape of the halo-forming 
 
 FIG. 35. Diurnal variation of rain and cloud in a cyclone. 
 
 portion of a cyclone. But when we observe in a great 
 number of cases that halo sky rarely lasts long, and is 
 exclusively formed in front of the regular cloud-area, then 
 we conclude that the ring of halo sky, such as we have 
 marked in our diagram of cyclone-prognostics, is very 
 narrow. The reason is, that as the cloud-area is propa- 
 gated at the same rate as the cyclone, which we may 
 take at twenty miles an hour, and that as a halo usually 
 
 N 
 
178 WEATHEE. 
 
 lasts, say, only half an hour, therefore the width of the 
 halo-ring will not be much more than ten miles. Of 
 course, if charts could be constructed for every hour of 
 the day, and at stations only five or ten miles apart, there 
 is nothing we learn from meteograms which we could 
 not also derive from charts ; but, as such observations are 
 impracticable, it is of the utmost importance to know 
 precisely how the continuous trace of instruments at any 
 one station can be collated with the intermittent observa- 
 tions at widely scattered localities. 
 
 The most striking example of the value of meteograms 
 in building up the nature of a cyclone is found in the 
 phenomena of the trough. These are confined to a line 
 only a mile or two wide, and it would be utterly impos- 
 sible from charts alone ever to learn the significance of 
 the turn of the barometer. We might look at fifty charts 
 of different cyclones, and it might happen that the trough 
 was not actually passing over any observing-station in 
 any one of them. 
 
 But if, in a large number of cyclones, it is found that 
 whenever the barometer turns to rise there is a squall, 
 this, being independent of the time of day, must be 
 referred to that part of the cyclone for its origin ; and 
 since this phenomenon occurs at all places over which the 
 cyclone-trough passes, however distant from its centre, if 
 a synoptic chart could be made with a large number of 
 stations close together, a line of squalls would be seen 
 under the trough of the cyclone, marking all the points 
 at which the barometer turned to rise simultaneously. 
 
 This inference may be derived either by taking the 
 history of the passage of a single cyclone, and observing 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 179 
 
 that a squall was associated with the trough at every 
 station, or else by observing at any one station that in 
 every cyclone which passed, the trough and a squall came 
 together. The latter deduction hangs on the assumption 
 that in a great number of cyclones no two need be sup- 
 posed to pass at the same distance from the station ; so 
 that, to a certain extent, a large number of different 
 sections of a single cyclone, and a large number of single 
 sections of different cyclones, give the same result. 
 
 The method of the meteorologist is, in fact, analogous 
 to that of the microscopist, who builds up his picture of 
 the organs of an animal by taking a series of their 
 sections, across any portion of it. 
 
 There are many other deductions which can be made 
 as to how the flexure of barogranas indicate the nature of 
 the gradients that are being propagated over any place ; 
 and as to how squalls and thunderstorms, and even single 
 gusts, each leave their characteristic mark on a baro- 
 graphic trace, which can be read off any time afterwards 
 by a practised observer. We have already explained 
 how some of the fluctuations of a thermograph tell 
 their own story about cold showers, or passing clouds, and 
 many other deductions can be made from these traces. 
 We could also point out how wind- traces reflect each 
 fitful gust in their own appropriate manner ; and also how 
 minute details of the relation of wind-direction to cyclone 
 and anticyclone centres, as well as minor diurnal varia- 
 tions both of force and direction, can be deduced more 
 accurately from anemograms than from charts. The 
 consideration of these is, however, unsuitable for an 
 elementary work, and the object of this chapter so far 
 
180 WEATHER. 
 
 will have been attained if we have conveyed to the reader 
 a clear idea that observations at any one station give a 
 section of the weather-changes which are shown in plan 
 on successive synoptic charts ; and that each self-recording 
 instrument writes in its own language, and, as it were, 
 in its own alphabet, the history of the weather for every 
 day. 
 
 DESCRIPTIVE, OR NON-INSTRUMENTAL, RECORDS. 
 
 So far we have discussed the significance of instru- 
 mental records ; but, however skilfully we may read those 
 written traces, it is evident that there is still a great 
 deal of weather about which they tell us nothing. No 
 mechanical registration of pressure, temperature, or wind 
 can ever make up for the want of a good verbal descrip- 
 tion of weather-sequence. No instrument can picture to 
 us the various ways in which a blue sky can become over- 
 cast ; whether the blue grows gradually pale and sickly, 
 or whether great snaky-looking clouds seem irresistibly 
 to embrace the whole heavens. Neither can it describe 
 the delicate distinctions which our senses enable us to 
 perceive in the way the wind blows. Our eyes tell us at 
 a glance that a south-west wind raises a long sea, while 
 a nor'-wester rakes the surface of the ocean into lines of 
 foam ; and that the fitful gusts of an impending shower 
 drive little eddies along the dusty road. 
 
 In like manner, no short cloud-symbols, such as 
 detached cloud, overcast, misty, or even the more detailed 
 words cirrus, cumulus, etc., can ever give more than a 
 lifeless picture of the sky as we know it. 
 
BAROGRAMS, THERMOGRAMS, METEOGRAMS. 181 
 
 The old myth-makers excelled in their descriptions 
 of weather. In their own peculiar figurative language 
 we see reflected a vivid picture of cloud and thunderstorm 
 which we can scarcely match in the more sober verbiage 
 of modern times. The Greek poets knew the difference 
 between the beneficent diurnal winds which sprang up at 
 dawn and the dangerous blasts of an approaching 
 thunderstorm ; and never mistook the wind which sighed 
 among the pinetops for the north-westerly squalls which 
 tumbled the trees over the cliffs. 
 
 All that instrumental traces could tell of this would 
 be deduced from seeing if the velocity-trace had some 
 connection with the time of day, or if it was fitful, and 
 that the direction-trace was also unsteady; or whether 
 some directions, such as the north-west, were associated 
 with higher velocities than others. 
 
 On the other hand, instruments not only give precision 
 to the general impressions derived from the senses, which 
 alone a savage can receive ; but also enable us to discover 
 some changes which our perceptions alone could never 
 detect. For instance, by measuring heat-curves we can 
 calculate the ordinary amount of daily range, and com- 
 pare the value in London with that in Berlin, or New 
 York ; and we can also draw deductions from certain 
 bends in the temperature-curve which would never have 
 entered into the head of semi- civilized man. In like 
 manner, there is in England a small increase of the wind- 
 velocity about 1 a.m. which has some scientific interest, 
 but which certainly would not have been discovered 
 without instrumental appliances. 
 
 But, in addition, the invention of the barometer has 
 
182 WEATHER. 
 
 given us another sense that is to say,, the appreciation 
 of the varying weight of the atmosphere, which was 
 denied to our ancestors ; and this book is the answer to 
 the question how much weather-knowledge can be derived 
 from observation of that instrument. 
 
 It will be found a distinguishing feature of this work 
 that we have endeavoured to describe the weather in 
 different shapes of isobars, so far as possible, in the 
 language of popular prognostics. This language, while 
 it contains many survivals of mythic speech, is still in 
 current use, and gives a much more accurate picture of 
 weather than more formal language. It is far more life- 
 like to talk of a cyclone-front as dirty and muggy than 
 to report sky overcast, humidity ninety-eight per cent. ; 
 or to say that the sun " draws water " in straight isobars 
 rather than c. 9 stratus (sky nine-tenths overcast, stratus- 
 cloud). We use, in fact, the phraseology of popular 
 weather-lore to translate, as it were, the indications of 
 instrumental readings into the language of common life. 
 
 At the same time, we have already examined most 
 carefully the minuter fluctuations of some instrumental 
 traces, and in various chapters we shall investigate the 
 precise significance of the results of various arithmetical 
 calculations which can be made from the numerical values 
 derived from thermograms, etc. 
 
 The problems which the meteorologist has to solve are 
 so complex and varied that he cannot afford to dispense 
 with any possible assistance from whatever quarter; and 
 our endeavour has been to convey to the reader the results 
 of every line of investigation, and to collate the old and 
 new meteorology into one compact science. 
 
( 183 ) 
 
 CHAPTER VI. 
 
 WIND AND CALM. 
 
 IN the preceding chapters we have ouly stated that in 
 most cases the force or velocity of the wind is roughly 
 proportional to the closeness of the isobars ; but we shall 
 now go into the details of the subject, and give the actual 
 numbers which connect wind and gradients. We shall 
 then point out various sources of variation which prevent 
 us from laying down any law of wind with mathematical 
 accuracy, and carry out the same idea with reference to 
 the relation of the angle between the direction of the 
 wind and the lie of the isobars. After that we shall 
 extend these and other general principles of wind to the 
 southern hemisphere, and conclude with a few general 
 reflections on the subject. 
 
 GRADIENTS. 
 
 The relative closeness of any two isobars is not 
 measured by the number of miles between them, but by 
 the steepness of the barometric slope which they indi- 
 cate. For instance, suppose that two isobars differ by 
 0'2 in. (5 mm.) of barometric level say 29'7 and 29'9 in. 
 
18-fc WEATHER. 
 
 (755 and 760 mm.) we do not measure their relative 
 proximity by saying that they are thirty or ninety 
 miles apart, but we think of the barometric slope with 
 a rise of two-tenths of an inch (5 mm.) in either thirty 
 or ninety miles. Then, to reduce this to a common 
 standard, we take a uniform distance in England fifteen 
 nautical miles, or seventeen statute miles and calculate 
 how many hundredths of an inch the barometer would 
 rise in those fifteen miles ; that is to say, we treat the 
 barometric slope like the slope of a hill, which is uni- 
 versally estimated by saying that the latter rises so many 
 feet in a mile. 
 
 The slope between two isobars is called the barometric 
 gradient, and, of course, it is measured square or at right 
 angles to the isobars, just in the same way that we 
 measure the slope of a hill between two contour lines. 
 
 For instance, suppose that in Fig. 36 the line A B, 
 drawn square to the isobars, is thirty nautical miles long, 
 and that the isobars denote differences of two-tenths or 
 twenty-hundredths of an inch ; then the rise in filteen 
 nautical miles would be ten-hundredths of an inch ; and 
 we should say that there was a gradient of ten between 
 the two stations A and B. If the distance between the same 
 two isobars at c and D was ninety miles, the gradient over 
 an observer at E would only be 0*2 X 100 X = 3'3 ; 
 and this last number Mould be the required gradient. 
 
 In practice we too often come across the error of taking 
 the difference of pressure at two places, F and G, and 
 calculating the gradient from the distance in miles between 
 them. This always gives a smaller gradient than the 
 real one, for the line of a gradient is always the shortest 
 
WIND AND CALM. 
 
 185 
 
 line which can be drawn between two isobars. In this 
 case G H, not G F, is the proper line to measure for the 
 gradient at G. The best method for measuring gradients 
 on a synoptic chart is to take the station E, whose gradient 
 
 .29.7 in. 
 
 755 
 
 FIG. 36. Barometric gradients. 
 
 is required, and then to draw the line c D through E on 
 the map, square to the isobars, and measure its length on 
 the scale of the chart. This is much quicker and more 
 accurate than trying to find two places which are nearly 
 square to the isobars. Gradients as thus measured are 
 rarely higher than four or five in Great Britain, though 
 much higher values than these are occasionally recorded 
 in very severe storms. In a general way, it may be said 
 that gradients are slight or moderate when below one, 
 and steep when above two. In Europe gradients are 
 measured by the number of millimetres of barometric 
 difference in one degree of a great circle of the earth ; 
 that is, in sixty geographical miles. For any slope the 
 numbers are nearly identical with those in England, for 
 one millimetre very nearly equals 0*04 in., and fifteen 
 nautical miles are exactly equal to one degree. 
 
186 
 
 WEATHER. 
 
 RELATION OF VELOCITY TO GRADIENT. 
 
 But it may be interesting to see what the velocity of 
 the wind actually is for any given gradient. 
 
 The following are the numbers obtained by Messrs. 
 Whipple and Baker at Kew, near London, both in English 
 and metrical equivalents : 
 
 . 
 Gradients 
 per fifteen 
 nautical miles. 
 
 Wind-velocity 
 in miles 
 per hour. 
 
 Gradient in 
 millimetres per 
 degree of latitude. 
 
 Velocity 
 in metres 
 per second. 
 
 0-2 
 
 50 
 
 203 
 
 2-23 
 
 0-5 
 
 7-0 
 
 508 
 
 3-13 
 
 0-7 
 
 7-5 
 
 711 
 
 3-35 
 
 1-0 
 
 9-2 
 
 1016 
 
 4-11 
 
 1-2 
 
 11-6 
 
 1-219 
 
 5-19 
 
 T5 
 
 12-6 
 
 1-524 
 
 5-63 
 
 1'7 
 
 15-0 
 
 1-727 
 
 6-70 
 
 2-0 
 
 16-5 
 
 2-032 
 
 7'38 
 
 2-2 
 
 19-1 
 
 2-235 
 
 8-54 
 
 2-5 
 
 22-0 
 
 2-540 
 
 983 
 
 27 
 
 22*2 
 
 2-743 
 
 9-92 
 
 3-0 
 
 25-2 
 
 3-048 
 
 11-40 
 
 Loomis has arrived at the following values in the 
 United States : 
 
 Gradient 
 
 Velocity 
 
 Gradient 
 
 Velocity 
 
 millimetres 
 
 in metres 
 
 millimetres 
 
 in metres 
 
 per degree. 
 
 per second. per degree. 
 
 per second. 
 
 !i 
 
 
 2-09 
 
 7*20 
 
 2-80 
 
 10-64 
 
 2-31 
 
 8-18 
 
 2-90 
 
 11-09 
 
 2-48 
 
 9-03 
 
 8-08 
 
 11-44 
 
 2-61 
 
 9-66 
 
 3-36 
 
 11-80 
 
 2-72 
 
 10-28 
 
 3-73 
 
 12-20 
 
WIND AND CALM. 187 
 
 These agree very fairly well with the British observa- 
 tions. 
 
 In the Atlantic,, Professor Loomis finds that for the 
 same gradient the velocity of the wind is forty per cent, 
 greater than in the^United States. This is doubtless due 
 to the influence of a certain number of sheltered stations 
 among the land-observatories. Nearly every place feels 
 some winds more than others,, and will therefore some- 
 times report comparatively little wind for a considerable 
 gradient. Thus, when the results are averaged, the mean 
 values will be lower than if every observer was equally 
 exposed to all winds, as in the open sea. 
 
 Wind is much stronger for the same gradients in the 
 tropics than in higher latitudes. In the Indian Ocean, 
 especially, the north-east and north-west monsoons- blow 
 steady with a gradient that would give variable winds in 
 temperate regions ; and the violence of the south-west 
 monsoon is out of all keeping with the steepness of 
 gradient according to European experience.. 
 
 VARIATIONS IN TELOCITY AND GRADIENT;. 
 
 There are various sources of variation from these 
 general laws of the relation of velocity to gradient, some 
 of which only can be explained in an elementary work 
 like the present. In Great Britain it is found that, for 
 any given (moderate) gradient, winds from north and east 
 points are stronger than those from south and west 
 points. For instance, Ley has found at Kew the follow- 
 ing differences: 
 
188 
 
 WEATHER. 
 
 Gradient per 
 fifteen nautical 
 miles. 
 
 Velocity in miles 
 for winds from 
 S.S.E. by S. 
 to N.W. 
 
 Velocity in miles 
 for winds from 
 
 N.N.W. by N. 
 to S.E. 
 
 006 
 
 4-14 
 
 6-89 
 
 009 
 
 6-41 
 
 8-63 
 
 012 
 
 8-37 
 
 10-93 
 
 015 
 
 11-21 
 
 14-27 
 
 016 
 
 13-56 
 
 16-98 
 
 reason -why the mean of these does not agree with 
 the mean velocity for the same gradients as deduced by 
 Mr. Whipple for the same station is readily explained. 
 The latter takes all winds from all directions for the 
 same gradient, and averages them up together. Mr. Ley 
 separates the two principal directions ; but, as one direc- 
 tion the south-west is much more frequent than the 
 north-west, his numbers would have to be weighted pro- 
 portional to the frequency of these two directions to give 
 the same numbers as Mr. Whipple. 
 
 Local variation of wind is too obvious to need much 
 comment. The only thing we have to consider here is 
 how it affects forecasting. If every station was equally 
 exposed to every quarter, it might be possible to issue 
 forecasts in which the amount of wind recorded by any 
 instrument might be approximately indicated ; but when 
 we have to deal with a gale which begins in the south- 
 east and works round to north-west, it is manifest that we 
 can only state the probable amount of wind in general 
 terms, as no place is equally influenced by wind from 
 these two quarters. 
 
 The relation of diurnal variation of wind- velocity to 
 
WIND AND CALM. 189 
 
 gradient has already been discussed in our chapter on 
 Meteograms ; and other diurnal phases of wind, such as 
 land and sea breezes or valley winds, will be most con- 
 veniently taken in our chapter on Diurnal Variations of 
 Weather. But, in temperate regions, by far the most 
 important elements of disturbance in the simple relation 
 of wind to gradient are squalls and thunderstorms. In 
 both of these the barometer usually rises suddenly, some- 
 times as much as one-tenth of an inch, from causes which 
 are at present obscure. And in both we find angry, 
 violent gusts, which bear no relation whatever to isobaric 
 gradients. Many of the discordant observations on this 
 subject are doubtless due to want of care in distinguishing 
 one kind of wind from another. 
 
 Loomis has called attention to the total want of 
 accordance between wind and gradient which he has 
 found during the " northers " of New Mexico, and the 
 author has found that in the " nortes " of Panama the 
 wind also is quite disproportionate to the gradient. 
 Finley has also discovered in the United States what are 
 called " straight-line gales," or long streaks of wind, two 
 or three miles across, blowing at the rate of sixty to eighty 
 miles an hour, and extending over eighty or one hundred 
 miles in length. These appear on the side of a cyclonic 
 depression, at some distance from the centre, and are not 
 associated with any deflection of the isobars. Other 
 winds that are not directly associated with isobaric 
 gradients have been noticed in other parts of the world, 
 and we are therefore led to the conclusion that, though 
 the great wind-circulation of the atmosphere is related to 
 isobars, still there are some winds that are impelled by 
 
100 WEATHER. 
 
 other causes than those which develop isobars ; and for 
 the sake of classification we will call them generically 
 " non-isobaric winds." They are most probably connected 
 with what we have before alluded to as non-isobaric 
 rains, 
 
 We cannot say what is the origin of the wind in 
 thunderstorms and non-isobarie winds, but it is certain 
 that the cause is quite different from that in cyclones. 
 We must therefore take care, in talking about wind, not 
 to mix up two kinds which really have little in common. 
 From all this we see the very fallacious results which 
 come of trusting blindly to instruments, and also that 
 any statistical values which are derived from mixing up 
 various sorts of wind can only give rise to discordant 
 deductions. 
 
 We may also remark that merely saying that a storrn 
 blew with such a force or velocity tells us very little 
 either of the true character of the wind or of the amount 
 of destruction which the gale might cause. An instru- 
 mental record of forty miles of wind in an hour may be 
 made up either of a steady weight of wind, or of violent 
 gusts alternating with quieter intervals. The damage 
 clone in the latter case would many times be greater than 
 in the former. 
 
 Then there are many minute differences in the way of 
 blowing which instruments cannot even detect. We all 
 know that most chimneys smoke more with an east wind 
 than with a west one. We have also just shown that 
 the velocities of these two winds is not the same for the 
 same gradients. 
 
 It has -been suggested, with a great deal of probability, 
 
WIND AND CALM. 191 
 
 that the difference may be due to the wind not blowing 
 horizontally, and that east winds are perhaps directed 
 a little downwards. Another very striking phase of wind 
 is the difference between the kind of sea raised by -the 
 south-west gale in front of a cyclone and the north-wester 
 in rear. The first raises a high sea with only a moderate 
 amount of white water, while the latter rakes the surface 
 of the ocean into long streaks of foam. There are other 
 reasons for believing that in front of a cyclone the wind 
 is rising, while in rear the air-currents have a slant down- 
 wards. If so, the cold, dry clearness of north-westers is 
 readily explained. The whirls of dust that precede 
 some kinds of rain are also familiar instances of the 
 specific character which belongs to different winds. 
 
 RELATION OF DIRECTION TO GRADIENT. 
 
 We will now consider the details of the relation of the 
 direction of the wind to gradients and to the lie or trend 
 of the isobar conjointly. 
 
 When we talk of gradient only, we get no indication 
 of the direction of the wind, for the barometric slope may 
 face in any direction or have any aspect. Following the 
 analogy of barometric gradients to hill-slopes, we will 
 call the direction towards which gradients slope the aspect 
 of the gradient, so as to keep the word direction for wind. 
 For instance, if isobars run east and west they may slope 
 either north or south, or we might say that the aspect of 
 the gradients was either towards the north or the south, 
 just as we should talk of a hill ; or, to take- the analogy 
 of geological terms, we might say that the strike of the 
 
192 WEATHER. 
 
 isobars was east and west, but that the gradients dipped 
 either north or south. 
 
 But by combining the idea of gradient with that of 
 aspect, and both with the Buy Ballot's law, we see at 
 once that if the isobars run or strike east and west, the 
 general direction of the wind will be westerly if the aspect 
 of the gradients is towards the north, but easterly if the 
 aspect is to the south. We therefore say that in the 
 former case we have gradients of such a value for westerly 
 winds, and in the latter gradients of such an amount for 
 easterly winds. This holds for every direction. In 
 Fig. 36 we have, as before explained, a gradient of ten 
 between A and B for velocity, and now we can say that 
 the gradient is also for north-westerly winds ; at E there 
 is a gradient of 3'3 for south-westerly winds. By this 
 simple method of expression, whenever we see a synoptic 
 chart, we can calculate at once both the probable direction 
 and force of the wind. 
 
 INCLINATION OF WIND TO ISOBARS. 
 
 Buy Ballot's law does very well for the general sweep 
 of the wind, but the subject is capable of much greater 
 refinement. The acute angle between the direction of the 
 wind and the lie of the isobar is called the inclination of 
 the wind to that isobar. Taking all kinds of winds and 
 all kinds of isobars, Whipple has found that the inclina- 
 tion amounts to 52 at Kew ; while Loomis has deduced 
 an angle of 42 in the United States. 
 
 But, by taking the inclination of the wind in different 
 shapes of isobars and different portions of each shape, 
 
WIND AND CALM. 19-3 
 
 Ley, Loomis, Hildebrandson, and others, have arrived at 
 a series of remarkable generalizations as to the general 
 circulation of the atmosphere. They find that the wind 
 is much more inclined and incurved in the right front of 
 a cyclone than in any other portion ; and that in the rear 
 the inclination is very small, if not occasionally reversed 
 that is to say, a little outcurved. 
 
 We have examined the details of these cyclone 
 surface-winds, as well as of those in an anticyclone in 
 our chapter on Clouds. There we treated each shape 
 separately, but we can connect both in a very striking 
 manner if we call attention to some general values obtained 
 by Loomis from observations over the Atlantic Ocean. 
 Taking an ideal cyclone, with an adjacent anticyclone, 
 he finds that, starting from the anticyclone, the inclina- 
 tion of the wind to the isobar begins at about 52, and 
 then gradually decreases to 25 near the centre of a 
 cyclone. Of course this is a generalized case, for we have 
 shown that the inclination is not the same on different 
 sides of a cyclone. The great thing to remember is that 
 in every shape of isobars each part has a wind velocity 
 and direction of its own relative to the gradients. 
 
 The only other material source of variation is diurnal. 
 We have already sufficiently explained, in our chapter- 
 on the Meteograms, that, whatever the inclination due to 
 any part of any shape of isobars may be, the diurnal 
 variation imposes a modification on that, but does not 
 alter the direction due to general causes. Land and sea 
 breezes we shall discuss in our chapter on Diurnal 
 Variations of Weather. 
 
194 WEATHEK. 
 
 CALMS. 
 
 We have already stated that calms are the product 
 of no barometric gradient. The most persistent calms 
 are found in the " doldrums," or the col of low pressure 
 near the equator between the north-east and south-east 
 trade winds all over the world. 
 
 In temperate regions the most persistent calms are 
 near the centres of stationary anticyclones ; but more 
 short-lived calms are found in the centres of cyclones, 
 along the crest of wedges, and in cols. 
 
 We do not think it necessary to give any special 
 examples of either gales or calms, for they are abundantly 
 illustrated by numerous charts in the course of the 
 work; we need only call attention to Figs. 65 and 66 
 of south-westerly gales in Great Britain, to Figs. 77 and 
 78 of easterly gales, and to Figs. 22 and 24 of calms. . 
 
 WINDS IN THE SOUTHERN HEMISPHERE. 
 
 So far we have confined our attention to winds in 
 the northern hemisphere only ; now, however, that we 
 thoroughly understand the nature of wind in that 
 hemisphere, we can easily follow the modifications which 
 occur south of the equator. 
 
 The great general principles that every shape of 
 isobars has a distinctive wind; that cyclones incurve, 
 while anticyclones outcurve ; that the velocity is mainly 
 determined by the gradient, and also the relation of 
 diurnal to general winds are the same in both hemi- 
 
WIND AND CALM. 195 
 
 spheres. What does differ is that portion of Buy Ballot's 
 law which gives the position of the nearest low pressure 
 to an observer who turns his back to the wind. For the 
 southern hemisphere the law is as follows : stand with 
 your back to the wind, and pressure will be lower on 
 your right hand than on your left. This is exactly the 
 converse of what holds north of the equator. 
 
 As a necessary consequence of this, the surface-wind 
 will rotate round a cyclone or anticyclone in the opposite 
 manner to what it does in the northern hemisphere. 
 That is to say, a cyclone rotates in the direction of the 
 motion of the watch-hands, but is incurved; while the 
 anticyclone turns against the watch-hands, but is still 
 outcurved, as in Europe. The general circulation of the 
 upper currents is exactly analogous to that of the northern 
 hemisphere, being nearly parallel to the isobars at the 
 level of the lower cumulus, and more or less outwards at 
 the higher cirrus level in cyclones, and inwards in anti- 
 cyclones. 
 
 For this reason, the vertical succession of the upper 
 currents is contrary to that of Europe. There the upper 
 currents always come successively more and more from the 
 left as you stand with your back to the wind ; whereas 
 they will come more and more from the right in the 
 southern hemisphere. For instance, if the surface-wind 
 was south, in Europe the lowest clouds would be south- 
 south-west, the next layer south-west, and the highest 
 cirrus perhaps from west. Whereas in Australia, with 
 the same surface-wind from south, the successive upper 
 currents would come from south-south-east, south-east, 
 -and east respectively. 
 
196 WEATHER. 
 
 The diurnal variation of wind-velocity will be fully 
 discussed in our chapter on that subject. 
 
 The sequence of wind as a cyclone drifts past an 
 Australian station would be different from that of Europe. 
 Anywhere south of the line, the wind goes round by the 
 north if the centre passes south, and round by south if 
 the centre passes north, of the observer, which is exactly 
 the converse of what happens in Europe. Mr. Ringwood 
 has pointed out that we can express both cases by one 
 general law, if we say that in both hemispheres the wind 
 goes round by the polar side when the centre passes on 
 the equatorial side of the station, and by the equatorial 
 side when the centre passes on the polar side. 
 
 This can all be better illustrated by a few actual 
 examples than by generalized diagrams, the more so as 
 the figures can then be made to show some other in- 
 teresting phenomena. 
 
 In Fig. 37 we give an example of a violent cyclone 
 which blew in the Indian Ocean on February 13, 1861, 
 between 10 and 20 south latitude, and about 80 east of 
 Greenwich, as deduced by Mr. Meldrum, of the Mauritius. 
 The general nature of the rotation of the wind with the 
 direction of the watch-hands will be very obvious, but we 
 should note that the incurvature in most places is very 
 considerable, especially to the west of the centre. To the 
 south of the centre the wind was south-east ; to the west, 
 south-west ; to the north, north-west ; and from north- 
 north-west to .north-east on the eastern edge of the 
 cyclone. The four feathered arrows denote a wind of 
 hurricane force, and there is nothing in the steepness 
 of the gradients to suggest such high velocities. 
 
WIND AND CALM. 
 
 197 
 
 The path of the cyclone is marked by dated crosses, 
 and we see that the motion, as usual in these latitudes, 
 was very slow. From the 12th to the 13th the travel 
 was a very little towards the east, at a rate of hardly 
 three miles an hour ; after that, the path was irregularly 
 towards the south, at a rather higher speed. 
 
 We have introduced this example from the South 
 
 FIG. 37 Tropical hurricane (south of the equator). 
 
 Indian Ocean partly to show that in all principal 
 characteristics a tropical does not differ from an extra- 
 tropical cyclone ; but we shall understand the antithesis 
 of the wind-sequence far better from an Australian 
 example, because the conditions of weather in that 
 country are more similar to those of Europe or the 
 United States than those of the lower latitudes. 
 
198 
 
 WEATHER. 
 
 In Figs. 38 and 39 we give the wind and isobars for 
 Australia on November 20 and 21, 1884. For these we 
 are indebted to Mr. Ellery, of the Government Observatory 
 at Melbourne. We find in the first chart (Fig. 38) that 
 the highest pressure is over Queensland, and that a 
 moderate-sized cyclone covers the Australian Bight. The 
 wind rotates round this in the usual manner of the 
 
 20. 1 1. 8 j. 
 
 jo.o 
 2 9-9 
 
 2.9-9 
 
 FIG. 38. Cyclone (Australia). 
 
 hemisphere, being north and north-east in front, and south 
 and south-west in rear. Land and sea breezes deflect the 
 winds along the coast-line, and in the interior of the 
 island variable winds are reported, owing to the slight 
 gradients which are there present. By next morning 
 (Fig. 39) only the fragment of the cyclone appears to 
 the south of Tasmania ; portions of two anticyclones lie 
 
WIND AND CALM. 
 
 199 
 
 over the north-east and south-west corners of Australia ; 
 and a sort of ill-defined V-depression runs up towards the 
 col which lies between the two anticyclones. The rotation 
 of the wind round this Y is from north-east and north in 
 front, to south and south-west in rear, so that the wind- 
 sequence at Melbourne for the day was from north-east 
 
 21.11.84 
 
 jo.o 
 
 JO. I 
 
 jo.o 
 
 -9-9 
 
 29.6 
 
 FIG. 39. V-depression (Australia). 
 
 by north to north-west and south-west. In London the 
 passage of the same isobars would have been associated 
 with a shift of wind from south-east by south to south-west 
 and north-west. 
 
 . 
 
 GENERAL REMARKS. 
 
 We may conclude this chapter with a few general 
 remarks on the subject of wind. 
 
200 WEATHER. 
 
 In the first place, let us notice how little influence 
 the rate of a cyclone's motion has on the velocity of the 
 wind. All that we know for certain about the influence 
 of the motion of a cyclone is that a high rate^increases 
 the general intensity of the wind and weather every- 
 where, but that it does not prevent the centre from being 
 calm, or the wind from being light on any side where the 
 gradients are slight. 
 
 In squalls the independence of the velocity of wind to 
 that of the squall as a whole is still more curious. The 
 latter may be travelling, perhaps, only twenty miles an 
 hour, but the first blast may come at the rate of sixty 
 miles an hour. This fact we must consider as due to an 
 impulse being propagated which induces wind of such a 
 velocity, but not as due to wind, or a gust, moving solidly 
 over the earth's surface. Such an impulse is found in 
 the trough of a cyclone or V-depression. 
 
 We have not thought it necessary to give the general 
 principles of the dependence of wind-circulation on the 
 earth's rotation, as that may be found in any text-book 
 of physical science. The modification of Halley's old 
 theory of north-east and south-west winds, which has 
 been proposed by Professor Ferrel, has been universally 
 adopted all over Europe and the United States. The 
 theory is hardly known in England, and is too mathe- 
 matical for this work. No doubt the earth's rotation is 
 the real cause of the general direction of circulation in 
 cyclones of either hemisphere, but what we cannot explain 
 is the inclination of wind to the isobars. Theoretically, 
 any small difference of temperature should set up a wind 
 from the cold to the hot area ; but we have seen already, 
 
WIND AND CALM. 201 
 
 and shall see still more in our next chapter on Heat and 
 Cold, that differences of temperature even over large 
 areas have wonderfully little influence on wind. The 
 most that local differences of heat and cold do is to set 
 up local breezes, such as land and sea, or valley winds. 
 Then, theoretically, this cold wind should flow nearly 
 straight toward the hot area, only a little deflected to the 
 right or left, according to circumstances, by the earth's 
 rotation. In like manner, any difference of pressure, from 
 the high to the low barometer, however caused, should 
 draw wind nearly straight. But, in our chapter on 
 Diurnal Weather, we shall find some land and sea breezes 
 which blow nearly parallel to the coast-line. 
 
 On the other hand, if we look at a cyclone purely as 
 a circulating mass of air, the wind should be parallel to 
 the isobar, perhaps even a little outcurved from centrifugal 
 force. Now, in practice the wind is always incurved, and 
 the depression of a cyclone is certainly not caused by 
 centrifugal force. The fiercest wind which ever blew 
 would only depress the barometer a few hundredths of an 
 inch, instead of which we find depressions of two inches 
 and more with no wind over fifty miles an hour. This, 
 of course, is on the supposition that whirling air acts like 
 a fluid. 
 
 The idea has been suggested that the friction of the 
 wind on the earth's surface is the cause of the incurvature, 
 and that without friction the wind would be parallel to 
 the isobars, as we find it at the level of the lowest cloud- 
 layers. It is extremely probable that this is at least 
 partially true, for several experiments can be devised 
 with whirling water, in which friction of small particles 
 
202 WEATHER. 
 
 on the bottom does cause them to be collected in the 
 centre, instead of being thrown out to the edges of the 
 vessel. 
 
 KELATION OF FORCE TO VELOCITY. 
 
 Lastly, we may say a few words about the relation of 
 force to velocity. The velocity of wind is a real quantity, 
 which is perhaps capable of measurement in the abstract, 
 though we are at present far from being able to gauge it 
 accurately. But it is quite certain that there is no such 
 thing as an absolute force which corresponds to a given 
 velocity. According to the theory of stream-lines, when 
 even an inelastic fluid meets an obstacle, if the angles of 
 the obstruction do not break the continuity of the fluid 
 so as to form eddies or vortices, the same amount of 
 pressure which is imposed on the body by the first 
 deflection of the fluid is given back again as the stream- 
 lines of the fluid close up behind the obstruction. For 
 instance, if a ship is lying at anchor in a current, the 
 same amount of strain which the current causes on her 
 cable when forced asunder by the bows, is given back 
 when the current closes in behind her ; so that the total 
 pressure which she experiences is only that due to the 
 friction of the water on her skin. This is, of course, on 
 the supposition that her lines are so easy that they do 
 not break the stream-lines so as to form little eddies or 
 vortices. 
 
 Now, the same thing holds with wind. If we put up 
 two square plates of different sizes, face to the wind, the 
 pressure on each is not proportional to the area, while in 
 
WIND AND CALM. 203 
 
 light breezes neither will record anything. The reason 
 is that, in light wind, a thin mobile fluid like air can 
 glide round even the sharp angles of a square without 
 forming eddies, and as there is no vacuum formed behind 
 the plate, there is no pressure recorded. In higher winds, 
 the stream-lines are broken, and every shape and every 
 sized plate of the same shape form a different series of 
 eddies round the rim of the obstacle. Then the amount 
 of rarification behind the various plates is neither identical 
 nor proportional, and therefore every shape and size of 
 anemometer indicates discordantly at every different 
 velocity. 
 
 From all this it follows that, though we might say 
 that the pressure on a board one foot square was twenty 
 pounds, and might compare this force with that on 
 another board of the same size and mounting, we should 
 not be justified in saying that the force of the wind was 
 twenty pounds per square foot in the abstract, because a 
 board ten feet square, even if of the same shape, would 
 have given a different number. 
 
204 WEATHER. 
 
 CHAPTEK VII. 
 
 HEAT AND COLD. 
 
 IN this chapter we purpose to go a little more into the 
 details of the manner in which changes of temperature 
 are produced. What are the causes of burning heats and 
 hard frosts ; why is the same day of the month hot in 
 one year and cold in another ; why at the same season do 
 hot and cold days follow one another without any apparent 
 sequence ; and why is England sometimes warmer than 
 France, though the latter is nearer the equator ? 
 
 All these questions we propose to answer, and to point 
 out how easily they can be explained by means of synoptic 
 charts. The difficulties of getting rid of annual and 
 diurnal variations have tempted many meteorologists still 
 to adhere to the old method of averages, which can only 
 lead to unsatisfactory, if not to delusive, results. 
 
 DIURNAL ISOTHERMS. 
 
 The question we have to solve is this. We know that 
 the sun is the principal source of all heat, and, if nothing 
 disturbed his rays, there would be a regular diminution 
 
HEAT AND COLD. 205 
 
 of temperature from the equator to the pole, which we 
 shall call a thermal slope. Every day, as the earth 
 turned under the sun, a well-defined wave of variation 
 would be imposed on this slope. The lines which mark 
 out this deflected slope we will call diurnal isotherms. 
 We have first to determine what the shape of the lines 
 would be at any moment over the globe, and then how 
 these diurnal isotherms would modify the appearance on 
 a synoptic chart of any local developments of heat or 
 cold. If we see on a chart that at eight o'clock in the 
 morning there is a curious patch of heat in front of a 
 cyclone, how are we to discover how much is due to 
 cyclonic causes and how much to diurnal variations? 
 In fact, how can we prove that the heat is due to the 
 cyclone, and not to the time of day ? 
 
 First, to form a conception of the diurnal distribution 
 of temperature over the world at any moment. The 
 author has shown that the general shape of the isotherms 
 in any latitude would be like the lines in Fig. 40, if 
 there was a uniform thermal slope from the equator to 
 the pole, and no disturbing influences, such as unequal 
 distribution of land and sea. 
 
 The diagram gives the ideal shape of the isotherms 
 at any moment. Noon is placed in the middle of the 
 diagram in longitude 180, and the lines represent the 
 diurnal variation in the latitude of each isotherm. 
 The scale on the right of the diagram is degrees of 
 latitude ; that on the left, degrees of temperature. For an 
 ideal diagram we have supposed that the general thermal 
 slope from the equator is 1 of temperature for 1 of 
 latitude. The principle on which the diagram is formed 
 
206 
 
 WEATHER. 
 
 is as follows. Suppose that in latitude 20 the tempera- 
 ture, is 60 Fahr. at midnight, and that by 6 a.m. the 
 temperature has fallen to 59 ; then we should have to go 
 one degree of latitude further south at that hour, if we 
 want to follow the position of the isotherm of 60. If, in 
 our undisturbed world, we could walk round the earth 
 in any latitude in twenty-four hours, the line marked 60 
 
 Longitude/ 
 
 4.5 
 
 90 
 
 135' 
 
 180 
 
 225 
 
 270 
 
 315 
 
 360 
 
 \\ 
 
 \\ 
 
 6 
 
 9 
 
 N 
 
 M 
 
 H 
 
 FIG. 40. Diagram illustrating the shape of diurnal isotherms. 
 
 on the chart represents what our journey would be if we 
 wanted to keep under a uniform temperature of 60 for 
 the whole day. Starting at midnight on the left of the 
 diagram, we should have to go sixty geographical miles 
 south and 90 east of longitude by six o'clock in the 
 morning. Between then and 3 p.m. we should have to 
 make 300 miles of northing and 155 of easting, if we 
 
HEAT AND COLD. 207 
 
 still wished to keep our thermometer at 60 ; and from 
 then till the second midnight we should have to make 
 240 miles of southing and 135 of easting, to follow the 
 isotherm of 60. Observe that the easting has to be ex- 
 pressed in degrees of longitude, for the number of miles 
 in a degree varies with the latitude. The diagram is also 
 based on the supposition that there is a pretty uniform 
 isothermal slope from the equator to the pole, and that 
 the diurnal range of temperature does not vary much 
 within 5 or 10 of latitude. 
 
 Then, if there were no irregularities caused by cyclones, 
 or the unequal heating of land or water, the diurnal ther- 
 inogram in every place would be very similar in shape to 
 the trace of any of the isotherms as plotted on a chart if 
 we turn longitude into time, and latitude into degrees of 
 heat on a suitable scale. In fact, we may conceive the 
 curves shown in the diagram to sweep round the world 
 with the earth's rotation, and suppose that the rise or fall 
 of temperature at any station was caused by the passage 
 of this shape of isotherms, just as the motion of the 
 barometer is the product of the propagation of different 
 shapes of isobars over any place. For instance, in the 
 diagram (Fig. 40) the strong horizontal line shows the 
 position of the section across the diurnal isotherms which 
 is propagated over any station in latitude 20 north. 
 Starting from the first midnight on the left of the diagram, 
 the thermometer would mark 60. By 6 a.m. the mer- 
 cury would have fallen to 59, as that isotherm descends 
 to latitude 20 at that hour. Between 6 a.m. and 3 p.m. 
 five isotherms are propagated over the station, so that the 
 instrument would register 64 at the latter hour. Then, 
 
208 WEATHER. 
 
 as lower isotherms begin to pass over the observer, the 
 temperature would fall at the rate shown in the figure, 
 till 60 was reached again by the second midnight. 
 
 How DIURNAL MODIFY GENERAL ISOTHERMS. 
 
 Now, assuming this typical distribution of heat, we 
 can readily see how the diurnal range of temperature 
 modifies any isotherms which we find on a synoptic chart. 
 
 But, first, let us define the aspect of the thermal slope 
 on the map of the world as the direction in which the 
 gradients look, if we suppose the isotherms really to 
 represent relative heights. For instance, in all curves 
 the aspect of the slope in the morning after six o'clock 
 is towards the north-west, while in the afternoon it is 
 towards the north-east. 
 
 Now, suppose that at any hour we find a certain shape 
 of isotherms on a synoptic chart : these lines represent 
 the diurnal isotherms as modified by local radiation, etc. ; 
 or we may say that we have on the map temperature- 
 distribution due to radiation or cyclonic causes lying on 
 a diurnal thermal slope. Then, so long as the direction 
 or aspect of the diurnal slope does not vary, the shapes 
 of the isotherms will not alter ; only the numbers which 
 are attached to them will change. That is to say, the 
 propagation of a uniform slope alters the level, but not 
 the shape, of the isotherms. 
 
 For instance, let the square A B I a, in Fig. 41, repre- 
 sent an area of, say, 30 latitude by 30 (two hours) of 
 longitude, anywhere on the surface of the earth, and let 
 the slanting dotted lines mark a very exaggerated after- 
 
HEAT AND COLD. 
 
 209 
 
 5-0 49 48 4; 
 
 f 6 * 45" 
 
 \ \ a 
 
 \ '* \ 
 
 \ 
 \ %N ' 
 
 \ 
 
 \ \ 
 
 \ \ 
 
 \ \ \ 
 
 \ 
 
 \ \ \ 
 
 J ! i 
 
 \ 
 \ 
 
 FIG. 41. Thermal slope, and shape of 
 isotherms. 
 
 noon thermal slope, from 45 to 50 at the rate of 1J per 
 hour. 
 
 Also, suppose that the wind, etc., of a cyclone within 
 the square had very much contorted the isotherms ; the 
 resulting shape would be compounded of the cyclonic dis- 
 turbance lying on the 
 simple diurnal slope 
 shown on the diagram. 
 
 Now, if, while the 
 cyclone stood still, and 
 the diurnal thermal 
 slope was propagated 
 over the square for two 
 hours, then the only B 
 effect would be to 
 leave the shape of the 
 
 isotherms absolutely unchanged, but to make each line 
 mark 3 lower. The isotherm of a would have arrived 
 at A, & at B, e at &, and so on. That is to say, the lines 
 marked 50, 49, 48 would be in the same places, but 
 would be numbered 47, 46, 45 respectively, though the 
 shape of the contortions would be the same. 
 
 If, on the contrary, the direction of the diurnal slope 
 had changed during these two hours, from north-east to 
 north-west that is, from an afternoon to a morning 
 aspect then the shape of the contorted isotherms would 
 have been much modified. 
 
 This conception of the propagation of a kind of diurnal 
 wave, and the superposition of cyclonic or anticyclonic 
 heat-disturbance on its slopes, explains most satisfactorily 
 what the author has so often observed in the United 
 
210 
 
 WEATHER. 
 
 States tri-daily weather-maps, viz. that the shape of the 
 isotherms always appears to change more between the 
 morning and afternoon than between the afternoon and 
 night charts; and also that between the two latter, the shape 
 often remained pretty constant, though the numbering 
 had changed. For instance, in Figs. 42, 43, 44 we give 
 
 FIG. 42. Diurnal and cyclone temperature (United States). 
 
 reductions of the United States charts at 11 p.m. on the 
 22nd of January, 1873, as well as those at 4.35 p.m. and 
 11 p.m. the following day. These are to serve a twofold 
 purpose first, to show why the distribution of temperature 
 was so different on two consecutive days at the same hour, 
 
HEAT AND COLD. 
 
 211 
 
 viz. 11 p.m. ; and, secondly, to illustrate the diurnal varia- 
 tion in the shape of the isotherms between 4.35 p.m. and 
 11 p.m. the second day. 
 
 We will consider the latter first. The isotherms which 
 we see on the 4.35 p.m. chart (Fig. 43) represent the dis- 
 tribution of temperature due to the influence of a cyclone 
 
 FIG. 43. Diurnal and cyclone temperature (United States). 
 
 on a general irregular thermal slope from the equator to 
 the pole, as modified by the diurnal range of the season. 
 The aspect of the diurnal gradient is towards the north- 
 east, because the temperature is falling. 
 
 By 11 p.m. the same day (Fig. 44) the centre of the 
 
212 
 
 WEATHER. 
 
 cyclone has scarcely moved, and the general shape of the 
 isotherms is also nearly identical; but the position of 
 the isotherms of 40, 50, 60 at 4.35 p.m. is taken broadly 
 by those of 30, 40, 50 at 11 p.m., and the place of 10, 
 20, 30 at 4.35 is less nearly approached by those of 
 
 FIG. 44. Showing diurnal aud cyclonic temperature in the United 
 
 States. 
 
 0, 10, 20 at 11 p.m. in the west. This means that the 
 diurnal range was less in the north-west than in the south. 
 The interpretation of this is, that the aspect of the 
 thermal gradients has not materially changed, though 
 the temperature has fallen generally nearly 10 ; so that 
 
HEAT AND COLD. 213 
 
 the shape and position of the disturbance of temperature 
 set up by the cyclone remains the same, but the number- 
 ing of the isotherms is changed nearly 10. 
 
 TEMPERATURE-DISTURBANCE OF A CYCLONE. 
 
 Now that we have eliminated the influence of diurnal 
 range, we can better understand the nature of the heat 
 developed by a cyclone. In the same three figures we 
 have got rid of diurnal range by two methods. By taking 
 the charts at the same hour 11 p.m. in the first and 
 third (Figs. 42 and 44), diurnal range is allowed for by 
 being equalized, so that the whole of the difference 
 between these two sets of isotherms is due to general 
 changes, not to diurnal variations. 
 
 Then, by our second method of inferring the influence 
 of diurnal slope on any shape of isotherms, we are enabled 
 to use a chart at the intermediate hour of 4.45 p.m. 
 (Fig. 43) for the same purpose of discovering the nature 
 of cyclone-heat. 
 
 In all these charts we see that the general nature of 
 the development of heat by a cyclone consists of a certain 
 wedge-shaped projection of the isotherms northwards in 
 front and on the southern side of the cyclone-centre, and 
 that this heat moves on along with the cyclone. Observe 
 that the local seasonal thermal gradient, from the cold 
 interior of the continent to the warm sea, slopes to the 
 north-west, while the aspect of the diurnal thermal slope 
 is towards the north-east in all the charts. The quality 
 of cyclone heat is very peculiar. It is not the pleasant 
 warmth of a fine day, but has that characteristic close, 
 
214 WEATHER. 
 
 inuggy, disagreeable feeling which we have before 
 described as coining before cyclones. This is the kind 
 of heat which develops neuralgia and similar troubles 
 in old wounds, and many of the prognostics which are 
 associated with the front of a cyclone. We could not 
 have a more striking instance of the necessity of adding 
 a descriptive account to all instrumental records of 
 weather. Neither a thermogram nor a synoptic chart can 
 distinguish between one kind of heat and another. 
 
 The cause of this heat is obscure. The author has 
 shown * that it is not altogether caused by that backing 
 of the wind towards the south which precedes the rainy 
 portion of a depression, and that the rise of temperature 
 seems due to some peculiar property of cyclone-action. 
 
 In an ordinary whirl of dust or leaves we find the 
 particles most compressed on the side where the direc- 
 tions of rotation and translation coincide ; that is to 
 say, if the whirl is against the watch-hands, and the 
 motion in any direction, the compression is always on the 
 right-hand edge of the eddy, looking towards the front. 
 
 If we reflect that a chart of cyclone-heat shows a 
 wedge projection of the isotherms on a general thermal 
 slope, we can readily understand how such a form may be 
 analyzed into a detached patch of heat lying on a general 
 thermal slope. We are thus led to the conception of a 
 patch of heat developed by the cyclone, and moving about 
 with it, like all the other characteristics of such a whirl. 
 
 For this reason, we often find exceptionally high 
 
 * Abercromby, " On the Heat and Damp which accompany 
 Cyclones," Quarterly Journal of the Meteorological Society, London, 
 vol. ii. p. 274. 
 
HEAT AND COLD. 215 
 
 temperature on the north of a cyclone-path in the rare 
 cases when the propagation of the depression is towards 
 the west instead of towards the east, as is generally the 
 rule. We shall recur to the importance of this fact, and 
 give an illustration, in our chapter on Forecasting by 
 Synoptic Charts. 
 
 But we may now describe in more detail the tempera- 
 ture-changes in the United States for the twenty-four 
 hours to which the chart refers. 
 
 In the first map (Fig. 42) the centre of an irregular 
 cyclone is near Memphis; the isotherm of 50 projects 
 into this depression ; the isotherm of 40 reaches nearly 
 as far north as St. Louis, and all the Mississippi valley 
 below that city is warm. By 4.35 p.m. the next day 
 (Fig. 43) the cyclone has moved in a north-west direction 
 to Indianopolis, and the isotherm which now projects 
 most is that of 40. Temperature has fallen all over the 
 Mississippi valley, from the cold winds in rear of the 
 cyclone. But what we have to notice most are the tem- 
 peratures recorded at the Ohio stations, just, in front of 
 the upward projection of the isotherm of 40, which were 
 as follows -.Toledo, 19 ; Cleveland, 27 ; Pittsburgh, 31. 
 By 11 p.m. the same evening (Fig. 44) the centre of the 
 cyclone had only moved a few miles, but that was suffi- 
 cient to bring the stations just mentioned more within 
 the range of higher isotherms than earlier in the after- 
 noon. That is to say, the thermometer rose at all those 
 stations between 4.35 p.m. and 11 p.m., although in an 
 ordinary way we expect to see the mercury fall with the 
 sun. The actual figures were Toledo, 3, Cleveland and 
 Pittsburgh 2 each, higher than in the afternoon. 
 
216 WEATHER. 
 
 But while temperature has been rising in Ohio, many 
 of the stations in the lower Mississippi valley have lost 
 from 5 to 8 from diurnal causes. Other stations, such as 
 Montgomery, Alabama, have lost no less than 13 from 
 a combination of diurnal and cyclonic influences. A 
 glance at the charts will enable us to see this at once, for, 
 while the Mississippi stations are in the same portion of 
 the cyclone at both hours, the latter station was in front 
 of the cyclone at 4.35 and in rear at 11 p.m. 
 
 A few years ago, no explanation could have been 
 given of this apparent anomaly of the air getting hotter 
 as the sun went down ; but now we see that it was due to 
 the temperature-disturbance of a cyclone overriding the 
 ordinary variation of diurnal influences. In our chapter 
 on Meteograms we showed how similar changes would 
 affect the trace of a thermograph; now, to complete a 
 comprehensive view of the subject, we have illustrated 
 the same phenomenon by the totally different method of 
 synoptic charts. 
 
 Nothing could show better the extreme facility with 
 which synoptic charts enable us to study temperature- 
 changes in spite of diurnal variation. But just as it is 
 not at all obvious at first sight how changes in the position 
 of isobars are reflected in a barogram, so nothing but a 
 good deal of experience will enable the meteorologist to 
 see readily how changes in the position and shape of the 
 isotherms would affect the indications of a single 
 thermometer, or to handle with any ease the idea of the 
 propagation of diurnal isotherms over a complicated 
 system of temperature-distribution. Our example is one 
 of the simplest which the author could find. In the first 
 
HEAT AND COLD. 217 
 
 and third charts we equalize diurnal influences by con- 
 structing the maps at the same hour each day. By this 
 means we can explain why the Atlantic states were colder 
 on the first day than on the second, and why the Missis- 
 sippi valley was colder on the second than on the first day. 
 
 By our second and third charts we illustrate the 
 manner in which general changes override diurnal varia- 
 tions when the latter are not very strong, as during the 
 winter months, as well as the characteristic nature of a 
 diurnal fall of temperature on an existing system of 
 isotherms. 
 
 There are nearly eighty stations in the United States 
 and Canada. During the six and a half hours in question, 
 changes in every direction of varying magnitude took 
 place at each ; but there is not one, however apparently 
 anomalous, which cannot be explained by means of the 
 principles which we have here laid down. 
 
 SOURCES OF HEAT. 
 
 We were obliged to introduce the question of diurnal 
 variation of temperature in the first place, so as to get 
 rid of any ideas of difficulty from that source of com- 
 plication ; but now, before we describe further the changes 
 in the isotherms from day to day, we must consider the 
 various sources of heat and cold with which we have to 
 deal. In all this we must never forget that the natural 
 distribution of temperature is an irregular thermal slope 
 from the equator to the pole, and that what we have to 
 explain are the divergences from that ideal distribution 
 which we find in practice. We shall find that places 
 
218 WEATHER. 
 
 far north are sometimes much warmer than others nearer 
 the equator, and that some parts of Europe are often 
 colder in March than in January. All these apparent 
 anomalies we can explain easily, but we must begin with 
 sources of heat. 
 
 The primary source of heat is, of course, the sun, so 
 that, other things being equal, we should get the greatest 
 heats where there is the least cloud ; that is to say, 
 generally in anticyclones. This, however, cannot be laid 
 down as a general rule without some modifications. In 
 the belt of anticyclones which surround the world about 
 the line of the tropics, some of the greatest known heats 
 are recorded, notably in the Sahara and in Australia. 
 But in higher latitudes the sun has a powerful enemy in 
 the cold space which surrounds the earth. In summer 
 the sun is the more powerful, and we get hot days with 
 cold nights. In winter-time, when the sun is low, radiation 
 into space overpowers the radiation from a low sun, and 
 clear weather is cold. When, then, we come to discuss in 
 general terms the influence of cloud on isotherms, we must 
 always take into consideration the time of year and 
 latitude. 
 
 Another very powerful source of irregular isotherms is 
 found in wind. Of course, speaking broadly, southerly 
 winds will deflect the isotherms northwards, and northerly 
 or easterly winds will bend them towards the south. 
 This too is, however, subject to many irregularities. The 
 great difference in the radiating power of land and water 
 at different seasons, makes a continental area colder in 
 winter and hotter in summer than a sea in the same 
 latitude. For this reason an easterly wind from a land- 
 
HEAT AND COLD. 219 
 
 area would blow warm into a neighbouring sea in summer, 
 and cold in winter. 
 
 We have already alluded to the idea of a specific 
 quality of heat which is developed by cyclones, and minor 
 local sources of variation, such as the descending winds 
 which probably constitute the "fohn," need only be 
 mentioned here. 
 
 When all these sources of heat are combined, a vertical 
 sun, a cloudless sky, a southerly wind, and an arid soil ; 
 when light, hot puffs fill the air with scorched particles of 
 sand, till the dulled sun appears to glow in a sea of 
 molten brass, and the poisoned breath of the simoon 
 sweeps fitfully across the desert, then the traveller may 
 well beware, and hasten for his life to the nearest shelter. 
 
 An example of great heat will be found in the charts, 
 Figs. 82 and 83, which illustrate the first burst of the 
 south-west monsoon in Hindostan. The dates, June 17 
 and 18, 1881, would coincide with the end of the hot 
 season in Northern India. Our diagrams show on both 
 maps a patch of heat over 100 Fahr. (38 C.) over the 
 Desert of Scinde ; and, as this would be at about half-past 
 five in the afternoon locally, it is certain that much higher 
 temperatures must have been recorded nearer midday. 
 The isobars show that what wind there was would be 
 southerly or south-westerly, and of course light from the 
 absence of gradient near the centre of the depression. 
 The soil there is saltish sand, and similar material has 
 been known to get heated up to nearly 200 in Australia. 
 In Scinde there is a dangerous wind at this season exactly 
 analogous to the simoon of Arabia and the Sahara. Both 
 are certainly allied to whirlwinds and tornadoes; but, 
 
220 WEATHEE. 
 
 unfortunately, no scientific observations have been made 
 on the reputed poisonous or fatal character of these blasts, 
 or of the dangerous quality of heat which they develop. 
 
 There is one type of warm weather in Europe for 
 which no explanation can be given at present. We have 
 seen that an anticyclone usually develops cold- radiation 
 weather, but sometimes we find an anticyclone with warm 
 air and a peculiar soft cloudy sky. This anticyclone 
 covers Continental Europe, and is always associated with 
 the eastward passage of distant cyclones on the northern 
 side. No reason can be assigned for this heat; all we 
 can do is to note the fact for future research. 
 
 SOURCES OF COLD. 
 
 The principal source of all cold is radiation into 
 space. The space which surrounds the earth has a 
 theoretical temperature of at least 226 below Fahr., 
 and it is the influence of this chilly envelope which we 
 feel. 
 
 The greater part of the influence is, however, indirect. 
 We do not feel the cold of space as if we were standing 
 near an iceberg, for all our greatest colds are produced by 
 radiation. Bodies on the earth's surface radiate into this 
 cold space till they lose a large amount of their original 
 temperature ; and air, which is a bad radiator itself, gets 
 cold by contact with the chilly soil. 
 
 For instance, on a calm winter night different bodies 
 say, a sheet of iron lying on the ground and a patch of 
 grass begin to radiate into space at different rates, 
 according to their own intrinsic properties. Iron radiates 
 
HEAT AND COLD. 221 
 
 very quickly, but is also such a good conductor that it 
 brings up an abundant supply of heat from the ground to 
 replace the loss by radiation, so that the plate does not 
 become very cold. The grass is a less good radiator, but 
 at the same time a very bad conductor ; so, though it parts 
 with heat slower than the iron, it cannot replace what it 
 has lost by conduction, and therefore, on the balance, 
 becomes much colder. This is the cold which we really 
 feel, and which sends down the thermometer. 
 
 A very striking result of all this is, that under these 
 circumstances, the air gets warmer as we ascend up to a 
 certain height, and this proves conclusively that we do 
 not feel directly the chill of space. Of course, the greatest 
 cold will be produced when the greatest number of causes 
 are combined which favour radiation. These are a still 
 air, a clear sky, and an absence of water- vapour in any 
 stratum of the atmosphere. This last condition is very 
 interesting. Professor Tyndall's researches seem to show 
 that water-vapour is a great absorbent of the quality of 
 heat which is radiated from the ground, so that when 
 much vapour is present the ground cannot lose its heat 
 so rapidly as when the air is dry. 
 
 All these conditions of great cold are fulfilled in the 
 most perfect manner in Siberia. There we have the 
 centre of a large dry continental area, which in winter- 
 time is persistently covered by an anticyclone ; while the 
 latitude is so high that the sun has little power. Here, 
 then, we find calm, dryness, and a feeble sun ; and here 
 the greatest known colds are reported, if we except some 
 in the north of Smith's Sound, many degrees further 
 north. A good illustration of this will be found in the 
 
222 WEATHER. 
 
 two charts which we give in our chapter on types of the 
 north-east monsoon (Figs. 80 and 81). In them we see 
 that the south of Siberia, which is covered by an anti- 
 cyclone, has stations in which the mercury marks more 
 than 30 below zero, Fahr. 
 
 If we take the less extreme cases which occur in 
 Great Britain, we find that all frosts in that country are 
 "home-brewed;" that is to say, that cold winds never 
 bring extremely low temperatures from the plains of 
 Europe or the mountains of Norway. But when shallow 
 gradients for east and north-east winds cover Great 
 Britain, and a dry chilly air favours nocturnal radiation, 
 then all the hardest frosts are developed. Then we often 
 find the temperatures 10 or 20 lower in the most inland 
 stations of England and Ireland, and the isotherms 
 gradually increase round these cold centres. When we 
 look at a synoptic chart of Europe for 8 a.rn., we find, on 
 these occasions, that England and Ireland are separate 
 islands of cold on the general thermal slope from a cold 
 continent to the warm North Atlantic. From the fact 
 that frost depends on radiation, we can readily explain 
 why cold is so local. Kadiation is very sensitive ; the least 
 breath of wind or any local shelter may interfere with 
 the free play of radiation, and so we find two places only 
 a few miles apart, one of which records 10 or 15 lower 
 than the other. 
 
 The next source of cold is found in wind. When this 
 blows from a frozen continent, then, of course, very low 
 temperatures may be recorded ; but this is not the same 
 kind of cold as radiation-frost. Here we have another of 
 the innumerable instances of the necessity of distinguish- 
 
HEAT AND COLD. 223 
 
 ing between different kinds of the same nominal pheno- 
 menon. The 1st of January may be cold in one year 
 from wind ; in another from radiation. These are the 
 products of totally different kinds of weather, and must 
 not be mixed up in scientific meteorology. 
 
 THE "BLIZZARD" AND THE "BARBER." 
 
 A very striking example of wind is found in the 
 " blizzards " of the United States. These are cold snaps 
 which come with a high wind, as opposed to the calm 
 frost of anticyclones. They are the result of the passage 
 of the rear of cyclones or of V-depressions in the winter 
 months, such as we see in Figs. 42 and 43. Then we get 
 high, strong, north-westerly winds, blowing off a frozen 
 continent with a temperature many degrees below zero, 
 and with surroundings which are very destructive to life. 
 The wind drives the cold into the bones even through 
 fur clothing, and raises a blinding dust of powdery snow. 
 Under these circumstances only are the western voyagers 
 ever lost. If wood cannot be found, nature can only resist 
 the cold for a certain number of hours, and the men are 
 frozen to death if no shelter can be reached. A very 
 curious circumstance attends these deaths. In almost 
 every case the victims are found to have begun to strip 
 themselves. When the body is nearly reduced to an 
 icicle, only a very little blood continues to circulate 
 languidly through the brain. Then delirium, sets in, with 
 a delusive sensation of heat, under the influence of 
 which the traveller begins to divest himself of his clothes. 
 
 Another disagreeable form of cold is found in the 
 
224 WEATHER. 
 
 St. Lawrence Gulf. Sometimes with a high wind the air 
 becomes much colder than the open water. The latter, 
 being relatively hot, begins .to smoke, and the vapour 
 freezes into peculiarly sharp spicules. The poudre snow- 
 crystals of the north-west are usually small, dry, six-sided 
 petals, and, though penetrating as sand, they are soft. The 
 latter kind of snow is so damp and sharp that, when driven 
 by a gale, it nearly cuts the skin off the face. Hence the 
 popular name of the " barber," which is applied to this 
 phenomenon. The same name of " barber " is applied to 
 another phase of cold along the coasts of Nova Scotia and 
 New England. When a vessel is caught by a gale of wind 
 in a cold arctic current, the spray freezes the moment it 
 touches the deck or rigging. Every block is turned into 
 a lump of ice ; men get coated with ice like an icicle ; and 
 sometimes such a weight of ice forms on the bow that 
 the stern is lifted out of the water, and the ship becomes 
 unmanageable for want of steering power. 
 
 The last source of cold which we need mention is 
 rain. All rain, of course, is not cold. In front of a 
 cyclone rain is warm, and a shower does not send down 
 the thermometer. In the rear, on the contrary, and in 
 thunderstorms and secondaries, precipitation is more or 
 less cold, and turns the mercury downwards. The influence 
 of this varies very much in different countries and at 
 different seasons of the year. In England, during the 
 summer, rainy weather is cold, because it cuts off the sun, 
 independent of any chill of its own. In winter, on the 
 contrary, rainy weather is warm, because an overcast sky 
 prevents loss of heat by radiation. In the tropics cloudy 
 weather is colder, as far as the thermometer is concerned, 
 
HEAT AND COLD. 225 
 
 than a bright day, because the rays of the sun are ob- 
 structed ; but if there is little wind, a cloudy day is more 
 oppressive to men than one with sunshine. Near the 
 equator there is very little diurnal radiation of any kind, 
 owing to the excessive amount of vapour in the air. 
 
 We may sum up all the effects of heat and cold 
 briefly thus : In winter wind, cloud, and rain in 
 temperate regions tend to raise the temperature, as they 
 check cold radiation ; calm, on the contrary, induces hard 
 frost. In summer wind, cloud, and rain are cooling 
 influences, as they check hot radiation; calm, on the 
 contrary, is then hot, because it allows full play for the 
 sun's rays. 
 
 We may, in fact, look at the opposing forces of hot 
 and cold radiation as in a state of constant conflict. 
 The rotundity of the earth always weakens the power of 
 the sun in the north. Water-vapour in some shape 
 forms, as it were, a blanket for the earth, and saves her 
 from being burnt up and frozen alternately. The incessant 
 circulation of the atmosphere sometimes eddies in a 
 cyclonic form, and develops dense cloud, which shields 
 the earth from the radiation of the season and latitude ; 
 at other times the circulation of the air eddies downwards 
 in an anticyclone, and the clear, dry, calm atmosphere 
 gives full play to radiation, and some extreme of heat or 
 cold is then developed. 
 
 The task of the meteorologist is to trace how the 
 varying forms of atmospheric circulation modify the 
 distribution of heat and cold over the world from day to 
 day, by the application of the general laws we have just 
 laid down. 
 
226 
 
 WEATHER. 
 
 EXAMPLES OF DAILY TEMPERATURE-CHANGES OVER 
 EUROPE. 
 
 For instance, let us consider the changes of tempera- 
 ture which occurred in Europe on the three days, 
 February 26-28, 1865 ; that is, during three of the days 
 for which we shall give synoptic charts in Figs. 68-70, 
 when discussing the westerly type of weather. We have 
 to explain now why European temperature varied as it 
 did on those days. 
 
 The isotherms for the period in question are given in 
 Figs. 45-47. In all of these there is a thermal gradient 
 
 Feb.26. 1865, 8am.Green.wLch,. Feb.23. 2866, 8cun.Breen.vnch. 
 
 FIGS. 45 and 46. Isotherms in Europe for three consecutive days. 
 
 from south-west to north-east instead of a slope towards 
 the north-west, as undisturbed natural isotherms should 
 have at eight o'clock in the morning the hour for which 
 the charts are constructed. The reason for this broad 
 feature is that in winter a continental area is always 
 
HEAT AND COLD. 
 
 227 
 
 colder than a sea-surface, and therefore, whatever smaller 
 variations may occur from day to day, the general slope 
 of temperature will always be from frozen Russia towards 
 moisture-bathed Portugal. This feature belongs to the 
 season, and is found in every chart ; what we have now 
 to explain is the fluctuation 
 in the position of the iso- 
 therms caused by the vary- 
 ing development of heat 
 and cold locally. 
 
 Glancing at both the 
 synoptic charts of pressure 
 and temperature, we see 
 that on the morning of 
 February 26 a Y-depression 
 covered Great Britain, with 
 
 warm SOUth-west winds in J i eb.28.l865,8am.Greenwuh. 
 
 FIG. 47. Isotherms in Europe for 
 three consecutive days. 
 
 front. Straight isobars lay 
 over Scandinavia, an anti- 
 cyclone stretched over 
 
 Western Europe from the Atlantic, and a calm col lay over 
 Russia. From all this England was warm, as shown by the 
 projection northwards of the isotherm, of 41 Fahr. (5 C.) ; 
 Continental Europe and Russia were very cold. In the 
 latter country, -4 Fahr. (-20 C.) is reported, and local 
 patches of cold as low as 4 Fahr. ( 15 C.) are reported 
 in different parts of France and Germany. These should 
 be noticed, for they are most characteristic of the abrupt 
 local variations of temperature which we often find are 
 caused by local differences in radiation. They are 
 identical with all the frosts which occur in Great Britain, 
 
228 WEATHER. 
 
 to which we have before alluded. Observe also that they 
 have no influence whatever as a cause of weather ; they 
 are the product of the general circulation of the atmo- 
 sphere, allowing free play for radiation, not a cause of 
 that circulation themselves, though the influence of the 
 general thermal slope from Kussia to Portugal is an 
 important factor in determining the path of the cyclones. 
 By next morning the British V and Scandinavian straight 
 isobars have formed a well-defined cyclone, some second- 
 aries appear over various parts of Europe, while a calm 
 wedge covers England. England is, therefore, colder than 
 on the previous day, because of the radiation of the wedge, 
 and the isotherm of 41 Fahr. (5 C.) has retreated 
 southwards. Russia and Continental Europe are much 
 warmer, because the cyclones and secondaries have 
 destroyed radiation. 
 
 By the morning of the third day the Scandinavian 
 cyclone has died out, but a new one lies over the north 
 of Scotland. Secondaries still cover the greater portion 
 of Europe, but in Eussia the weather would be calmer. 
 From this it results that England is warmer, so that the 
 isotherm of 41 Fahr. (5 C.) projects northwards again; 
 Continental Europe is a little colder, \vithout many local 
 frosts ; Eussia is a great deal colder, but not so cold as 
 the first day, for the conditions are less favourable to 
 radiation. None of these cyclonic changes reach so far 
 south as Spain, and therefore we see the isotherm of 
 50 Fahr. (10 C.) scarcely alters its position during 
 the three days. 
 
 We may also put the changes of temperature over 
 Europe in a very striking light by looking at the isotherm 
 
HEAT AND COLD. 229 
 
 of 32 Fahr. (0 C.). On the first day it stretches from 
 Belgium to the Black Sea ; the second day it has been 
 driven back almost to the Gulf of Bothnia and to Poland ; 
 the third day it has advanced again, but not so far as on 
 the first day. So on the conflict would go between the 
 frost and sun till the sun at last drove that isotherm out 
 of Europe. In the autumn the battle would be renewed ; 
 but then the sun would be beaten, and frost remain 
 supreme for several months in the more northern portions 
 of that continent. 
 
 Had our limits permitted, we would have given 
 examples of the reversal of radiation effect which occurs 
 in summer, when an anticyclone means heat instead of 
 cold. Then we may often find England hotter than 
 France, for if the calm centre of an anticyclone lies over 
 the former country, the sun's rays have more power there 
 than in the more windy southern edge, which would cover 
 France under these circumstances. 
 
 We may, however, refer again to Figs. 21 and 22, 
 which relate to the same day of May the 17th in two 
 different years, and in which diurnal variation is allowed 
 for by constructing the charts at the same hour. On the 
 first day (Fig. 21) a cyclone covers Great Britain, and 
 the isotherm of 50 Fahr. (10 C.) reaches to the north 
 of Scotland and Denmark, under the influence of southerly 
 winds and a cloudy sky. 
 
 On the second day (Fig. 22) the isotherm of 50 Fahr. 
 (10 C.) runs north and south down England, and a corner 
 of the line of 40 Fahr. (5 C.) appears over Northern 
 Germany. This shows that to the west of the isotherm 
 of 50 the temperature rises towards 60 Fahr. (15 C.), 
 
230 WEATHER. 
 
 and therefore that part of England and Ireland is warmer 
 than on the same day of another year, when no place 
 recorded anything as high as 50. This is the product 
 of the calm blue sky of an anticyclone ; while the 
 diminished temperature over Germany is due to the 
 general thermal slope of the season, for Continental 
 Europe does not get warm till the month of June. 
 
 If we combine all these with the other examples 
 we have already given of temperature ranging from 
 100 Fahr. (40 C.) to -30 Fahr. (-35 C.) in Europe, 
 Asia, and America, the reader will have a very fair idea 
 of the nature of temperature- changes. 
 
 FORECASTING TEMPERATURE. 
 
 From all this it will be very evident that, though we 
 can lay down some general laws of temperature-changes, 
 still the modifications which occur in practice are endless. 
 The forecaster in every country has to learn by experience 
 the qualities of the different winds, and the power of the 
 different radiations at each season of the year. 
 
 For instance, in Great Britain he soon learns to dis- 
 tinguish between the uniformly warm, close heat of 
 winter cyclones ; the oppressive, sultry heat of a summer 
 thunderstorm ; and the clear, cold air, with a hot sun, of 
 a spring anticyclone. Any doubt which can arise as to 
 the future course of temperature-changes depends on the 
 same points which always make any forecasting uncertain 
 viz. the difficulty of knowing what the future path of 
 the cyclones will be, or whether any new distribution 
 of pressure is likely to set in suddenly. If the forecaster 
 
HEAT AND COLD. 231 
 
 judges rightly as to the future movements of pressure- 
 distribution, he rarely makes a mistake as to the nature 
 of temperature-changes which accompany them. 
 
 PRIMAKY AND SECONDARY EFFECTS OF HEAT. 
 
 We will conclude with one important reflection. We 
 know that heat is the prime mover of all atmospheric 
 circulation ; why, then, do the great local differences of 
 temperature have so little influence on the sequence of 
 weather? The greatest diurnal ranges are found in 
 anticyclones, which are also associated with the steadiest 
 weather ; and in wedges, where we find strong contrasts 
 of heat and cold, these local differences of temperature 
 are certainly not the cause of the cyclone and rain which 
 follow soon. At the same time, it is certain that the 
 persistent anticyclone over Siberia during the winter 
 months is caused by the radiation cold of that country. 
 That is to say, we may conceive that in the general 
 circulation of the hot air of the equator towards the pole, 
 the direction of the currents will be profoundly modified 
 by the surface-temperature of the earth, and that it is 
 perhaps easier to flow over a cold surface at one season 
 and a warm one at another. 
 
 However that may be, we are met by the apparent 
 contradiction that, though the daily variations of tempera- 
 ture are undoubtedly the product of the motion of 
 cyclones, etc., the broad situations of the areas of cyclone 
 activity are themselves due to radiation. 
 
 The truth probably is that both inferences are correct 
 in a modified degree, and that in this, as in every other 
 
232 WEATHER. 
 
 meteorological problem, we have to deal with a balance 
 of influences which act and react on one another in a 
 very complicated manner. 
 
 We have already explained the stability of a cir- 
 culatory system such as a cyclone or anticyclone, and 
 the idea that diurnal variations may merely affect the 
 rapidity, but not the form, of the vortex system ; but 
 one observation may perhaps be noted here which pro- 
 bably has some bearing on the question. Our synoptic 
 charts give surface-temperature only, but we have taken 
 no notice of the heat of upper currents. Now, it has 
 been discovered that over cyclones temperature diminishes 
 from the surface upwards at the rate of about 3 Fahr. 
 (1*5 C.) in one thousand feet ; in anticyclones, on the 
 contrary, when radiation produces frost, the air gets 
 warmer as we ascend to a short distance, after which the 
 temperature begins to fall as we go higher up. 
 
 What the precise significance of this may be we 
 cannot tell, but it is interesting in connection with the 
 manner in which pressure decreases at a slower rate over 
 cyclones as compared with anticyclones. Further research 
 can alone solve these problems, to which we have merely 
 alluded to carry out our purpose of giving a picture of 
 the state of meteorology at the present day. 
 
( 2G3 ) 
 
 CHAPTER VIII. 
 SQUALLS, THUNDERSTOEMS, AND NON-ISOBARIC RAINS. 
 
 IN this chapter we propose to introduce the reader to 
 details of weather totally different from any that we have 
 hitherto described. So far we have dealt with the 
 phenomena of wind and rain which are associated with 
 cyclones and rapid changes of barometric pressure ; now 
 we intend to discuss changes of weather which are con- 
 nected but indirectly with the distribution of surrounding 
 pressure, and in which, if the mercury moves at all, the 
 direction is upwards. Isobars which have been our 
 unerring guide through the most complicated cyclonic 
 weather will now totally fail us ; and, under the heading 
 of non-isobaric rains, we shall discuss certain rainfalls, to 
 the origin of which we have at present but little clue. 
 In addition to the interest which attaches to such striking 
 manifestations of nature as squalls, thunderstorms, tor- 
 nadoes, and whirlwinds, a great deal of research has been 
 bestowed of recent years on these subjects which has not 
 yet found its way into popular literature, and which at 
 present is scarcely known beyond the limited circle of 
 
234 WEATHER. 
 
 professional meteorologists. We now propose to explain 
 some of the most remarkable results which have thus 
 been obtained. 
 
 SIMPLE SQUALLS. 
 
 If we watch the stages of gradually increasing wind, 
 we find that as the strength rises the tendency is more 
 and more to blow in gusts. Gradually these gusts get 
 still more violent, and in their highest development come 
 with a boom like the discharge of a piece of heavy 
 ordnance. This is what sailors call " blowing in great 
 guns," and these are the gusts which blow sails into 
 ribbons, and dismast ships more than any amount of 
 steady wind. These gusts only last a few minutes, but 
 they seem to be very closely allied to the simplest form 
 of squalls. In a true, simple squall the wind generally 
 need not be of the exceptional violence which causes 
 " guns ; " but after it has rather fallen a little, the blast 
 comes on suddenly with a burst, and rain or hail, accord- 
 ing to intensity, or other circumstances, while the whole 
 rarely lasts more than five or ten minutes. At sea one 
 often sees two or three squalls flying about at a time. 
 Then we readily observe that over the squall there is firm, 
 hard, cumulus cloud ; that the disturbance only reaches 
 a short distance above the earth's surface ; that the 
 squall moves nearly in the same direction as the wind ; 
 and that there is little or no shift of the wind before or 
 during the squall. We also see that the shape of the 
 squall is merely that of an irregular patch, with a 
 tendency rather to be longer in the direction of the wind 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 235 
 
 than in any other quarter ; and that the motion of the 
 squall as a whole is much slower than that of the wind 
 which accompanies the first blasts. If, at the same time, 
 we watch our barometer closely, we find that if the squall 
 is sufficiently strong, the mercury invariably rises some- 
 times as much as one-tenth of an inch and returns to its 
 former level after the squall is over. No difference is 
 observed in this sudden rise, whether the squall is accom- 
 panied with rain, hail, or thunder and lightning; and 
 though we are unable exactly to explain why the wind 
 sometimes takes this irregular method of blowing, we 
 have still to do with a comparatively simple phenomenon. 
 
 THUNDER-SQUALLS. 
 
 The simplest kind of thunderstorm may more properly 
 be described as a squall accompanied by thunder and 
 lightning, instead of only with wind and rain. In Great 
 Britain these thunder-squalls are very common on our 
 extreme west and north-west coasts during the winter 
 months, while they are very rare in Central or Eastern 
 England at any season of the year. On a wild, stormy 
 day, with common squalls, one or two of these, which are 
 exceptionally violent, will be accompanied by one or two 
 claps of thunder with lightning. The principal interest 
 which attaches to this type of thunderstorm consists in 
 the proof which is afforded that there is no essential 
 difference between a common squall and another which 
 may be associated with electrical discharge, except 
 intensity. The look and motion of the clouds, and the 
 sudden rise of the barometer, are identical in both cases. 
 
236 WEATHER. 
 
 We can readily conceive, since the formation of cumulus 
 above the squall points unmistakably to the presence of 
 an ascensional current, that when the uptake is only 
 moderate, the condensation of vapour may take place so 
 gradually that none of the electricity which there is 
 reason to believe is given off under these circumstances- 
 is discharged disruptively ; but when the uprush is so 
 violent as to inject the moist air into strata which are so 
 cold and dry that the electricity cannot pass off silently, 
 then a disruptive discharge with thunder and lightning 
 will be produced. In Western Europe this class of 
 thunderstorm is much more common in winter than in 
 summer, which is the reverse of what takes place with all 
 other kinds of thunderstorm. So much is this the case 
 that in Iceland there are no summer thunderstorm?, but 
 only winter ones, of this simple squall type. In Norway 
 both types occur ; and the winter ones are there found 
 to be the most destructive, because they are lower 
 down, and therefore the lightning is the more likely to 
 strike buildings. In that country, however, the summer 
 thunderstorms are not nearly so violent as in more 
 southern latitudes. 
 
 BAROMETER IN SQUALLS AND THUNDERSTORMS. 
 
 We have just mentioned that the barometer usually 
 rises just as the rain of a squall or thunderstorm strikes 
 a place, and this is as true on the Equator as on the 
 Arctic Circle. Since this fact is of great importance in 
 the discussion of the more complicated phenomena that 
 are called line-squalls, we will devote a few paragraphs to 
 
SQUALLS; THUNDERSTORMS, NON-ISOBARIC RAINS. 237 
 
 the elucidation of the details of these barometric fluctua- 
 tions. We can do this best RBBSSSH^^B9| 
 by an actual example. 
 
 In Fig. 48 we give a pho- 
 tographic engraving of the 
 barometer-trace given by the 
 author's barograph in Lon- 
 don, on May 18, 1878. The 
 original was recorded on 
 smoked paper, and is here 
 reproduced by photography, 
 absolutely untouched by the 
 engraver. By this means 
 the most delicate fluctuations 
 are faithfully rendered, and 
 those who are familiar with 
 sensitive self-recording in- 
 struments will readily recog- 
 nize that characteristic un- 
 easiness of the whole trace, 
 which can never be copied 
 by hand. In the figure the 
 vertical lines represent inter- 
 vals of six hours, while the 
 horizontal lines indicate a 
 difference of 0'5 inch of mer- 
 cury. Confining our atten- 
 tion to the right-hand portion 
 only of the diagram, we have 
 to note, soon after midnight of May 17, a small curious 
 dip of the barometer, followed immediately by a sudden rise. 
 
238 WEATHER. 
 
 This is marked a, and it occurred during a thunderstorm. 
 Just before 6 a.m., and for some time after, we find the 
 still more remarkable fluctuations marked b. These were 
 also associated with a series of thunderstorms, none of 
 which were particularly violent. Still later, about 8 a.m., 
 we see the singular dip marked c. This occurred with 
 gloomy, threatening weather, but neither with wind, rain, 
 nor thunder, at the place of observation in London. 
 
 The chart for the day at 8 a.m. showed that a series 
 of small secondaries lay over Great Britain, but there 
 were no bends in the isobars that would explain such 
 curious barometric oscillations. 
 
 The origin of this characteristic rise of the barometer 
 in squalls and thunderstorms is at present unknown. It 
 has been suggested that it is due to a rush of air, carried 
 down by the rain. That such is partially the cause is 
 extremely probable, for we sometimes see a small rise 
 under a heavy splash of rain without either thunder or 
 wind. But it is equally certain that this downrush does 
 not entirely explain the phenomenon, for sometimes a 
 rise occurs without any rain at all, or of an amount which 
 bears no relation to the heaviness of the fall. Still more 
 puzzling are the small dips of the mercury which we 
 occasionally find with thunderstorms, and of which some 
 examples are given in our last figure. These dips are 
 more rare than the rises, and though in most cases they 
 are, as in this example, more or less associated with the 
 rises, still they occasionally occur alone. In the first dip 
 shown in our last figure, about 1 a.m., the depression was 
 associated with a storm ; while in the second case, about 
 8 a.m., no storm or rain occurred locally, though un- 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 239 
 
 doubtedly storms were in existence not far off. We are, 
 therefore, almost driven to the conclusion that some of 
 these curious fluctuations of the barometer must be due 
 to a sort of true wave-action, through which the disturb- 
 ance, caused perhaps by falling rain, may be propagated 
 by the elasticity of the air to some distance from the 
 place of original disturbance. In connection with this 
 idea of air being brought down by falling rain, we may 
 notice that very striking effects are sometimes observed 
 in avalanches of snow, which always bring down an 
 immense amount of imprisoned air with them. It is 
 usually found that persons caught in the blast of the 
 avalanche have their clothes torn into ribbons. The 
 suggestion has also been made that if rain is the product 
 of the condensation of an ascensional current of air, then 
 the more violent the uptake, the greater must be the 
 reaction downwards; but, unfortunately, our knowledge of 
 the dynamics of air in motion is not sufficiently advanced 
 to enable us to say exactly what the nature of pressure 
 would be under these circumstances. 
 
 But though we cannot altogether explain the origin 
 of these barometric fluctuations, we know enough to say 
 that they are of a totally different nature from any 
 motion of the mercurial column due to the action of 
 cyclones or the propagation of isobars over any station. 
 When, then, we see on a barogram these peculiar 
 irregularities, we can at once infer that they are the 
 product of squalls or thunderstorms, and not of cyclones, 
 and so far we are enabled to increase our knowledge of 
 the method of reading barograms, to which we have 
 already given so much attention. These dips and rises 
 
240 WEATHER. 
 
 may, in fact, be taken as another letter in that barographic 
 alphabet by which a skilled meteorologist can read the 
 history of the weather from his barometric trace. Another 
 very important inference which this knowledge gives us 
 is that, as these rises are entirely different from those 
 due to cyclonic motions, we cannot make the same deduc- 
 tions from the one that we would from the other. For 
 instance, if in rear of a cyclone the mercury rose at the 
 rate of two-tenths of an inch in an hour, that would be an 
 exceptional rate anywhere in Europe, and we should expect 
 that it would be associated with a violent gale. But the 
 mercury might rise at two or three times that rate in a 
 thunderstorm of no exceptional intensity, with which there 
 would be no more than a few irregular gusts. For the 
 same reason, if we have to include any barometric readings 
 of these peculiar rises in the construction of our synoptic 
 charts, we must not draw the same conclusions from the 
 lie of the isobars as we should in ordinary weather, be- 
 cause the origin of the isobars is not the same in both 
 cases. The error which we have to avoid is not to take 
 as the same two phenomena that are really totally distinct, 
 but which have one common property namely, a rise of 
 the barometer. 
 
 LINE-SQUALLS. 
 
 We have already explained that the line of the trough 
 of a cyclone or V-depression is associated with a line of 
 squalls, and that we must picture to ourselves a long, 
 narrow, thin band of rain and wind sweeping across the 
 country, broadside on, like a wall or curtain, at the same 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 241 
 
 speed as the depression itself. This rate bears no relation 
 to the velocity of the wind in the squall. In practice the 
 rate of the depression will be much slower than the wind 
 in the first gust. The former will probably not exceed 
 forty miles an hour ; the latter may mount up to seventy 
 or eighty miles an hour. We will now go into some very 
 interesting details of this class of atmospheric disturbance, 
 which, for the sake of classification, we will call " Line- 
 squalls." 
 
 The nature of this class of squall will be best ex- 
 plained by an actual example of the squall which capsized 
 an English man-of-war the Eurydice and caused one of 
 the greatest disasters which has befallen the British navy 
 for many years. In Fig. 74, under weather- types, we give 
 a chart of a large portion of the northern hemisphere for 
 March 24, 1878, at 0.43 p.m., Greenwich, and we may 
 just glance at it now to see the general distribution oi 
 pressure over Europe on that day. The squall which we 
 have now to consider belonged to one of the numerous 
 secondaries, which hardly show on the large chart ; but in 
 Fig. 49 we give the details of pressure, wind, and weather 
 over Great Britain and France at the same hour on a 
 larger scale. In this diagram we see an extremely com- 
 plex distribution of pressure. What concerns us most is 
 the bend in the isobars, along which we have run a dotted 
 line that is marked " trough " at one end. This bend is 
 a small V-depression, in some way secondary to the ill- 
 defined fragment of a cyclone that covers the southern 
 portion of the Scandinavian peninsula. During the 
 course of the day this cyclone appeared to circle round 
 another which lay in the morning over the Carpathian 
 
242 
 
 WEATHER. 
 
 Mountains ; and, in connection with these greater changes, 
 the trough of the V wheeled round a point near the Scaw, 
 in Denmark, like the spoke of a wheel. Fig. 49 shows 
 the position of the trough at 0.43 p.m. ; the front line of 
 the crescent-shaped shaded area in Fig. 50 shows approxi- 
 mately the position of the trough at 3 p.m.; and by 6 p.m. 
 
 FIG. 49. The Eurydice squall. 
 Isobars and wind at 0.43 p.m. 
 
 FIG. 50. The Eurydice squall. 
 Area covered by squall at 3 p.m. 
 
 the trough passed in a curved line from Yarmouth, through 
 the Straits of Dover, into Normandy. By reason of this 
 wheeling motion, different portions of the trough moved 
 with very different velocities. Between the hours just 
 named, the northern portion of the trough moved across 
 England at the rate of only thirteen miles an hour, while 
 the extreme south-westerly edge traversed the country at 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 243 
 
 the rate of no less than forty- eight miles an hour. The 
 portion which struck the Eurydice was going at the rate 
 of thirty-eight miles an hour. 
 
 So far for the motion of the V as a whole. In Fig. 49 
 the wind was from about west in front, and from north-west 
 in rear of the V, but no well-defined area of rain was then 
 developed. By 3 p.m., however, the depression had so 
 much increased its intensity that Mr. Ley was able to 
 construct the diagram given in Fig. 50. In that figure 
 the shaded portion shows the area over which rain or 
 snow was falling at the moment ; the solid arrows give 
 the general sweep of the surface- winds, the dotted ones 
 those of the upper currents. The author has further shown 
 that the front of the rain-area was essentially coincident 
 with the trough of the V, which we see about three hours 
 earlier in Fig. 49, so that we evidently have to deal with 
 a V of that class in which the rain is in rear of the 
 trough. At every station, after the wind had been from 
 the west, with a cloudy sky in the morning, the clouds 
 gradually banked up ominously to the north-west ; then 
 rain or snow came on with a tremendous squall, while 
 the wind jumped round to north-west. After the first 
 burst had moderated, rain or snow continued for a longer 
 or shorter interval till the sky cleared again. 
 
 H.M.S. Eurydice was a full-rigged corvette, homeward 
 bound from the West Indies. At 3.45 p.m. three- 
 quarters of an hour later than that for which the chart 
 given in Fig. 50 was constructed she was off Ventnor, in 
 the Isle of Wight, running free before a nearly westerly 
 wind, with all sail set. At that moment she was struck 
 by a squall from the north-west; before sail could be 
 
244 WEATHER. 
 
 shortened, she went on to her beam-ends, and, as the lee 
 ports were open, she filled and foundered. 
 
 On the whole we have, therefore, to idealize a band- 
 shaped area of rain, bounded in front by a line of squalls 
 in this case more than four hundred miles long sweep- 
 ing broadside on across Great Britain at a rate varying 
 from thirteen to nearly fifty miles an hour. From this 
 we can readily see how places many miles apart can be 
 struck simultaneously at the same hour, and how ap- 
 plicable is the name of line-squalls, which we have applied 
 to this class of disturbance. 
 
 Though this class of line-squall is uncommon in 
 Great Britain, it appears to be very frequent in other parts 
 of the world. For instance, in Iowa, a similar kind of 
 squall is peculiarly characteristic of summer weather. 
 There it generally occurs after a spell of continued hot, 
 rather sultry weather, the wind having blown steadily 
 but moderately from the south or south-west, the 
 barometer not changing much. In the north-west the 
 storm-front will make its appearance ; threatening, dark, 
 towering clouds, or at times an immense roll-like cloud, 
 will approach ; the air cools rapidly as the storm-front 
 comes nearer ; and, with a high, straight blow, bending 
 young trees to the ground and driving the rain nearly 
 level, the fierce storm passes over, while the barometer 
 rises rapidly. Such a blow does not last long, but may 
 be repeated with gradually weakened force at intervals. 
 A steady pouring rain generally follows, after which the 
 sky clears, and the storm-wind wheels back to the south- 
 east, the weather being as hot as before the storm. This 
 description, which we have taken from Dr. Hinrichs, of 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 245 
 
 Iowa City, together with, the maps which he gives to 
 illustrate them, point very clearly to line-squalls, asso- 
 ciated with that class of V-depression in which the rain 
 follows the trough. His maps do not exhibit the 
 shape of the rain-area like the one we have just given, 
 but they show the squalls sweeping across the State 
 with a crescent-shaped front exactly like the Eurydice 
 squall. 
 
 When we compare this class of squall with the pure 
 and simple squall which we first described, it will be 
 obvious that the two kinds have little in common except 
 the name. The former kind seems to be simply a local 
 intensification of a general sweep of wind. The latter, on 
 the contrary, is associated with a very definite but com- 
 plex phase of aerial circulation, which we shall understand 
 better when we have described a precisely analogous class 
 of thunderstorms. 
 
 THUNDERSTORMS ASSOCIATED WITH LINE- SQUALLS. 
 
 Squalls are rarely of sufficient importance to attract 
 the notice of enough observers to enable the details of 
 their shape and progress to be properly determined ; but 
 thunderstorms are such a striking manifestation of weather 
 that they are much more easily traced, and an enormous 
 amount of work has been done in late years in marking 
 out the hourly advances and development of such dis- 
 turbances. 
 
 Many, but not all, European thunderstorms have been 
 found to be precisely similar to the line-squall which 
 we have just described. Some of Bezold's diagrams of 
 
246 WEATHER. 
 
 Bavarian thunderstorms, which give the shape of the 
 area covered by the storm at successive hours, show long 
 narrow bands sweeping broadside on across the country 
 exactly analogous to the squall-area which we drew in 
 Fig. 50. 
 
 But the remarkable point is, that though some of these 
 storm-bands are associated with the troughs of cyclones 
 and V's, precisely similar bands are more often found 
 either in front or in rear of the cyclone, where we can 
 connect them (the bands) with no particular part of the 
 cyclone, except that the front of the band is usually 
 perpendicular to the line of progress of the depression in 
 which it is formed. 
 
 We will first give an example of the storm and 
 thunderstorm associated with the trough of a V-depres- 
 sion. On July 16, 1884, at about 6.15 p.m., the trough of 
 a V-depression swept over Hamburg, and brought, as 
 usual, a violent squall and heavy rain, with much thunder 
 and lightning. This was only a section, as it were, of a 
 line-thunderstorm. Dr. Sprung, by combining the section 
 of barometric and other curves at Hamburg with the 
 records of other observatories and the synoptic charts of 
 Germany, at 8 p.m. the same evening, in the manner that 
 we explained in our chapter on Meteograms, has built up 
 the beautiful diagram of this storm, which we reproduce, 
 with a few trifling modifications, so as to assimilate it 
 with our other illustrations, in Fig. 51. There the isobars 
 are given both in millimetres arid their approximate 
 equivalents in inches : t marks the line of the trough ; 
 the shaded line r the position and dimensions of the rain- 
 stripe ; the long dotted arrow the direction in which the 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 247 
 
 whole system was being propagated ; and the small solid 
 arrows the direction and force of the wind across any 
 section. The whole is evidently, in the main, one of those 
 V's in which the rain begins just after the passage of the 
 trough ; but the curious projection upwards of the isobars 
 just under the rain-stripe 
 is unlike anything we 
 have seen before; and 
 the strong west wind, with 
 four feathers on the arrow, 
 is quite unconformable 
 with the isobars accord- 
 ing to our usual expe- - 
 rience. From whence 
 comes all this? First, 
 for the upward projec- 
 
 ,. P j.1 i AH FIG. 51. Line-thunderstorm, t, Trough 
 
 tlOn Of the isobars. All of ^depression ; r, rain stripe. 
 
 the barographs showed, 
 
 about seven minutes after the passage of the trough, a 
 sudden rise exactly similar to those marked b in Fig. 48 
 just as the heavy rain began, and this rise, as usual, was 
 quite distinct from the general increase of pressure due to 
 the rear of the V. If we were to superimpose a long, 
 narrow, isolated ridge of high-pressure on the rear of 
 a V, we should get an inverted V, or wedge, exactly like 
 that which we find in the diagram under the rain-stripe ; 
 but we must not treat this like the ordinary wedge- 
 shaped isobars which we have before described. The 
 form is the same, but the cause is different. 
 
 Then for the wind-sequence. We find in front of the 
 V a light south-east wind ; then, just before and at the 
 
248 WEATHER. 
 
 commencement of the rain, a very violent squall (called 
 "boe" in Germany) from the west; and, finally, a light 
 south-west wind in rear of the whole disturbance. The 
 westerly squall is nearly perpendicular to the isobars and 
 to the lie of the rain-stripe ; but, as the isobars here are 
 not the lines of general atmospheric circulation, but are 
 partly due to purely local causes, we need not be surprised 
 that Buy Ballot's law does not hold. 
 
 Temperature was, as usual, very high in front ; very 
 low for the season in rear of the trough. If we could 
 have drawn the isotherms, we should have found them 
 running almost north and south, nearly parallel to the 
 rain-stripe. It is impossible to determine at present how 
 much of this cold is due to the mechanical transport of 
 cold air with the heavy rain, and how much to the 
 general descent of cold air in rear of the V as a whole. 
 
 The rain-stripe, we see, was in this instance a long 
 narrow band, moving broadside on, and manifestly con- 
 nected with the trough of a V-depression. 
 
 We will now give an example of line-thunderstorms 
 which are not associated with the trough of either a V or 
 a cyclone, though they also move broadside on, nearly 
 perpendicular to the depression with which they are in 
 some way associated. In Fig. 52 we give synoptic charts 
 for France at 9 a.m. and 9 p.m. (Paris time) on August 21, 
 1879. The full lines are isobars, a few arrows show the 
 general direction of the wind, and the one dotted line in 
 each marks the mean position of the thunderstorms which 
 were raging at that moment. In France the mean of the 
 time between the first and last thunder is taken to give 
 the position of the storm at a given hour. For instance, 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC BAINS. 249 
 
 if the first thunder was heard in a place at 8 a.m. and the 
 last at 10 a.m., then 9 a.m. would be marked as the hour 
 at which the storm passed the station. This method is 
 manifestly inferior to that of noting the times of first and 
 last thunder, and then plotting the shape of the storm on 
 a chart. The barometric changes during the day were 
 
 Aug. 2ii8^q. q.p.m. 
 FIG. 52. Thunderstorms in France. 
 
 really much more complicated than might appear from 
 an inspection of the maps over the limited area of France. 
 The large secondary cyclone whose' centre lay over the 
 west of France in the morning, appears to have crossed 
 that country in a north-easterly direction, and to have 
 merged in a complicated manner with a larger depression 
 which lay to the north-west of Ireland in the early morn- 
 ing. By 9 p.m., however, the whole of the south-west of 
 France was covered by an anticyclone, whose origin we 
 cannot trace. 
 
 In Fig. 53 we give a diagram of the positions at every 
 alternate hour of two sets of thunderstorms that traversed 
 
250 
 
 WEATHER. 
 
 if jr. p.m. 
 
 France during that day, which are marked A and F 
 respectively. Such lines are called " isobrontons," or 
 lines of equal development of thunder. Those in the 
 figure may be taken as typical of the march of this class 
 of thunderstorm all over Europe a long narrow line, 
 
 advancing broadside on 
 ir.p.m. towards the east or north- 
 east, pretty nearly indepen- 
 dent of the shape of isobars 
 with which it is associated. 
 
 The first storm, marked 
 A, struck the coasts of the 
 Bay of Biscay at five o'clock 
 in the morning. The shape 
 of the front was bent, the 
 ends being most in advance : 
 this is very often the case in 
 France. The position of the front of this storm is givenTor 
 every two hours' interval until 1 p.m., when the disturb- 
 ance appears to have died out near Calais. The relation 
 which the position of the thunder bore to the secondary 
 cyclone may best be seen by reference to the preceding 
 chart (Fig. 52). There the dotted line shows that the front 
 of the storm A lay to the north-east of the centre of the 
 cyclone ; and, as far as the small number of wind-arrows 
 allow us to judge of the nature of the disturbance, there 
 seems to be a small local deflection of the wind in rear of 
 the storm. The wind in rear should have been from 
 south-east with the shape of isobars there represented, 
 whereas the chart shows that in some places the direction 
 was from the west and north-west. 
 
 FIG. 53. Track of thunder- 
 storms. 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 251 
 
 The second series of thunderstorms, marked B, com- 
 menced at Biarritz at three o'clock in the afternoon, and 
 moved irregularly in a more westerly direction than the 
 morning storm. The storm, in its course during the day, 
 seems to have increased enormously, for at 9 p.m. we find 
 the front reaching nearly from Brussels to Perpignan in 
 the Pyrenees, a distance of six hundred miles. This will 
 perhaps enable us to realize the disastrous character of 
 hail and thunderstorms in France. Here we have a line of 
 destruction six hundred miles long, and from ten to twenty 
 miles broad, sweeping like a curtain across the country at 
 a rate of about thirty miles an hour, wrecking in a few 
 minutes vineyards which are worth many thousands of 
 pounds, and destroying at the last moment the husband- 
 man's labour for the whole year. 
 
 But now we come to one of the most puzzling points 
 connected with thunderstorms. If we look at Fig. 52, 
 where we have marked by a dotted line the position of 
 the storm-front B at 9 p.m., it is very difficult to see any 
 connection between the shape of the isobars and the 
 position of the thunder. So far as we can see, there is no 
 trace of the trough either of a cyclone or of a V-depression, 
 and all the general indications would have been for 
 improving weather after the passage of the secondary 
 cyclone. Whether more numerous observations at 
 stations nearer to one another would have shown the 
 presence of secondaries, we cannot say ; but this is by no 
 means an isolated instance in France, and similar cases 
 occur in other countries. For the present, therefore, the 
 explanation of the nature of this class of thunderstorms 
 must await future research ; all that we can do here is to 
 
252 WEATHER. 
 
 note them as an apparent exception to the general course 
 of weather which we have already explained. We must 
 also specially note them as cases where rain falls with 
 a steady rising barometer; and also understand that, 
 although a forecaster could not have pointed out in the 
 morning exactly when or where thunderstorms would 
 -strike, he could have said with certainty that storms 
 would occur during the day in many parts of France. 
 Secondaries can never form in summer without some 
 electrical disturbance. 
 
 The details of rain and cloud in line-squalls and 
 thunderstorms are extremely interesting, and for them 
 we are chiefly indebted to the careful researches of Dr. 
 Koppen. The approach of a thunder-squall usually 
 announces itself by the rapid crowding up of heavy 
 clouds. We see a dark, black border, often looking like 
 a long roll or wreath, and beyond this a peculiar light 
 grey uniform sky. The dark, low band passes overhead, 
 and heavy rain commences as the light grey cloud comes 
 on. The first burst of rain is usually the heaviest, and 
 after a longer or shorter period the rain usually clears 
 gradually off. This is the rain with which the sudden 
 rise of the barometer is observed. The wind, which has 
 fallen very light from south-east or south as the clouds 
 begin to bank up, comes in a violent squall from the 
 west, about the time the dark wreath passes overhead, 
 and falls again shortly after the commencement of the 
 heavy rain. A good illustration of a very pronounced 
 cloud-wreath will be found in Fig. 56 in the next chapter 
 under "Pamperos." We give an ideal diagrammatic 
 section of such a squall in Fig. 54. We may suppose 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIG RAINS. 253 
 
 that from general causes, such as the trough of a V-de- 
 pression, a cold westerly current meets a warmer one 
 from the south-east or south. The latter rises, as shown 
 by the small arrows, and curls over where the black 
 wreath (w) of cloud is found, and then the commingling 
 of the two currents forms a gigantic dark vault (v) of 
 cloud, from which heavy rain (r) pours down. The light 
 
 FIG. 54. General circulation and cloud-vault of line-squall. 
 
 grey cloud which an observer sees behind the black 
 wreath is really a peep into the rain falling from this 
 great vault. The big drops of rain bring down mechani- 
 cally with them a vast amount of cold air, which rushes 
 straight out in front of the squall it has no time to 
 pick up anything from the earth's rotation and produces 
 the squall q, marked by a long arrow. 
 
 Our section, then, of a squall is that of a vertical 
 whirl, the whole system perhaps not one mile high 
 by one and a half mile across, while the length of the 
 front of the storm may be two hundred miles ; and our 
 picture of the whole must be a long, nearly straight 
 horizontal axis, moving broadside on, round which the 
 
254 WEATHER. 
 
 wind whirls vertically in a direction opposite to that of 
 the watch-hands. It is always an episode, as it were, in 
 the history of some general form of atmospheric circula- 
 tion. We can, perhaps, see how a broad-fronted west 
 current can meet a southerly stream of air in the trough 
 of a V or cyclone ; but we are unable at present to form 
 any conception of a straight-fronted current advancing 
 across the curved isobars of an anticyclone, as in Fig. 52 
 for French thunderstorms. 
 
 Line-squalls and thunderstorms of a different type 
 are very common in the tropics. The author has observed 
 a very striking instance at the junction of the south-east 
 trade and north-west monsoon in the Indian Ocean. 
 There was no doldrum, but the two currents met along a 
 line whose position was marked by a long dark, black 
 cloud, with heavy rain and squall. 
 
 In like manner, the daily thunderstorm which occurs 
 in so many countries at the time when the sea-breeze 
 comes in, charging, as it were, the prevailing wind over 
 the land, is due to a long vertical whirl where the two 
 currents meet, and the whole length can sometimes be 
 watched gradually advancing inland from the coast. The 
 so-called north-westers at Calcutta, during the hot season, 
 belong to this last type. 
 
 THUNDERSTORMS WITH SECONDARIES. 
 
 We must now just mention a class of thunderstorms 
 which are more complicated than a simple squall, and 
 yet differ in many ways from line-thunderstorms. They 
 are associated with secondary cyclones, and are much 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 255 
 
 commoner in England than line-thunderstorms, but 
 none have been tracked over a sufficiently long area to 
 allow us to say anything about their shape or motion. 
 All we know is, that as surely as we see a secondary on 
 the charts in summer, so certainly will thunderstorms 
 occur during the day, though we cannot say in what 
 portion of the small depression. 
 
 The general features of this kind of disturbance will 
 be best understood by refer- 
 rence to Fig. 55, where we 
 give a typical example of 
 that distribution of pressure 
 which is associated with a 
 summer thunderstorm in 
 Great Britain, The isobars, 
 wind-arrows, and weather- 
 symbols give the synoptic 
 conditions of North-western 
 Europe at 8 a.m. July 3, 
 1883. The broad features 
 of pressure-distribution are 
 found in an anticyclone over 
 Scandinavia, in the fragment 
 of a large depression to the 
 west of Ireland, and in a 
 complicated mass of second- 
 aries over Great Britain and the north of France. The 
 general direction of the wind is from the south, but both 
 the bends in the isobars, which mark out the position of 
 the secondaries over Central England and the north 
 of France, are associated with a considerable deflection of 
 
 55. Conditions of thunder- 
 storms. 
 
256 WEATHER. 
 
 the wind. In both instances we see a partial circulation 
 of the wind round a central spot of calm ; the latter 
 stations are marked by the symbol of a circle with a dot 
 in the centre. At the moment when the chart was con- 
 structed, three thunderstorms were in progress : two on 
 the east coast of England at Shields and Spurn Head 
 with one in France, at L'Orient. All these stations are 
 marked with the symbol t. 
 
 The special features of this class of thunderstorm are 
 the calm sultry weather with which they are associated, 
 so different from the squall of a line-thunderstorm, and 
 the limited rotation of the surface-wind during the pro- 
 gress of the storm. Another very remarkable feature is 
 that this surface circling of the wind extends only a very 
 short distance upwards, and whenever a glimpse can be 
 caught of the drift of the upper clouds, they are found to 
 move in the same direction throughout the whole period 
 of the disturbance. 
 
 This is the familiar class of thunderstorm which we 
 associate with sultry weather, and with the thunder coming 
 against the wind. As the secondary approaches any 
 station, the wind draws more or less in towards the centre, 
 and recovers its former direction after the depression has 
 passed. In a case like that figured in the diagram, the 
 motion of the storm as a whole would be towards the 
 north-east, and the rotation of the wind at any place 
 would depend on which side of the centre the station lay. 
 . Squalls, troughs, and secondaries do not by any means 
 exhaust all the conditions under which thunder and 
 lightning may be developed, even within the limits of 
 Europe, but any attempt to examine into these other 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 257 
 
 causes would be beyond the scope of this work. We 
 have, therefore, only indicated the three principal sources 
 of electrical discharge, which, however, will account for 
 more than eighty per cent, of European thunderstorms. 
 The great thing is to realize that there are several dif- 
 ferent kinds of thunderstorms, each of which has certain 
 distinctive features. 
 
 GENERAL KEMARKS. 
 
 We may, perhaps, conclude this short notice of 
 thunderstorms with a few general remarks on the subject. 
 One of the first things which must strike everybody is, 
 that even in the temperate zone some countries are far 
 more ravaged by thunderstorms than others. For instance, 
 France suffers more than any other part of Europe, and 
 England the least. We may probably find at least two 
 causes which modify the development of thunderstorms. 
 In the first place, the geographical position of the country 
 relative to the great seasonal areas of high and low 
 pressure. From this point of view we can readily see 
 that France is far more exposed to the influence of small 
 secondaries, which come in from the Atlantic, and which 
 die out before they reach Central Europe, than any other 
 portion of that continent. If we look at the large charts 
 which we give in our chapter on Weather Types, we can 
 easily understand that whenever a cyclone leaves the 
 Atlantic anticyclone to go towards the north-east, there 
 must be left a somewhat irregular col of pressure over 
 France, and this we know is most conducive to the 
 formation of thunderstorms. 
 
258 WEATHEE. 
 
 The second cause which may modify the formation of 
 storms is the amount of vapour in the air. We know by 
 experiment that the discharge of frictional electricity is 
 very much modified by the hygrometric condition of the 
 atmosphere. In England, if we turn a machine on a 
 damp day, the electricity will escape as fast as it is made, 
 and no discharge can be obtained ; on a dry day, on the 
 contrary, sparks can easily be procured. From this 
 analogy we can easily conceive that the same atmospheric 
 disturbance which causes a violent thunderstorm in the 
 dry climate of France, would either discharge its elec- 
 tricity noiselessly, or at all events occasion but a very 
 feeble storm, in the damper atmosphere of Great Britain. 
 
 No doubt, insulated damp air is just as good a non- 
 conductor as dry air ; and the silent discharge of a 
 frictional machine in damp weather is due to the con- 
 densation of a thin layer of water on the surface of the 
 insulating supports. But in the atmosphere we cannot 
 conceive absolute insulation. The air is full of ice or 
 water-dust, and we know that somehow or other electrical 
 discharge is easily propagated from one cloud to another. 
 
 We cannot say absolutely, in our present state of 
 knowledge, how far electricity is only a secondary pro- 
 duct of atmospheric disturbance, or whether it ever plays 
 a primary part in making a storm. As far as we can see, 
 however, the evidence is entirely in favour of the idea 
 that electricity is only a secondary phenomenon. We 
 cannot suppose that an abnormal amount of electricity 
 can ever be developed without some definite cause, and 
 it is also an obvious fact, that in thundery weather there 
 are often a great many showers without thunder, which 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 259 
 
 only differ from, those with thunder and lightning in 
 violence or intensity. 
 
 On the other hand, it is almost certain that the dis- 
 charge of electricity is in some manner associated with 
 the formation of hail and very large rain-drops ; but at 
 present no complete explanation can be given of the 
 relation between these two phenomena. 
 
 NON-ISOBARIC BAINS. 
 
 We have now to deal with the most unsatisfactory 
 branch of modern meteorology the nature of those falls 
 of rain that are not associated with any definite shape 
 of isobars, and which are therefore called " non-isobaric " 
 rains. We have already described the very remarkable 
 case of line-thunderstorms whose position cannot be 
 detected by any inspection of isobars ; and numerous 
 other cases of a less striking nature are of constant 
 occurrence. The importance of this classification of rains 
 in any comprehensive treatise on meteorology may be 
 judged from the fact that in Great Britain, though the 
 bulk of winter rain is cyclonic, a great deal of summer 
 rainfall is non-isobaric; in Continental Europe a still 
 larger proportion is of the latter character ; so are most 
 tropical rains, except the downpour of hurricanes ; while 
 the whole of the heavy rain on the equator, and all that 
 falls in the doldrums, is also absolutely non-isobaric, 
 
 THE SOUTH-WEST MONSOON. 
 
 But by far the most striking non-isobaric rain in the 
 world is the burst of the south-west monsoon in the 
 
260 WEATHER. 
 
 Indian Ocean. Let us consider the course of the seasons 
 in Ceylon, and correlate them with changes of pressure 
 over India. In the month of February we find a very 
 shallow stationary depression not a cyclone over Lower 
 Bengal, a belt of high pressure stretching across the 
 Bay of Bengal from Madras to Rangoon, and a general 
 diminution of pressure from that belt to the equator. 
 From this we might reasonably expect what we find 
 light south-west wind over Lower Bengal, variable breezes 
 over Madras, and a light north-east monsoon across 
 Ceylon ; but it is not so obvious why the south-west wind 
 should be so fine and dry as it is. The low pressure over 
 Bengal gets gradually more pronounced, and spreads with 
 its accompanying south-west wind slowly southwards, till 
 Ceylon is embraced within its sphere. These conditions 
 are most pronounced towards the end of May, and we get 
 the dry, nearly cloudless, hot season of India and Ceylon, 
 with a light south-west wind. Then the sky begins to 
 cloud over, and suddenly rain bursts in a series of terrific 
 thunderstorms, and the bad wet weather continues for 
 two or three months. The rain begins in Ceylon, and 
 then works slowly up the west coasts of India and Burinah 
 omitting Madras till Calcutta and Lower Bengal are 
 reached, three or four weeks later than Colombo. Then 
 we are met by the strange fact that this, the most striking 
 weather-change in the whole year, is associated by no 
 change in the shape of the isobars. We give in our 
 chapter on Weather Types two diagrams (Figs. 82 and 
 83) of Indian isobars just after the monsoon has burst 
 over Bombay and Calcutta. The only difference in the 
 isobars then and a fortnight previously, when the hot dry 
 
SQUALLS, THUNDERSTORMS, NON-ISOBARIC RAINS. 261 
 
 season prevailed, is that the level of pressure is a little 
 lower, that the position of lowest barometer has moved 
 a little higher up the Ganges, and that the distortion of the 
 isobars by secondaries is more pronounced. When the 
 monsoon is fairly established, we can, no doubt, see certain 
 slight fluctuations in the shape and intensity of the 
 isobars which accompany what is called " a break in 
 the rains," and sometimes exceptionally heavy rain falls 
 during the passage of a small cyclone from the Bay of 
 Bengal up country ; but we cannot find any change in 
 the isobars to account for the sudden change of weather 
 which is called in common parlance, " the burst of the 
 monsoon." The quality of the rain, if nothing else, dis- 
 tinguishes the monsoon from cyclonic precipitation. The 
 rain in front of a Bengal cyclone seems to grow out of 
 the air, while that of the monsoon falls in thunderstorms 
 and from heavy cumuloform clouds. The only rational 
 suggestion which has been made to account for this burst 
 of rain, would look to a sudden inrush of damp air from 
 the region of the doldrums as the source of the change in 
 weather, but not of the direction of the wind, or of the 
 shape of the isobars ; for the burst is apparently almost 
 coincident with the disappearance of the belt of high 
 pressure to the south of the Bay of Bengal. 
 
 No satisfactory clue has, however, yet been discovered 
 either to the cause, or still less to the quantity, of non- 
 isobaric rainfalls. They are the bugbear of every 
 European forecaster, though in Japan, curiously enough, 
 they find rain easier to announce than the direction of 
 the wind. Mr. Finley, of the United States Signal Office, 
 has made some very interesting studies on local rains 
 
262 WEATHEK. 
 
 that do not show on the isobaric charts. He takes a map 
 of the United States, and puts in all the wind-arrows 
 without any isobars. Very often he finds some large 
 areas swept by a generally southerly wind, and others by 
 a generally northerly wind, and he draws lines to mark 
 out the tracts of country where these currents meet, and 
 where they diverge. Then he finds that there are always 
 local rains over the first areas, and rarely any over the 
 latter. This would undoubtedly point to local vertical 
 whirls between the meeting currents as the source of rain. 
 Whether this is universally the case, or whether the 
 conditions of all rains could be analyzed into small Y's 
 or secondaries if the isobars were constructed from stations 
 sufficiently close together, we cannot at present say. 
 The important thing is not to mix up all kinds of rain 
 together when we want to discuss general meteorological 
 problems. 
 
( 263 ) 
 
 CHAPTER IX. 
 
 PAMPEKOS, WHIRLWINDS, AND TORNADOES. 
 
 WE will now describe two remarkable kinds of storms 
 which occur in La Plata and in the United States 
 respectively. 
 
 PAMPEROS. 
 
 The word " pampero " is, unfortunately, used in a very 
 vague manner in the Argentine Republic and neigh- 
 bouring states. Every south-west wind which blows from 
 off the pampas is sometimes called a pampero ; and there 
 is a still further confusion caused by calling certain dry 
 dust-storms pamperos sucios, or dry pamperos. The true 
 pampero may be described as a south-west wind, ushered 
 in by a sudden short squall, usually accompanied by rain 
 and thunder, with a very peculiar form of cloud-wreath. 
 We will describe these as given by D. Christison in the 
 Proceedings of the Scottish Meteorological Society, No. Ix. 
 p. 330, and then we shall have no difficulty in recognizing 
 a line-squall as the source of the pampero. 
 
 The barometer always falls pretty steadily for from 
 
2G4 WEATHEK. 
 
 two to four days before the pampero, and always rises 
 for some days after the squall. There are not enough 
 barometric observations to allow of any generalizations 
 as to the precise position of the squall relative to the 
 trough of the general depression, but in two recorded 
 cases the mercury began to rise some hours before the 
 storm burst. 
 
 Temperature is always very high before the squall, 
 and then the sudden change of wind sends the thermo- 
 meter rapidly down, sometimes as much as 33 in six 
 hours. 
 
 Thunder accompanies about three out of four pamperos ; 
 but more or less rain always falls, except in the rarest 
 cases. 
 
 The wind before this class of pampero almost in- 
 variably blows moderately or gently for some days from 
 easterly points, and then with a sudden burst the south- 
 west wind comes down with its full strength, and, after 
 blowing thus from ten to thirty minutes, either ceases 
 entirely or continues with diminished force for a certain 
 number of hours. In all cases but one the upper wind- 
 currents have been seen to come from the north-west both 
 before, during, and after the pampero. 
 
 The general appearance of a pampero will be best 
 understood by a description of an actual squall. " In the 
 early morning of a day in November, the wind blew 
 rather strongly from the north-east. The sky was cloudy, 
 but not overcast, save in the south-west horizon. The 
 clouds were moving very slowly from the west, or a little 
 south of it, throwing out long streamers eastwards. 
 About 8 a.m. the threatening masses in the south-west 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 265 
 
 had advanced near enough to show that at their head 
 marched two dense and perfectly regular battalions of 
 cloud, one behind the other, in close contact, yet not 
 intermingling, and completely distinguished by their 
 striking difference of colour, the first being of a uniform 
 leaden grey, while the second was as black as the smoke 
 of a steamer. On arriving overhead, it was seen that the 
 front, although slightly sinuous, was perfectly straight in 
 its general direction, and that the bands were of uniform 
 breadth. As they rushed at a great speed under the 
 other clouds without uniting with them, preserving their 
 own formation unbroken, their force seemed irresistible, 
 as if they were formed of some solid material rather than 
 vapour. The length of these wonderful clouds could not 
 be conjectured, as they disappeared beneath the horizon 
 at both ends, but probably at least fifty miles of them 
 must have been visible, as the ' Cerro ' commands a 
 view of twenty miles of country. Their breadth was not 
 great, as they only took a few minutes to pass overhead, 
 and appeared to diminish from the effects of perspective 
 to mere lines on the horizon. At the instant when the 
 first band arrived, the wind which was still blowing, and 
 something more than gently, from the north-east went 
 round by north to south-west ; at the same time a strong, 
 cold blast fell from the leaden cloud, and continued to 
 blow till both bands had passed. From neither of them, 
 however, came lightning or rain, but, filling up the sky 
 in rear of the regular army, followed a confused rabble of 
 clouds, with a constant rumbling of thunder, and from 
 which evidently rain was falling. It was not, however, 
 till fifteen minutes after the passage of the two regular 
 
266 
 
 WEATHER. 
 
 bands that rain fell where the observations were taken. 
 The storm, passing on, obscured the whole sky, wind, rain, 
 and thunder continuing for some hours, but only to a 
 moderate degree." The diagram (Fig. 56), taken from a 
 sketch made at the time, represents the northerly half of 
 the storm-clouds while still at some distance from the 
 spectator, and advancing from a westerly direction. 
 
 From all this it is manifest that the changes of wind, 
 the rapid alternations of temperature, and the typical 
 cloud-wreaths are identical in character with the class of 
 
 FIG. 56. Cloud-wreath iu pampero. 
 
 disturbance we have described in the previous chapter 
 under the heading of Line Squalls and Line Thunder- 
 storms. We must remember, however, that being south 
 of the equator, north-east, south-west, and north-west 
 winds are equivalent to those from south-east, north-west,, 
 and south-west in the northern hemisphere. 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 267 
 
 WHIRLWINDS. 
 
 A whirlwind may be described as a mass of air whose 
 height is enormously greater than its width, rotating 
 rapidly round a more or less vertical axis. A moderate 
 whirlwind may be two hundred feet high, and not above 
 ten feet in diameter. The dimensions, however, are very 
 variable, for a whirlwind may vary in intensity from a 
 harmless eddy in a dusty road to the destructive tornado 
 of the United States. As the latter are the most terrific 
 manifestation of weather in the whole range of meteoro- 
 logy, we shall devote a few pages to their consideration. 
 
 TORNADOES. 
 
 A tornado is simply a whirlwind of exceptional 
 violence ; if it were to encounter a lake or the sea, it 
 would be called a waterspout. Its most characteristic 
 feature is a funnel, or spout, which is the visible manifes- 
 tation of a cylinder of air that is revolving rapidly round 
 a nearly vertical axis. This spout is propagated through- 
 out the northern temperate zone in a north-easterly 
 direction at a rate of about thirty miles an hour, and 
 tears everything to pieces along its narrow path. 
 
 The diameter of the actual spout often does not 
 exceed a few yards, and the total area of destructive 
 wind is rarely more than three or four hundred yards 
 across. The height of the spout is that of the lowest 
 layer of clouds, which are then never high ; and, as in 
 thunderstorms, the upper currents are unaffected by the 
 violent commotion below. 
 
268 WEATHEK. 
 
 The spout as a whole has four distinct motions : 
 
 1. A motion of translation generally towards the 
 north-east at a variable rate, but which may be taken to 
 average thirty miles an hour. 
 
 2. A complex gyration. The horizontal portion of 
 this rotation is always in a direction opposite to that of 
 the hands of a watch that is to say, in the same manner 
 as an ordinary cyclone. But in addition to this there is 
 a violent upward current in the centre of the cylinder of 
 vapour or dust which constitutes the spout, and sometimes 
 small clouds seem to dart down the outer sides of the 
 funnel whenever these float in close proximity. There 
 are, however, no authentic instances of any object being 
 thrown to the ground by the individual effort of a down- 
 ward current. The slight downward motion of a few 
 small clouds is probably only a slight eddying of a violent 
 nprush. 
 
 3. A swaying motion to and fro like a dangling whip, 
 or an elephant's trunk, though the general direction of 
 the spout is always vertical. 
 
 4. A rising and falling motion, that is to say, that 
 sometimes the end of the funnel rises from the surface of 
 the ground and then descends again, and so on. Owing 
 to this rise and fall, the general appearance of the tornado 
 changes a good deal. When the bottom of the spout is 
 some distance above the ground, the whole is somewhat 
 pointed, and does comparatively little harm as it passes 
 over any place. As the spout descends, a commotion 
 commences on the surface of the ground. This latter 
 gradually rises so as to meet the descending part of the 
 spout, ami then the whole takes the shape of an hour- 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 269 
 
 glass, as in Fig. 57. This is the most dangerous and 
 destructive form, because the ground gets the whole force 
 of the tornado. 
 
 The general appearance of the cloud over a tornado 
 or whirlwind is always described as peculiarly smoky, or 
 
 FIG. 57. Tornado-cloud. 
 
 like the fumes of a burning haystack. The tornado is 
 also never an isolated phenomenon ; it is always associated 
 with rain and electrical disturbance. 
 
 The destructive effects of the tornado are very curious, 
 from the sharp and narrow belt to which the injury is 
 confined. It appears that in the passage of some 
 tornadoes wind-pressures of various amounts, from eighteen- 
 
270 WEATHER. 
 
 to a hundred and twelve pounds per square foot, have 
 been demonstrated by destruction of bridges, brick build- 
 ings, etc. The upward pressures are sometimes as 
 great as the horizontal, and even greater. Downward 
 pressures or movements of wind have not been clearly 
 proved. Upward velocities of 135 miles per hour seem 
 not to be unusual, and horizontal velocities of eighty 
 miles have been recorded with the anemometer. The 
 destructive wind- velocities are confined to very small 
 areas. A destruction of fences, trees, etc., is often visible 
 over a path many miles long and a few hundred yards 
 wide, but the path of greatest violence is very much 
 narrower. The excessive cases above referred to are 
 observed only in small isolated spots, less than a hundred 
 feet square, unequally distributed along the middle of 
 the track. Thus, in very large buildings, only a small 
 part is subject to destructive winds. In different parts 
 of this area of maximum severity, the winds are simul- 
 taneously blowing in different, perhaps opposite, direc- 
 tions, the resultant tending not to overturn or carry off 
 or crush in, but rather to twist round a vertical axis. 
 Buildings are generally lifted and turned round before 
 being torn to pieces. As the chances are very small that 
 a building will be exposed to the violent twisting action, 
 it is evidently the average velocity of rectilinear winds 
 within the path of moderate destruction that it is most 
 necessary to provide against in ordinary structures. These 
 winds may attain a velocity of eighty miles an hour over 
 an area of a thousand feet broad, and generally blow from 
 the south-west ; the next in frequency blow from the 
 north-west. The time during which an object is exposed 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 271 
 
 to the more destructive winds varies from six to sixty 
 seconds. An exposed building experiences but one stroke, 
 like the blow of a hammer, and the destruction is done. 
 Hence, in a suspension-bridge, chimney, or other structure 
 liable to be set into destructive rhythmic vibrations, the 
 maximum winds do not produce such vibrations. The 
 duration of the heavy south-west or north-west winds 
 over the area of moderate destruction is rarely over two 
 minutes. The motion of translation of the central spout 
 of a tornado, in which there is a strong vertical current, 
 is, on an average, at the rate of thirty miles an hour. 
 
 Tornadoes mostly occur on sultry days and either in 
 the south-east or right front of cyclones, or in front of 
 the trough of V-depressions. 
 
 The relative frequency of tornadoes is, in order of de- 
 creasing frequency, June, July, April, May, . . . January. 
 In the geographical distribution of 247 tornadoes from 
 1794 to 1878, the largest figures are obtained from New 
 York (24), Indiana (20), Illinois (20), Ohio, and Georgia 
 (16 each), etc. ; but the records are fragmentary, and now 
 Kansas is the most tornado-stricken state. The largest 
 number of tornadoes apparently occur between 5 p.m. and 
 6 p.m. ; the next between 4 p.m. and 5 p.m. 
 
 The following example will illustrate all the principal 
 features of tornadoes. It is taken from one of the reports 
 of the United States Signal Office, and the author is 
 indebted to the chief signal-officer for supplying him 
 with the materials. The most important part of the 
 work has been done by Mr. Finley of that office, who 
 for many years has levoted his special attention to the 
 subject. 
 
272 
 
 WEATHER. 
 
 In Fig. 58 we give a synoptic chart of the meteoro- 
 logical conditions of a portion of the United States on 
 May 5, 1879, at 4.35 p.m., Washington time. The course 
 of the great rivers Mississippi, Missouri, and Ohio, together 
 with the outline of Lake Michigan, are clearly marked 
 
 Local. 
 
 3-55-P-- 
 
 FIG. 58. Conditions and paths of tornadoes. 
 
 by thin lines ; and the position of the cities of Chicago, 
 St. Louis, Cincinnati, Cairo, and Montgomery will be 
 readily recognized by their initial letters ; so that there 
 is no need to indicate the boundaries of any state. The 
 isobars are shown by bold black lines, and the arrows 
 
PAMPEROS, WHIRLWINDS, AND TOENADOES. 273 
 
 fly with the wind at the different stations. No less than 
 eleven tornadoes ravaged the Western States during that 
 day; the course of each of these is shown on the map 
 by a line of small circles, joined together, and those 
 numbered 1, 2, 3, and 4 respectively were apparently 
 actually in progress at the moment to which the chart 
 refers. Local time is indicated by figures on the top of 
 the diagram. 
 
 The broad features of pressure-distribution are suffi- 
 ciently simple. One area of high pressure lies over 
 Manitoba ; another over the southern states ; a col lies 
 between them. On the previous day this col had been 
 filled by a V, which by this morning had partially 
 developed into the secondary cyclone which now lies over 
 the Western States. A portion of the original V is still 
 seen just to the west of Lake Michigan. The trough of 
 the V is undoubtedly connected with the trough of the 
 secondary, but the latter appears to be made up of several 
 subsidiary depressions ; the general direction of the wind 
 is typical of such isobars. In front of the V and secondary 
 the general sweep of the wind is from the south a little 
 more south-east in some places, and a little more south- 
 west in others. 
 
 In rear of the trough, the general direction of the 
 wind is northerly a little more north-east in some places, 
 and a little more north-west in others. The formation 
 of tornadoes appears to have been associated everywhere 
 with the secondary, but at the moment for which the 
 chart is constructed, the tornado marked 4 seems to have 
 been produced by a different disturbance from that which 
 caused numbers 1, 2, and 3. If we had more barometric 
 
274 WEATHER. 
 
 observations, we should most probably find that the 
 projection of the isobar of 29 ? ins. which surrounds 
 number 4 tornado, was really a separate subsidiary 
 depression. It should also be noticed that all the eleven 
 tornadoes moved in the same general direction, which was 
 practically identical with that of the whole system of 
 depressions. 
 
 The general character of all tornadoes is so similar 
 that the description of one will do for all. We shall 
 therefore give some of the description furnished by an 
 eye-witness to the United States Signal Office, of the 
 tornado marked 3 in the chart (Fig. 58), which is de- 
 scribed in the reports as the " Delphos " tornado. 
 
 " On Friday morning, May 30, 1879, the weather was 
 very pleasant, but warm, with the wind from the south- 
 east, from which direction it had blown for several days. 
 The ground was very dry, and no rain had fallen for a 
 number of weeks. About 2 p.m. threatening clouds 
 appeared very suddenly in the west (against the wind), 
 attended in a few minutes by light rain, the wind still in 
 the south-east. It stopped in about five minutes, and 
 then commenced again, wind still the same, accompanied 
 by hail, which was thick and small at first, but rapidly 
 grew less in quantity and larger in size, some stones 
 measuring three and a half inches in diameter, and one 
 was found weighing one-fourth of a pound. This last 
 precipitation continued for about thirty minutes, after 
 which a cloud in the shape of a water-spout was seen 
 forming in the south-west, and moving rapidly forward to 
 the north-east. The cloud from which the funnel de- 
 pended, seen at a distance of eight miles, appeared to be 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 275 
 
 in terrible commotion ; in fact, while the hail was falling, 
 a sort of tumbling in the clouds was noticed as they 
 came up from the north-west and south-west, and about 
 where they appeared to meet was the point from which 
 the funnel was seen to descend. There was but one 
 funnel at first, which was soon accompanied by several 
 smaller ones, dangling down from the overhanging clouds 
 like whip-lashes, and for some minutes they were appear- 
 ing and disappearing like fairies at a play. Finally one 
 of them seemed to expand and extend downwards more 
 steadily than the others, resulting at length in what 
 appeared to be their complete absorption. This funnel- 
 shaped cloud now moved onward, growing in power and 
 size, whirling rapidly from right to left, rising and 
 descending, and swaying from side to side. When within 
 a distance of three or four miles, its terrible roar could be 
 heard, striking terror into the hearts of the bravest." The 
 eye- witness judged that the funnel itself would reach a 
 height of about five hundred feet from the ground. As 
 the storm crossed a river, a cone-shaped mass came up 
 from the earth to meet it, carrying mud, debris, and a 
 large volume of water (Fig. 57). The cloud then passed 
 the observer's house very near to 4 p.m. The progressive 
 velocity at the time was considered to be about thirty 
 miles per hour, although at Delphos, three and a half 
 miles distant, it had slackened down to near twenty miles. 
 A few minutes previous to and during the passage of the 
 funnel, the air was very oppressive ; but ten minutes 
 after, the wind was so cold from the north-west that it 
 became necessary to wear an overcoat when outside. 
 As the tornado struck the house, another member of 
 
276 WEATHER, 
 
 the family says, " We think it is coming near us. We 
 can now see its fury. Shall we leave the house ? No ; 
 for we are not certain on which side it will pass. We are 
 apparently as safe here as elsewhere. The windows are 
 nailed fast. Three of us lean against the door which is 
 nearest the storm; the rest go into the cellar. It is 
 about 4 p.m. A moment of breathless suspense, and the 
 storm strikes us. The timbers creak, the sides of the 
 house sway in and out; surely they cannot outlast it. 
 We hear no well-defined roar now, for on the outside 
 boards and other debris are fiercely crashing. All is dark 
 within. In about fifteen minutes the storm is over. We 
 leave the house. The centre of the storm has passed to 
 the west of us, and we can see its dark form moving away 
 in a north-east direction." 
 
 The actual diameter of this storm appears to have been 
 only forty-three yards. On the right of the track, destruc- 
 tive winds extended to a further distance of from one to 
 two miles, sensible deflected winds for another mile and a 
 half, beyond which only the usual wind of the day was 
 experienced. On the left or northern side of the tornado 
 path, the damage did not extend quite so far, for the 
 width of the belt of destructive winds was not more than 
 twenty-eight yards across, and that of sensibly deflected 
 winds one mile and a quarter. 
 
 As a specimen of the damage done, a large two-horse 
 sulky plough, weighing about seven hundred pounds, was 
 carried a distance of twenty yards, breaking off one of 
 the iron wheels attached to an iron axle one and three- 
 quarter inches in diameter. A woman was carried to the 
 north-west two hundred yards, lodged against a barbed- 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 277 
 
 wire fence, and instantly killed. Her clothing was entirely 
 stripped from her body, which was found covered with 
 black mud, and her hair matted with it. A cat was 
 found half a mile to the north-west of the house, in which 
 she had been seen just before the storm, with every bone 
 broken. Chickens were stripped of their feathers, and 
 one was found three miles to the north-west. 
 
 A few miles further on, another eye-witness says, 
 " The dark, inky, funnel-shaped cloud rapidly descended 
 to the earth, which reaching, it destroyed everything 
 within its grasp. Everything was taken up and carried 
 round and round in the mighty whirl of the terrible 
 monster. The surrounding clouds seemed to roll and 
 tumble towards the vortex. 
 
 " The funnel, now extending from the earth upwards 
 to a great height, was black as ink, excepting the cloud 
 near the top, which resembled smoke of a light colour. 
 Immediately after passing the town, there came a wave 
 of hot air, like the wind blowing from a burning building. 
 It lasted but a short time. Following this peculiar 
 feature, there came a stiff gale from the north-west, cold 
 and bleak, so much so that during the night frost 
 occurred, and water in some low places was frozen." 
 
 EELATION OF WHIRLWINDS TO CYCLONES. 
 
 Before concluding this chapter, we may make a few 
 remarks on a very interesting question which here pre- 
 sents itself. Commencing with a whirlwind only two or 
 three feet across, we find every gradation of size till 
 we come to the destructive tornado. From the small 
 
278 WEATHER. 
 
 secondary which deflects the wind in connection with a 
 thunderstorm, there seems to be every gradation of size 
 into the secondary which is so large that we can hardly 
 say whether it should not be called a primary cyclone. 
 
 In both the whirlwind and cyclone series we have 
 certain common features a horizontal rotation, and more 
 or less uptake near the centre of the gyration. But is 
 there any intermediate series between the whirlwinds and 
 the cyclones, or can the former ever develop into the 
 latter ? We believe not, though the opposite opinion has 
 often been propounded. 
 
 In the first place, we are unable to find any connecting 
 link between the two types of rotation. Under certain 
 conditions, the wind seems to have a tendency to form 
 little eddies, which under favourable circumstances may 
 grow into complete cylinders of rotating dust. In our 
 chapter on Prognostics we have shown that in England 
 whirling dust is a well-known precursor of showers of 
 rain, but not of the true cyclone-rain. In other countries, 
 such as the Punjab and on the Isthmus of Suez, regular 
 whirlwinds are of daily occurrence at certain seasons of 
 the year ; but these never by any chance grow into even 
 the smallest secondary. We have just seen that the 
 terrific tornadoes of the Western States of the American 
 Union are merely an episode in the conflict of opposing 
 currents near the trough of larger depressions, but the 
 whirlwinds never give rise to any larger disturbance. In 
 every one of the eleven tornadoes which occurred on the 
 day we have just described in some detail, it was found 
 that rain and hail invariably preceded the tornado-cloud 
 from ten to thirty minutes, and that the tornado was 
 
PAMPEROS, WHIRLWINDS, AND TORNADOES. 279 
 
 only, as it were, a local accident in a very large dis- 
 turbance. 
 
 In our chapter on Weather-types, we shall give 
 abundant illustrations of the manner in which both 
 primary and secondary cyclones are formed without the 
 presence of two such opposing currents as are found in 
 front and rear of the trough of a Y-depression, and we 
 shall see how the two kinds of cyclones may either 
 develop or degrade from one into the other. 
 
 We have also already seen the very small circulation 
 of the wind which accompanies some kinds of thunder- 
 storms, but in no case do we find any transitional link 
 between the whirlwind and cyclone types of rotatory 
 motion. 
 
 At the same time, we may note that the inner core 
 and very deep central depression of a tropical hurricane 
 approximates more nearly to the tornado type than the 
 cyclones of temperate regions ; but the absence of tran- 
 sitional forms seems conclusive against the identity of 
 tornadoes and cyclones. In both the destructive fury is 
 out of all keeping with any forces that we are acquainted 
 with ; and their true nature remains to be discovered by 
 future research. 
 
 . 
 . 
 
280 WEATHER, 
 
 CHAPTER X. 
 
 LOCAL VARIATION OF WEATHER. 
 NATURE AND PRINCIPLES. 
 
 THE object of this chapter is to explain what is known as 
 the local variation of weather. This term groups con- 
 veniently a large class of dependent phenomena, which 
 owe their origin to the influence of local obstacles or 
 peculiarities on the development of weather. We know 
 that in the same country some places are much colder or 
 wetter than others; that some are more exposed to 
 destructive gales; and that others are more frequently 
 ravaged by disastrous hailstorms. We will now en- 
 deavour to show why this should be so, and how the pro- 
 ducts of this variation are related to the general principles 
 of the dependence of weather on the distribution of 
 atmospheric pressure which we have already described so 
 fully. 
 
 If we watch the actual occurrence of any local 
 peculiarity of weather, we shall soon find out that in every 
 instance it is the intensity, and not the general character, 
 which is altered. For instance, two places a few miles 
 
LOCAL VARIATION OF WEATHER. 281 
 
 apart may differ by 10 of temperature on a frosty 
 morning. No local cause has formed the distribution 
 of pressure which gives the necessary calm. That stillness 
 has been developed by general causes, while it is local 
 peculiarities of exposure, etc., which have enabled radia- 
 tion to be so much more powerful in one place than 
 another. Similarly, an inch of rain may fall in one place, 
 and only a few drops in another not far distant. But if we 
 think of the day on which this occurred, we shall remember 
 that it was cloudy and showery naturally. The difference 
 of actual rainfall was either due to one place catching a 
 heavy shower which did not affect the other, or else some 
 local peculiarity of the one, which increased the amount 
 of precipitation that would otherwise have been induced 
 by a cyclone of any given intensity. 
 
 These instances might be multiplied indefinitely, and 
 it is from the observation of innumerable cases that we 
 are enabled to lay down the general law that the primary 
 character of all weather is given by the general distribu- 
 tion of surrounding pressure ; the local variation modifies 
 but never alters this general character. By this means 
 we are able to steer our way through many intricacies of 
 weather which would otherwise present hopeless diffi- 
 culties, and to explain many phenomena which would 
 otherwise be inexplicable. Hence we see the appropriate- 
 ness of the word " variation," as applied to the modification 
 due to local causes. 
 
 The cases which present themselves in practice are 
 endless. Every country, every part of a country, has a 
 set belonging to itself, and the local meteorologist has 
 to work out the details for his own neighbourhood, just as 
 
282 WEATHER. 
 
 the geologist explains the local peculiarities of his own 
 scenery by the combination of general and local causes. 
 Local is also like diurnal weather, in so far that the 
 observed weather is the sum of the local variations and 
 general causes. When the general are strong, the local 
 are entirely masked; when the general are weak, then 
 the local become of primary importance. We shall con- 
 fine ourselves to a few examples relating to cloud, rain, 
 and hail, so as to exemplify the general principles- 
 involved. 
 
 LOCAL CLOUD. 
 
 By far the most important and difficult source of 
 local variation of weather is found in the development 
 of cloud, rain, and other forms of precipitation by the 
 influence of seas, lakes, rivers, hills, and valleys ; some of 
 these phenomena are so interesting that we propose to 
 devote a few pages to their consideration. 
 
 We will commence with cloud, though we must re- 
 member that in most cases cloud is only undeveloped 
 rain, and that the same cause which, when slight, gives 
 cloud will give rain when more intense. In our diagram 
 of cyclone-weather and prognostics (Fig. 2), we have 
 marked cumulus-cloud in rear of the trough. In England 
 this holds all through the year, but in the drier climate 
 of Continental Europe it is only true during the summer 
 months. The reason is simply that, in cold climates, 
 there is only sufficient vapour to develop that form of 
 cloud in summer. What we have specially to note is,, 
 that when cumulus does not form, it is not replaced by 
 
LOCAL VARIATION OF WEATHER. 283 
 
 any other kind of cloud, but the sky clears without any 
 cloud at all. For instance, no local variation could ever 
 turn the cumulus of the rear into the cirrus-stratus of the 
 front of a cyclone ; the quantity, not the quality, could 
 alone be changed. Similarly for rain. We shall see 
 directly that the actual amount may vary enormously 
 from local causes, but no peculiarity can turn the drizzling 
 rain of a cyclone-front into the heavy, big-dropped shower 
 of a thunderstorm, or vice versa. 
 
 A very striking illustration of the influence of local 
 peculiarities in the formation of cloud was once observed 
 by M. Flammarion during a balloon-voyage from Paris to 
 the Ehine. He saw one afternoon that cloud was formed 
 over the rivers and woods, but not over the open plains. 
 The synoptic charts for that evening show that Eastern 
 France was then covered by a large cyclone of moderate 
 intensity ; and the explanation of the whole is, that all 
 the air in that part of Europe was then in a rising con- 
 dition, but that it was only over rivers and damp woods 
 that enough vapour was present to condense into cloud. 
 A more intense cyclone would have developed cloud 
 everywhere, and rain only over the rivers and forests ; 
 another still more intense would have brought rain every- 
 where. In our chapter on Prognostics, we alluded to mist 
 being formed over rivers in fine frosty weather. Here, 
 too, we have local variation, but of a contrary nature to 
 that which we have just considered. This example will 
 serve to call attention to the great importance of dif- 
 ferentiating between the various kinds of condensed 
 vapour. 
 
 Another very common local cloud is that which rests 
 
284 WEATHER. 
 
 on or over hilltops, when blue sky covers the plains. 
 This, of course, is due to the horizontal currents of the 
 air being deflected upwards, and, if sufficient vapour is 
 present, cloud is formed by condensation. The most 
 interesting thing about these clouds is that they remain 
 stationary as a whole, though their outlines and con- 
 stituent particles are in constant motion. Their prog- 
 nostic value has been already explained in a previous 
 chapter. 
 
 LOCAL KAIN. 
 
 We shall now explain a few of the principal causes 
 which affect the quantity of rainfall. One of the com- 
 monest and most obvious is that, when the wind which 
 blows over water first meets the land, rain will be pre- 
 cipitated. For instance, in England, with a cyclone of 
 moderate intensity and a westerly wind, rain will only 
 fall on the western coasts and on the high ground inland. 
 With an east wind, on the contrary, the fall will be 
 confined to the most exposed portions of the east coast, 
 and in a less degree to high inland stations. A similar 
 effect is found all over the world. For instance, in 
 Ceylon the rainy seasons on the two sides of the island 
 are in different months, which depend on the time when 
 each coast is exposed to the prevailing monsoon. The 
 south-west monsoon brings rain to the exposed west side 
 of the island, and the dry season to the east coast, which 
 is then a lee shore. The north-east monsoon, on the 
 contrary, first strikes the east coast, and develops abundant 
 rainfall there; while the west coast then enjoys its dry 
 
LOCAL VARIATION OF WEATHER. 285 
 
 season. A similar sequence is observed on the opposite 
 coasts of the island of Luzon, in the Philippines, and for a 
 similar reason. 
 
 But though the sea may assist in the development of 
 rain under suitable circumstances, the presence of water 
 alone will not cause rain. The rainless districts of Peru 
 and Arabia both border on the sea-coast, but no rain falls 
 in either country. In the former the persistent anti- 
 cyclone which habitually covers that country does not form 
 the ascensional current necessary for rain, though the air 
 is damp enough to deposit very copious dew; in the 
 latter, though the isobars are sometimes curved cycloni- 
 cally, the rising currents never seem to be sufficiently 
 strong or vapour-laden to produce rain. 
 
 Allied to the influence of water in supplying an 
 abundant source of vapour, is the presence of a damp 
 surface, such as a thick forest. The leaves of the trees 
 retain so much moisture that the air is always damper 
 over wood than over earth. Then, as we have just 
 explained in talking about the development of cloud, 
 when an ascensional impulse is propagated over the 
 country, rain will sometimes fall over the woodlands 
 when it would not be precipitated over the cultivated 
 soil. For instance, in the State of Iowa, Dr. Hinrichs has 
 shown that the amount of mean rainfall is very materially 
 influenced by the position of the timber-line, and numerous 
 similar cases have been recorded in other parts of the 
 world. 
 
 But observations on this point are very discrepant. 
 In some parts of Germany the influence of forests is said 
 to be enormous; in India the effect appears to be less 
 
286 WEATHER. 
 
 marked ; while the Swedish meteorologists can find little 
 relation between rainfall and the covering of the earth. 
 The facts are doubtless as they have been described ; and 
 the apparent discordance is only another example of the 
 great principle that all meteorological results are the 
 balance of various circumstances. The difference in 
 humidity over trees or soil will be much less in a cold 
 country like Sweden, than under a blazing Indian sun ; 
 and a great deal will depend on whether the principal 
 rainfall is induced by cyclones or secondaries. 
 
 MOUNTAIN BAIN. 
 
 So far we have treated of water and forest as merely 
 supplying the material for rain ; now we must consider a 
 little more in detail how hills and valleys will affect the 
 precipitation of any current. In England, roughly speak- 
 ing, if a range of hills under about fifteen hundred feet 
 obstructs the prevailing westerly wind, the greatest 
 amount of rain will fall on the east side of the range. 
 This is because, though the moist current is deflected 
 upwards on the west side, the condensed vapour is blown 
 over the top of the hill and falls on the opposite slope. 
 If the range is over fifteen hundred feet, then the rain 
 cannot blow over, and the greatest rain will fall on the 
 west side of the hills. No rule can, however, be laid 
 down except in very general terms, for every hill and 
 every valley has its own local peculiarities in the manner 
 in which it develops rain with different winds. Every 
 country, and every part of each country, must be worked 
 out in detail on the spot. 
 
LOCAL VARIATION OF WEATHER. 287 
 
 The amount which will be deposited at different 
 heights will also vary from a number of circumstances. 
 For instance, on the west coast of Scotland, which is con- 
 stantly exposed to south-west winds, the rainfall on the 
 low western islands is only about forty inches in the year ; 
 while along the watershed, which forms the backbone of 
 Scotland, the precipitation exceeds one hundred inches in 
 many places. In Ceylon, to which we have already 
 alluded, the fall on some coast stations does not rise above 
 thirty-four inches, while some of the mountain stations 
 record no less than two hundred and nine inches. The 
 difference is readily accounted for when we consider the 
 relative altitudes of the respective mountains and the 
 greatly increased quantity of vapour which an air-current 
 of 90 temperature can carry, compared with one of only 
 40. These two latter numbers represent about the mean 
 temperatures of the two countries ; and while the water- 
 shed of Scotland rarely rises above two thousand five 
 hundred feet, many of the mountains of Ceylon attain an 
 altitude of six thousand feet. 
 
 VALLEY EAIN. 
 
 It is this property of mountains in developing rain 
 which gives truth to the well-known saying, " Hills draw 
 rain." But there are two sources of rain which are 
 intensified by valleys the rain of thunderstorms and 
 tidal showers. These we must now consider. In Great 
 Britain it is a common remark that thunder-showers have 
 a tendency to run along the course of rivers. The only 
 -class of thunderstorm which does not follow this rule is 
 
288 WEATHER. 
 
 that particular kind which occurs in the winter months 
 on the exposed western coast of Scotland and Ireland. 
 These are certainly thunder-squalls, which belong to large 
 cyclones, much developed by mountains. In France and 
 other countries an immense amount of labour has been 
 expended in tracking thunderstorms, as we have already 
 mentioned in a preceding chapter. Though the storms 
 as a whole travel in a north-easterly direction for short 
 distances, the course of rivers is found to exercise a very 
 powerful influence both on their path and still more on 
 their intensity. Forests and hills also modify the 
 development of thunderstorms to a less extent, so that we 
 may conveniently consider them all together. 
 
 LOCALIZATION OF HAILSTORMS. 
 
 
 
 But first we may give an example of the actual facts. 
 Since hail may be considered as the most intense form of 
 a thunderstorm, we have given in Fig. 59 a reduction of 
 a chart illustrating the distribution of hail in the French 
 Department of Loiret. 
 
 The river Loire will be readily recognized running 
 across the diagram from right to left, as well as some of 
 its smaller tributaries. A well-known conventional symbol 
 marks the limits of the forest of Orleans, while the small, 
 round points indicate the number of years in which any 
 .commune has been attacked by hail during the thirty 
 years 1836-1865. The scale of miles shows what a small 
 area we have to deal with ; but see what a difference in 
 the number of hailstorms. In the town of Orleans, 011 
 the river Loire, sixteen destructive hailstorms have been 
 
LOCAL VARIATION Ofr WEATHER. 
 
 289 
 
 recorded ; and at the village of Jangeau, a little higher up 
 the river, twelve serious falls. Yet within a couple of 
 miles of both places, other communes, such as Clery, have 
 not been visited by more than two or three storms. Then 
 observe the influence of the mass of forest to the north of 
 
 Cler 
 
 FIG. 59. Localization of hail iu Loiret. The points indicate the 
 number of years in which any commune has been attacked by hail 
 during the thirty years 1836-1865. 
 
 the river. All the villages to the west of the forest area 
 have a large number of points, while those inside the' 
 forest enjoy almost complete immunity from destructive 
 hailstorms. 
 
290 WEATHER. 
 
 The forest seems to act, in fact, as a breakwater, against 
 which the violence of the storms expends itself for a time, 
 till it can gather fresh force. Almost all the French 
 observers are agreed as to the origin of this development 
 and protection. We must recall the fact which we 
 mentioned in our chapter on thunderstorms, that the wind- 
 circulation of thunderstorms is always confined to the low 
 strata of the atmosphere only. 
 
 Hail is produced when two clouds are superimposed at 
 a certain distance. A storm is never isolated. The 
 ordinary French ones are often formed by partial deriva- 
 tions taken from the south-west winds, and occasioned by 
 the passage of cyclones. When the thick mass of cloud 
 which marks the existence of a storm meets a valley, the 
 lower clouds are diverted from the general route ; a 
 portion follows more or less exactly the contours of the 
 valley. Thus it happens that the clouds passing at a 
 great height from the ground cross those which, entangled 
 in the valley, have been deflected by the successive bends 
 of the river into a direction different from the south-west. 
 It is then that hail falls. 
 
 In the same way, when the lower clouds meet such an 
 obstacle as a forest, or even a mountain, eddies are formed. 
 The masses of cloud come back on themselves, and seem 
 to be repelled and dispersed by the forest. When the 
 clouds have succeeded in passing the obstacle, their force 
 is exhausted, and they only precipitate rare or inoffensive 
 hail, and do not regain their intensity for some time 
 afterwards. 
 
 From this we can readily understand the manner in 
 which hills and valleys develop their respective rains. 
 
LOCAL VARIATION OF WEATHER. 291 
 
 In a large, deep cyclone, hills deflect the moist currents 
 upwards on a large scale, and the greatest rain falls in the 
 mountains. In shallow secondaries, with slight general 
 wind-force, the rivers and forests give rise to local eddies, 
 which for some reason develop the precipitation of rain, 
 and especially of hail. 
 
 TIDAL SHOWERS. 
 
 Tidal showers are of very little practical importance, 
 but they may advantageously be mentioned as belonging 
 to the class of rain which hugs the valleys, and not the 
 hills. These showers are so called because they are 
 brought up by the tide, either along the coast or up tidal 
 rivers. How the rising water should develop rain, we 
 cannot explain, but the character of the influence is very 
 obvious. On a cloudy day, when showers or heavy masses 
 of vapour are flying about, it is frequently observed that 
 after the tide turns to rise, and the stream is running 
 upwards, the weather begins to get worse, so that what 
 was merely a mass of cloud before will now precipitate 
 rain. This rain is quite local, and does not extend far 
 from the river-banks. 
 
 In calm weather, a wind also often comes up with the 
 tide, or if the flow of the tide assists the general direction 
 of the wind, the latter will be much increased in force 
 and gustiness. If the day is really fine, of course the 
 tide will not bring up any rain, though it may modify 
 the wind. 
 
 From this description, the general nature of tidal 
 action on weather will be sufficientlv obvious. For some 
 
292; WEATHER. 
 
 reason or other the rise of the tide increases the intensity 
 of the existing system of weather. If this is tending 
 towards precipitation, the tide will give just the last impulse 
 which is required, and rain will fall. If the ascensional 
 impulse is strong enough of itself, it will rain independently 
 of the tide ; and if the natural impulse is downwards, as 
 in an anticyclone, no tide is sufficient to invert the general 
 character of the weather and cause rain. Tidal influence 
 on weather is found all over the world. Professor Hazen 
 has found a marked increase of thunderstorms with a 
 rising as opposed to a falling tide in the United States ; 
 and the author has observed a well-defined tidal variation 
 of the trade wind in tropical Fiji. 
 
 We can, therefore, sum up the contents of this chapter 
 very easily. All over the world local influences modify, 
 but do not make, the general character of the weather. 
 When the latter is weak, local weather may be the 
 prominent feature of the climate of any place ; when it is 
 strong, then local influences may be entirely obliterated. 
 
( 293 ) 
 
 CHAPTER XL 
 
 DIURNAL VARIATION OF WEATHER. 
 
 IN this chapter we propose to explain how to collate the 
 variations of weather that are found in many places to 
 depend on the hour of the day, with the great principles 
 of the relation of weather to the distribution of atmo- 
 spheric pressure which lie at the bottom of all modern 
 meteorology. In many places the direction of the wind 
 changes reguiaiiy at certain hours, or cloud and rain 
 gather at the same time day after day ; how can all this 
 be reconciled with the laws of the dependence of wind on 
 gradient, and of weather on the shape of isobars ? 
 
 The cases which arise in practice are endless. Every 
 country, every season, has its characteristic diurnal 
 weather, and a complete account of these variations in 
 some climates would more than fill the whole of this 
 volume. We must, therefore, content ourselves with a 
 statement of the general principles which are found by 
 observation to regulate all diurnal variations, and with 
 illustrations of a few typical examples from various parts 
 of the globe. 
 
294 WEATHER. 
 
 INDEPENDENCE OF DIURNAL VARIATIONS AND GENERAL 
 CHANGES. 
 
 The great principle which underlies all diurnal 
 weather is that diurnal variation modifies but never 
 alters the general character of the weather, which is 
 determined by the distribution of surrounding pressure. 
 In England the amount of cloud or rain in a cyclone 
 will vary at different hours, but the kind of cloud and 
 quality of the rain will never be altered. In like manner, 
 the land and sea breezes at Bombay will veer or back 
 with the sun, but the general character of the wind due 
 to the monsoon of the season will never be lost. This law 
 of weather not only enables us to explain many phenomena 
 of weather which would otherwise present a chaos of 
 discordant observations ; but it also serves to guide our 
 research into the great problems of weather- forecasting. 
 When once we know that we may safely neglect all con- 
 siderations of diurnal variations when we wish to study 
 the motion of depressions and the consequent changes 
 of weather, our task is thereby much simplified. If we 
 were to discuss the statistical values of meteorological 
 elements, we should find that all the voluminous results 
 which have been obtained by the method of averages are 
 of no use in forecasting, and that the diurnal, values of 
 wind or rain which have thus been obtained have nothing 
 to do with weather-changes. 
 
 DIURNAL TEMPERATURE. 
 
 In this independence of the general changes of 
 weather, diurnal are very like local variations ; but there 
 
DIURNAL VARIATION OF WEATHER. 295 
 
 is one important difference that diurnal variations intro- 
 duce us for the first time in this book to the consideration 
 of the true nature of all meteorological periodicities. 
 From the variations which run through their entire 
 course in one day, we can readily pass to those whose 
 period is one year, or even a longer cycle. 
 
 We shall therefore commence with an account of the 
 nature of the diurnal variations of heat, as that is the 
 most obvious of all meteorological phenomena. It is 
 evident to all the world that, whatever the temperature 
 of the day or season may be, the nights are in a general 
 way colder than the clays ; this is the ordinary instinctive 
 idea of diurnal variation. We also know almost by 
 instinct whether it is a generally cold or hot day ; and 
 equally instinctively we allow for the difference of day 
 and night. Put into the formal language of ordinary 
 meteorology, the general heat or cold of the day is ex- 
 pressed by the number which gives the mean temperature 
 of the day ; while the diurnal variation or range is given 
 by the numbers which denote how much the thermometer 
 was above or below the average at each hour. In a 
 climate like that of England, most people have a vague 
 idea that sometimes in winter the weather gets warmer in 
 the evening, after a white frost in the morning ; but 
 when we come to examine self-recording thermograms, 
 we find that irregular changes of temperature are much 
 more common than is usually supposed. W"e have already 
 alluded to this subject in our chapter on Meteograms, but 
 we give here, in Fig. 60, the thermogram at Kew, 
 December 7 to 9, 1874 that is, for the same period as 
 the meteogram and charts in Figs. 25, 26, and 27 ; and 
 
296 
 
 WEATHER. 
 
 in addition to this we give in Fig. 61 the mean annual 
 curve of diurnal variation of temperature at Kew, as 
 deduced by Mr. Eaton. The point which most concerns 
 
 i I j I i I 
 
 5 
 
 jo 
 
 Dec 
 
 TT 
 
 1874 
 
 Noon. 
 
 I I I I I i 1 
 
 FIG. 60. Thermograms. 
 
 us here is to understand exactly what the significance of 
 the curve of mean temperature is. The thermographic 
 trace for three days at Kew, in Fig. 60, shows very fairly 
 ( Noon f~ Q o what temperature-changes are really 
 like from day to day. In spite of 
 endless irregularities, there is a gene- 
 ral tendency in the first two days for 
 the hottest time of the day to occur 
 in the afternoon, and the coolest in 
 the early morning; while on the third 
 day there is little trace of diurnal 
 , variation, but a steady general fall 
 
 FIG. 61. Mean diurnal _ J ' 
 
 range of temperature, of temperature due to general causes. 
 
 When the mean temperature for every 
 
 hour is taken on a great many days, the irregularities 
 
 balance out, and the daily tendency only is reflected in the 
 
 mean curve of diurnal range. 
 
 If we lived in a vapourless climate, the sun would 
 
DIURNAL VARIATION OF WEATHER. 297 
 
 impose every day a similar record on the thermograph, 
 only varying a little in amount according to the season. 
 But in practice the sun has a daily struggle with the 
 wind and cloud. Some days a shift of wind to the south 
 in the afternoon will cause the thermometer to rise 
 steadily as the sun goes down, and midnight may be hotter 
 than noon; on other days a dense layer of mist will 
 completely shut off the influence of the sun's rays, and 
 the instrument will leave a straight horizontal line as its 
 record of temperature-variation for twenty-four hours. 
 
 All that the mean curve of temperature signifies is, 
 ;that in a general way, allowing for all sorts of irregu- 
 larities, there is a diurnal solar influence, which has its 
 greatest and least values at such and such hours. But we 
 must most carefully avoid two conclusions: first, that 
 the mean temperature represents any abstract entity, 
 called mean diurnal range, which might be applied as a 
 correction to the observed temperature at any hour so as 
 to deduce the mean temperature of the day ; and, second, 
 that because we do not see a diurnal variation on the 
 trace for every day, therefore there is no such thing 
 as solar diurnal influence. Because the thermogram for 
 December 9 (Fig. 60) shows no diurnal maximum and 
 minimum, that does not prove that there is no such thing 
 as diurnal variation at all. The importance of this last 
 conclusion will be evident in our next chapter on Cyclical 
 Variations of Weather. 
 
 What more immediately concerns us now is to note 
 how diurnal affects general heat. This can be better 
 accomplished by trying to recollect the history of any 
 particular day's weather than by the inspection of dia- 
 
298 WEATHER. 
 
 grams. If we think of any day, we shall remember that 
 if the wind did not shift, the general character of the heat 
 did not alter. Suppose, for instance, that we had been in 
 a large cyclone with a north-west wind, which lasted for 
 the whole twenty-four hours. We should have known 
 instinctively that it was a cold day for the season, and 
 though there would have been a very considerable differ- 
 ence between the temperatures recorded by night and by 
 day, the quality of heat which belongs to north-west 
 winds would have remained the same. But suppose that, 
 after a sharp white frost in the morning, about midday 
 the sky had begun to thicken and the wind to back to 
 the south, then, as before mentioned, the temperature 
 would rise while the sun was going down, but every one 
 would have recognized that the quality of the heat and 
 the general character of the weather had changed. We 
 thus see the correctness of the phraseology which calls 
 such changes general, which have their origin in the 
 great movements of atmospheric pressure with wind- 
 shifts, and those changes variations which are imposed 
 on the general character by the sun's daily influence. In 
 fact, we realize the great principle that the diurnal 
 variations are superimposed on the general changes, but 
 never alter the character of the latter. Temperature, 
 like every other element of weather, is the balance of the 
 general changes and diurnal variations; so that when 
 the general are strong the diurnal are masked, when the 
 former are weak the latter are predominant. 
 
DIURNAL VARIATION OF WEATHER. 299 
 
 DIURNAL CLOUD. 
 
 From the comparatively simple nature of diurnal heat, 
 we must now turn to the far more complicated variations 
 of fog, cloud, and rain. The simplest case of the diurnal 
 precipitation of vapour is found in the regular formation 
 of valley or river mist in fine weather. In settled climates, 
 at certain seasons, the sky is always blue by day, but after 
 dark, fog or mist begin to form in the low-lying ground, 
 from the influence of nocturnal radiation. By sunrise 
 the valleys will be filled with mist, which rises to such a 
 uniform level that, viewed from a height, the hollows 
 look as if they were filled with water. After the sun is 
 up, the vapour gradually rises and disperses, till the sky 
 resumes its usual blue appearance. The general anti- 
 cyclonic character of the weather is the same throughout, 
 but the day impresses a variation on the face of the sky. 
 
 Diurnal weather-variations are so intricate, and vary 
 so much, not only in different countries, but at different 
 seasons, that we can only give a few illustrations of 
 general principles. Every one of the seven fundamental 
 shapes of isobars has a type of diurnal weather peculiar 
 to itself. Only that of the two great shapes has been 
 worked out for Great Britain by the author.* He has 
 shown that in their diurnal variations cyclones and anti- 
 cyclones present the same antithesis as they do in all 
 their other special characteristics. The broadest features 
 of the diurnal variation in a cyclone is that, starting 
 from the early morning, the amount of cloud and the 
 
 * Quarterly Journal of the Meteorological Society, London, vol. iv. 
 p. 4. 
 
300 WEATHER. 
 
 general severity of the weather gradually increase till 
 about 2 p.m., and then gradually decrease till past mid- 
 night. In anticyclones, on the contrary, a misty or cloudy 
 morning is followed by a great diminution of cloud as the 
 day goes on, while later on, in the evening, mist and 
 cloud are sometimes formed again. 
 
 If, besides these broad features of diurnal variation, 
 we consider some of its more minute changes, we observe 
 that, in cyclones, a cloudy morning often has a short 
 break about 10 a.m. ; that the weather then becomes 
 much worse, but has a marked tendency to clear up again 
 about 4 p.m. In anticyclones, on the contrary, a clear 
 morning at 4 a.m. is frequently very cloudy at 10 a.m., 
 after which the cloud again decreases till about 4 p.m., 
 when more cloud or mist are often again formed, and last 
 till quite late in the evening. We find, in fact, not only 
 traces of a semi-diurnal variation, or of one which runs 
 through its course twice in a day, but also that the 
 intervals of this variation are obviously connected with 
 the hours of diurnal maxima and minima of pressure. 
 
 The increase of cloud during the day in cyclones may 
 be generally described as an accession of intensity which 
 accompanies the diurnal increase in the velocity of the 
 incurving wind. Then, as more air is being poured into 
 the centre of the cyclone, the ascensional currents must 
 also be stronger, and therefore more cloud will be formed. 
 In anticyclones, the morning mist has already been shown 
 to be due to radiation, and the marked clearing of the 
 sky during the day must be due to the increased strength 
 of the descensional currents near the centre, caused by 
 the increased velocity of the outcurved wind during the 
 
DIURNAL VARIATION OF WEATHER. 301 
 
 day. From this we can easily understand how two 
 different shapes of isobars can have such very different 
 diurnal variations. In one, the influence of the sun 
 modifies a rising current ; in the other, a descending one. 
 But whatever modification the diurnal variation may 
 impose on a cyclone or anticyclone, the general character 
 of either is never lost. No diurnal variation can make 
 the cirrus clouds in front of a cyclone either like the 
 cumulus in rear, or like the clouds formed from the rising 
 mist of an anticyclone. The variation is, as it were, a 
 modifying influence superimposed on the more general 
 features. 
 
 DIURNAL BAIN. 
 
 The diurnal variation of rain is one of the most 
 difficult questions in meteorology. Not only does the 
 variation differ in each shape of isobars, but there is a 
 tendency of small secondaries to form at particular hours ; 
 and moreover, during the winter months, there is a marked 
 tendency of large cyclones to come in from the Atlantic 
 with increased intensity during the night. Besides a 
 diurnal variation of the intensity of a rainy shape* of 
 isobars, there is a diurnal period both of the formation 
 and motion of cyclones. 
 
 The error which we have most carefully to avoid in 
 treating this branch of the subject, is the supposition that 
 all rain is cyclonic, or that the diurnal period of thunder- 
 storms and secondaries is the same in every country. From 
 this it is evident that if one shape of pressure-distribution 
 predominates in summer, and another in winter, then the 
 
302 WEATHER. 
 
 two seasons will have a totally different type of diurnal 
 rain-variation. 
 
 We can get a most striking illustration of these 
 principles from the tropics. At Calcutta Mr. H. F. 
 Blanford finds* that the weather is divided into three 
 seasons : 
 
 The rains, June to October, when the diurnal frequency 
 curve of rain begins to rise soon after midnight to a small 
 maximum about 6 a.m. and a small minimum about 8 a.m. 
 Then the frequency rises rapidly to its principal maximum 
 at 2 p.m., and falls quickly to the principal minimum at 
 1 a.m. The mode of the formation of the rain-cloud of 
 the summer monsoon is essentially cumulus. 
 
 The hot season, March to May, when the diurnal 
 epoch of minimum is not very distinctly indicated, but 
 would appear to occur about sunrise. There is, however, 
 little variation from midnight up to 9 or 10 a.m. ; and 
 after this, only a slow rise up to 2 p.m., when the increase 
 becomes more rapid. About two hours before sunset, 
 there is a sudden rise of about fifty per cent., and the 
 hour of maximum raininess occurs between 7 and 8 p.m. 
 Compared, however, with the maximum of the rainy 
 sea'son, it is very small. This very striking feature of the 
 hot season is due to evening storms, known as north- 
 westers, which are so called because they commonly 
 originate in the north-west, and are probably connected 
 with the diurnal variation of wind near the coast. 
 
 Lastly, the cold season from November to February. 
 In this, falls of rain are pretty evenly distributed through- 
 out the day, with a decided diminution during the two or 
 * Asiatic Society, Bengal, xlviii,, part ii., 1879. 
 
DIURNAL VARIATION OF WEATHER. 303 
 
 three hours before and after midnight. These seasons 
 are, of course, associated with different types of pressure- 
 distribution. The rainy, cold, and hot seasons belong to 
 the periods of the south-west and north-east monsoons, 
 with an intermediate period respectively. When these 
 are all combined into a yearly curve, the result is a curve 
 which gives the true variation of no season. In this case, 
 as the rainy season curve is very pronounced, and that of 
 the two other seasons much less marked, the annual curve 
 differs but little from that of the rainy season, though 
 some of the minor flexures are altered. Under other 
 conditions the mean annual curve might be very different 
 from that of any one season. 
 
 In some countries, where land and sea breezes are 
 tolerably constant, rain often falls at the turn of the wind, 
 but the details vary indefinitely ; and in many parts of 
 the tropics, inland as well as on the sea-coast, thunder- 
 storms form regularly at the same hour every day. In- 
 land this does not usually occur before 2 p.m., and usually 
 later ; but no rule can be laid down. These storms are, 
 of course, totally non-isobaric. 
 
 All over England the mean diurnal curves of rain are 
 so irregular that they do not show any real variation ; 
 for there are so many kinds of rain in that country each 
 with its own variation that the curve of all mixed up 
 together has no physical significance at all. In Prague, 
 Professor Augustin finds three types of diurnal frequency 
 for winter, summer, spring and autumn, respectively; 
 while in many parts of Europe, and in Japan, there is a 
 tendency to develop morning and evening maxima of 
 rain, with day and night minima. Every country has its 
 
304 WEATHER. 
 
 own peculiarities ; and each set must be explained on its 
 own merits. 
 
 In our detail of the cyclonic variation of cloud, in 
 England, we pointed out that the diurnal variation does 
 not alter the general character of the sky or cloud, or we 
 might say of the weather as a whole.. The same great 
 principle holds for every other shape of isobars and for 
 every other climate. The diurnal variation of rain in 
 Calcutta during the rainy season is enormous, but at all 
 hours the general character of the south-west monsoon is 
 always the same. Similarly, during the cold weather of 
 the north-east monsoon the diurnal variation is only a 
 modification, not a real change of weather. 
 
 DIURNAL WIND. 
 
 In our chapter on Meteograms we have partially 
 explained the general idea of diurnal wind-variation, but 
 we now wish to give some additional developments of the 
 subject, 
 
 DIURKAL VELOCITY. 
 
 The commonest feature of diurnal wind in most places, 
 temperate as well as tropical, is an increase of the velocity 
 from daybreak to about 2 p.m., and then a decrease to 
 its lowest point about 4 a.m. Besides this, there is in 
 many places a smaller series of variations whose turning 
 points occur about the same time as the maxima and 
 minima of diurnal pressure. For instance, in Great 
 Britain, the principal maximum is about 2 p.m., and the 
 principal minimum about 4 a.m. But in addition to this, 
 
DIURNAL VARIATION OF WEATHER. 305 
 
 there is a small minimum about 10 p.m., with a small 
 maximum between that hour and the great minimum at 
 4 a.m., besides a well-marked minimum about 10 a.m., 
 just at the hour when cyclones and anticyclones develop 
 their most characteristic difference of diurnal formation 
 of cloud. A similar tendency is found all over the world, 
 both north and south of the equator. The details vary 
 indifferently, and cannot be said to have been fully worked 
 out in any one country. 
 
 We can, however, safely say that the variations are 
 all of the diurnal type, and have nothing to do with the 
 character or amount of wind- velocity, which depends on 
 the distribution of surrounding pressure. Just as with 
 weather, the general character of wind and the relation to 
 gradient depend primarily on the isobars; the diurnal 
 variation/' in spite of its great complexity, is purely 
 secondary. 
 
 Buchan has made the very important observation that 
 the diurnal variation is almost nothing over the sea, when 
 away from the influence of land, and he has also connected 
 this with the fact that the diurnal variation of temperature 
 is very small over the sea compared with that over land, 
 so that in some way the diurnal amount of wind- velocity 
 appears to depend on the temperature of the floor over 
 which it blows. 
 
 Many of the details of wind-variation are both interest- 
 ing and puzzling. In some places the wind falls about 
 noon, probably from some local influence ; and in Great 
 Britain, Ley has shown that the diurnal variation of 
 velocity is greater with west than with east winds. This 
 again coincides with an observation of Hamberg's, that 
 
 x 
 
306 WEATHER. 
 
 the stronger the wind, the greater the amount of diurnal 
 increase of velocity, because as a rule in England west 
 winds have a higher velocity than east ones. 
 
 So far we have only thought of the wind on the earth's 
 surface, but on high mountain peaks the variation of wind 
 velocity is almost exactly the opposite ; for there the 
 maximum is in the early morning, and the minimum about 
 or just after noon. The general speed of the wind is, 
 however, much greater at high altitudes than at low 
 levels. This principle appears to hold equally in both 
 hemispheres. 
 
 DIURNAL DIRECTION. 
 
 The diurnal variation of direction is less marked than 
 that of velocity and much more difficult to detect. In 
 the northern hemisphere, however, there is a well-defined 
 tendency to veer a little in the morning, and to back 
 again in the afternoon ; so that the times of greatest veer- 
 ing and backing .correspond to the hours of greatest and 
 least velocity. That is to say, if the general direction 
 of the wind, is from the west, it will be a little more from 
 the north-west by day, and a little more from the south- 
 west by night ; the greatest northing being about 2 p.m., 
 and the greatest southing about 4 a.m. There are also 
 traces of a semi-diurnal variation exactly similar to that 
 which we described under the head of velocity; but we 
 cannot give complete details for any one place. 
 
 On mountain-tops the daily oscillation of the wind is 
 on a contrary system, for there the wind backs by day 
 and veers towards evening. For instance, a generally 
 
DIURNAL VARIATION OF WEATHER. 307 
 
 westerly wind will back towards west by south till the 
 afternoon, and then veer towards west by north at night- 
 fall. We demonstrated in our chapter on Clouds, under 
 the heading of "Vertical Succession of Air-Currents," 
 that in the northern hemisphere successive vertical strata 
 of wind came more and more from the left of an observer 
 standing with his back to the wind on the surface. If, 
 then, the surface current veers, and the upper ones back 
 during the day, the result will be that, both in cyclones 
 and anticyclones, the difference in direction between the 
 upper and lower currents will be greater by night than 
 by day. 
 
 In the southern hemisphere we have not a sufficient 
 number of observations to enable us to generalize on the 
 nature of diurnal wind-variation ; but as far as they go 
 they point to an exactly similar law to that which holds 
 in the northern hemisphere. The surface winds veer, 
 and the upper currents back with the course of the sun. 
 But observe that the course of the sun is opposite in the 
 two hemispheres, so that a westerly surface wind would 
 veer towards north in the northern hemisphere, and 
 towards south in the southern hemisphere. As some 
 may prefer to see the laws of diurnal wind exhibited in 
 their relation to absolute direction, as given by the hands 
 of a watch, we may state these results thus 
 
 FORENOOX AFTERXOOX. 
 
 Northern hemisphere Surface ... With watch-hands ... Against. 
 
 Hilltops ... Against ... With. 
 
 Southern hemisphere Surface ... Against ... With. 
 
 Hilltops ... With ... Against. 
 
 The following is a fair illustration of the nature and 
 amount of diurnal wind-variation, both of velocity and 
 
308 
 
 WEATHER. 
 
 direction, as ordinarily observed in Great Britain. In 
 Fig. 62 we give a copy of the anemographic record at 
 Kew, near London, for the three days, August 6th to 8th, 
 1874. 
 
 DtrcctuTrv 
 
 KEW 
 
 Noon Nooiv JSoow 
 
 FIG. 62. Anemographic curves for Kew, August 6th to 8th, 1874. 
 
 The synoptic conditions for these three days were the 
 commonest in that country. A series of large cyclones of 
 moderate intensity were passing to the north of Great 
 Britain, so that, although there was a good deal of cloud 
 and wind, there was not the marked shift of wind which 
 would occur if the cyclone centres had passed nearer the 
 station. Taking the velocity first, in the first two days 
 the tendency of the wind to rise during the day is very 
 obvious ; but on the third day the ordinary variation is 
 completely masked by violent squalls. Two of these 
 occurred at the times marked q on the diagram. The 
 smaller semi-diurnal variations are also almost completely 
 obliterated ; they are only observed in calm, summer, 
 anticyclonic weather. 
 
 Then, looking at the direction-traces, the general 
 
DIUKNAL VARIATION OF WEATHER. 309 
 
 westerly direction of the wind due to the isobars is 
 sufficiently obvious, but superimposed on that we find 
 every day an irregular tendency for the wind to veer a 
 little towards the north-west during the day, and to back 
 again during the night. We also see another feature of 
 British winds, viz. the increased gustiness by day com- 
 pared with the night. This is shown by the more 
 irregular trace during the day hours. 
 
 From a simple case of this sort we can readily see 
 both the amount of diurnal variation as well as the 
 manner in which it can easily be masked. When the 
 general features of the weather are feebly marked, then 
 the diurnal variations are strong, and may be the 
 prominent character of the day. This is common in 
 many tropical countries, especially in those which are 
 habitually covered by anticyclones. In variable climates, 
 like that of Great Britain, on the contrary, the diurnal 
 variations are only obvious in the finest settled summer 
 weather ; and in winter, when the general changes are 
 very intense, the diurnal features are often completely lost. 
 
 We may then lay down as a general rule that the 
 prominence of the diurnal variations of weather are a 
 measure of the settled or unsettled character of the 
 climate of any place. 
 
 Any attempt to discuss all the details of diurnal wind, 
 or the different theories which have been suggested, with 
 more or less probability, to account for it. is beyond the 
 scope of this work. All that we wish to do is to give a 
 clear sketch of the general nature of diurnal variation, 
 and especially of the manner in which it is subordinated 
 to the great laws of dependence of weather on isobars. 
 
310 WEATHER. 
 
 GENERAL VIEW OF THE SUBJECT. 
 
 It is most interesting to note the unity which runs 
 through the whole class of diurnal variations. The 
 principal maxima and minima of temperature, wind, and 
 partially weather at 2 p.m. and 4 a.m. respectively, are 
 strictly analogous to each other, while the semi-diurnal 
 features of wind and weather are analogous to the diurnal 
 variation of pressure. The latter, to which we have 
 scarcely alluded, has two minima at 4 a.m. and 4 p.m., 
 and two maxima at 10 a.m. and 10 p.m. respectively. The 
 single diurnal variations are undoubtedly due to the direct 
 influence of the sun's heat ; but the question how an 
 influence such as that which runs its course only once in 
 the twenty-four hours can induce a variation which has a 
 semi-diurnal period, has, up to the present time, baffled 
 the skill of meteorologists. It is, however, perfectly certain 
 that no one is the cause of the others ; all are equally 
 the products of the same influence, and no comprehensive 
 theory of diurnal variations will ever be complete which 
 does not explain and co-relate all together. When we 
 look at a series of synoptic charts for several consecutive 
 days, we see that many cyclones go on their course often 
 for two or three weeks, quite independent of diurnal 
 changes. We may, therefore, perhaps suggest the follow- 
 ing broad view of the relation of diurnal variation to the 
 general character of weather. The whole atmosphere is 
 circulating between the equator and the poles. Some- 
 times this flow of air takes the form of eddy known as a 
 cyclone, sometimes that known as an anticyclone, and 
 almost always one of the seven fundamental forms of 
 
DIURNAL VARIATION OF WEATHER. 311 
 
 circulation. Every day, as the sun rises and sets on this 
 system, he impresses either directly or indirectly a series 
 of complex variations on every meteorological element, 
 but does not change the intrinsic nature of any form of 
 circulation. 
 
 The results of this chapter may therefore be summed 
 up as follows. In every part of the world the diurnal 
 variation is superimposed on the general character of the 
 weather, which is due to the distribution of surrounding 
 pressure. The resulting weather is the balance of the 
 general character and diurnal variation ; the prominence 
 of the diurnal is a measure of the settled nature of the 
 climate of any place. 
 
 All over the world there is a tendency to form both a 
 single diurnal variation, which varies only once in the 
 twenty-four hours, and a semi-diurnal variation, which 
 has two maxima and two minima in the same time. The 
 origin of the first is undoubtedly the direct action of the 
 sun ; that of the latter cannot be at present explained. 
 No diurnal variation has any effect on general weather, 
 and can be neglected in all questions which relate to 
 forecasting general changes. This independence is one 
 of the most important principles in meteorology. 
 
312 WEATHER. 
 
 CHAPTEE XII. 
 ANNUAL AND SECULAR VARIATIONS. 
 
 SEASONAL APPEARANCE OF THE SKY. 
 
 THE term " seasonal variation " is used in a twofold sense. 
 In the simpler case, it refers to the minute differences in 
 the appearance of the sky which are found at various 
 seasons in cyclones, etc. For instance, the rear of a 
 cyclone does not form cumulus cloud in the dry winter 
 months of Continental Europe ; only blue sky is seen. In 
 damp England, cumulus is formed at all seasons ; but is 
 much denser and more strongly marked in summer than 
 in winter. In like manner, a secondary which would 
 develop thunder in summer in Great Britain would only 
 produce heavy rain in winter. In this way seasonal is 
 exactly analogous to diurnal variation, for it modifies but 
 never changes the general character of the weather. The 
 intensity alone is ever altered. 
 
 EECURRENT TYPES OF WEATHER. 
 
 But of far more importance is that form of seasonal 
 variation which applies to the occurrence or recurrence of 
 
ANNUAL AND SECULAR VARIATIONS. 313 
 
 similar weather about the same date every year. The 
 nature of recurrent weather in the temperate region of 
 variable pressure may be best illustrated by looking at 
 the connection between the variable European types and 
 the regular annual changes which take place in the 
 tropics. In most equatorial and tropical climates there 
 are only two or three seasons, which correspond to two or 
 three positions of the equatorial low pressure and tropical 
 belt of anticyclones. The monsoons of the Indian Ocean 
 are the most striking and best known instances of weather 
 that recurs at the same season of every year. 
 
 The English word " monsoon " is, in fact, derived 
 from an Arabic word meaning "season." But in the 
 regions which lie between the tropical and temperate 
 zones the author has found that there are recurrent 
 periods, intermediate both in their duration and the 
 certainty of their return to the monsoons of India and the 
 recurrent spells of European weather. One of the best 
 known of these is the " Khamsin " (the fifty days), a 
 hot, sandy south-east wind which blows regularly in 
 Egypt from the end of March and through April for 
 about fifty days. Klunzinger has given a whole series of 
 persistent and recurrent types for the whole year in 
 Kosseir, on the Ked Sea, about one hundred miles south 
 of Suez on the Egyptian side. The following may be 
 considered a list of the chief recurrent periods of weather 
 in Great Britain and North-Western Europe generally. 
 
 February 7 to 10. A spell of cold weather, associated 
 with the northerly type. This is the first of a series of 
 six cold and three hot periods discovered by A. Buchan. 
 He also noticed that during the cold periods the pressure 
 
31 4 WEATHER. 
 
 was higher to the north of Scotland, and lower to the 
 south, and that during the warm periods pressure was 
 higher over Scotland than in places to the north. This 
 means that the cold periods were the result of the 
 occurrence and persistence of either the northerly or 
 easterly types of weather. We have been unable to find 
 any allusion to this spell in European weather lore. 
 
 March. The proverbial east winds of this month are 
 mostly due to the northerly type of weather. The 
 occurrence of equinoctial gales about the 21st of the 
 month is almost universally believed. It is, however, a 
 curious fact, as has been pointed out by K. H. Scott, that 
 the records of the British observatories give no decided 
 indications of exceptionally strong winds at either equinox. 
 Whether equinoctial gales really occur in the Mediter- 
 ranean, and the idea has been carried from thence to 
 England by the monks, or whether the weather in Great 
 Britain might not be more properly called broken than 
 stormy, we cannot say. The author, however, rather 
 inclines to the latter view ; for it is almost impossible to 
 believe that an idea which has obtained such universal 
 credence can be altogether destitute of some real founda- 
 tion. The difficulties of settling a question like this 
 bring forcibly before us the uncertainty of any numerical 
 estimate of climate or weather. 
 
 April 11 to 14. A cold spell; Buchan's second 
 period, which he has identified with the popular '' weather 
 saw " of the " borrowing days." 
 
 May 9 to 14. A cold spell; Buchan's third period. 
 This is the most celebrated of the cold periods, as it 
 occurs over the greater part of Europe. A good many 
 
ANNUAL AND SECULAlt VARIATIONS. 315 
 
 sayings connected with it are found in many European 
 prognostics, such as those relating to the frost saints. 
 This period is of some interest on account of the strange 
 theories which have been propounded to explain the 
 origin of the cold. One of the most popular has been the 
 idea that about the middle of May the earth encountered 
 a stream of meteors which were so numerous as to act 
 like a cloud of dust and cut off some portion of the sun's 
 heat. We need hardly say that such an occurrence would 
 diminish the temperature all over the world, and that 
 there is nothing to give countenance to this. Besides, 
 the passage of the sun's rays through such a stone-strewed 
 space could not fail to give rise to some kind o blur of 
 light round his disc, as when he shines through big drops 
 of condensed vapour. Nothing can be more certain than 
 that this cold period is usually due to the setting in of a 
 spell of the easterly or northerly type over Europe. At 
 any other time of the year the same types bring similar 
 weather. 
 
 June. A cold spell in the second or third week is 
 associated with the northerly type. 
 
 June 29 to July 4. A cold spell; Buchan's fourth 
 period. Curiously enough, we have been unable to find any 
 reference to these thermometric periods in the weather 
 lore either of Great Britain or of any other part of Europe. 
 
 July 12 to 15. A warm period ; Buchan's first. 
 
 July 15. St. Swithin. The popular legend of this 
 saint, and other rainy saints like St. Medard, receives an 
 easy explanation from synoptic charts. 
 
 August 2 to 8. A wet period ; the " Lammas floods " 
 of Scotland. 
 
316 WEATHER. 
 
 August 6 to 11. A cold period; Buchan's fifth. 
 
 August 12 to 15. A hot period ; Buchan's second. 
 No prognostics are associated with these two latter 
 periods. 
 
 September. The easterly and northerly types are 
 rare during this month ; the gales or broken weather at 
 the equinox are almost invariably of the westerly type. 
 About the 30th a fine period is experienced for a few 
 days the " Indian Summer " of North America. 
 
 October. About the second or third week a spell of 
 the easterly type of moderate intensity is common. 
 
 October 18. A fine quiet period about this time 
 "St. Luke's Summer." This and the other summers 
 which occur at this season have sometimes been stated to 
 be due to the liberation of heat during the condensation 
 of vapour, and formation of ice, which begins to take 
 place on a large scale in the polar regions soon after the 
 autumnal equinox. According to this theory, the opposite 
 phenomenon of cold in April and May is supposed to be 
 caused by the absorption of heat due to the melting of 
 ice. Both ideas are purely fanciful. The spring cold we 
 have already explained ; the autumn summers are due to 
 the recurrence of tranquil periods at that season. 
 
 November 6 to 12. A cold spell; Buchan's sixth, 
 associated with the northerly type. The llth is " St. 
 Martin's Little Summer," popularly considered in the 
 Mediterranean to be a period of warm, quiet weather. 
 
 December 3 to 9. A warm period ; Buchan's third. 
 
 The general explanation of all these periodicities is 
 identical. They all depend for their origin on a tendency 
 of certain types of pressure-distribution to recur about 
 
ANNUAL AND SECULAR VARIATIONS. 317 
 
 the dates just mentioned. The cold periods all require 
 the presence of the northerly or easterly types; the 
 warm periods, either of the southerly type in winter, or 
 of anticyclones in summer ; while wet or broken periods 
 indicate the recurrence of intense cyclones of any type. 
 
 Eeturning to our old illustration of a globe surrounded 
 by a circulating atmosphere, we can readily suppose that 
 at the same date every year, when the sun is in the same 
 place, the motion of the air will tend to reproduce the 
 same kind of eddies in the same localities. 
 
 VALUE IN FORECASTING. 
 
 But now we have to consider how the knowledge of 
 these recurrent periods can be utilized in forecasting. In 
 our last chapter on Diurnal Variations we called attention 
 to the nature of the daily period of heat. In this, the 
 most obvious of all meteorological phenomena, we found 
 that though there is a powerful heating influence present 
 every day, still that other causes are sometimes so powerful 
 as to obliterate or invert the action of the sun. As a 
 consequence of this we cannot affirm absolutely that the 
 night will be colder than the day, though, of course, such 
 is generally the case. 
 
 If we were to attempt to forcast the heat at any hour 
 by reference to the mean curve of diurnal range, we should 
 sometimes give most erroneous forecasts ; if, on the con- 
 trary, we looked at the chart for 8 a.m., we could often 
 safely predict that the ensuing night would be warmer 
 than the day. From the temperature diagrams which 
 were there given, we also drew the important inference 
 
318 WEATHEK. 
 
 that, because we do not see a diurnal range every day, we 
 must not infer that there is no such thing as a diurnal 
 solar influence. If, then, such a powerful influence as the 
 direct rays of the sun can be so easily masked, we can 
 readily understand that a weaker influence, such as the 
 declination of the sun on any particular day, can readily 
 be obliterated. We can safely say that the change in 
 the altitude of the sun is of secondary importance, because 
 we see every day great changes in the distribution of 
 pressure, which are certainly in no way related to the 
 seasonal change in the declination of the sun. We need 
 not, then, be surprised that the types of heat or cold do 
 not recur absolutely every year, only that there is an 
 undoubted tendency to do so. When once we have 
 realized this, we can easily understand the following 
 statement of [the use of a knowledge of recurrent annual 
 types in forecasting. 
 
 A forecaster is not justified in saying that any period 
 will occur absolutely ; still, when about the time of its 
 usual recurrence the synoptic charts show signs of the 
 expected type, then the forecasts for a few days ahead can 
 be issued with greater confidence. For instance, suppose 
 that about the 6th of November the charts begin to show 
 traces of the northerly type, then but not before there 
 would be good grounds for saying that a period of cold 
 weather, which usually occurs at this season, has already 
 set in, and may be expected to last for five or six days. 
 The forecaster is thus enabled to issue a much longer 
 forecast than he can as a rule safely attempt. 
 
ANNUAL AND SECULAR VARIATIONS. 319 
 
 CYCLICAL PEEIODS. 
 
 By these we mean periods which run through their 
 whole course in any time other than a day or a year. 
 Many investigators have thought that they have detected 
 traces of periodicities of temperature of about twelve days 
 and of 25*74 days, the latter apparently connected with 
 the time of the sun's rotation. Others have endeavoured 
 to detect periodicities of rain or heat for longer epochs, 
 especially one of ll'l years, which would coincide with a 
 period of sun-spots. As this is the one of most importance 
 and greater interest, and as a discussion of it will serve to 
 illustrate the whole nature of periodicities, we shall con- 
 fine our attention to a short notice of this cyclic period 
 only. 
 
 SUN-SPOTS AND WEATHER. 
 
 Ever since the year 1775, we have more or less com- 
 plete records of the relative extent of black spots on the 
 sun's surface. These records show a most unequivocal 
 recurrence of sun-spot maxima at intervals of about 
 eleven years ; but the actual amount of surface covered 
 at each maximum is very irregular, In the lower part of 
 Fig. 63 we give a reduction of diagram which shows the 
 relative extent of black spots on the sun as plotted by 
 Professor Balfour Stewart. If we were to draw over this 
 curve another which showed the mean daily range of 
 magnetic declination for the same year, we should find 
 that there was an unmistakable similarity between the 
 
320 
 
 WEATHER. 
 
 two curves, and that both the times and magnitudes of 
 the maxima and minima agreed wonderfully well. 
 
 In like manner a curve which showed the number of 
 auroras observed in each year would also show a striking 
 likeness to the curve of sun -spotted area. This curve is 
 not so valuable as that of magnetic declination, because 
 auroras cannot be seen in cloudy weather, while magnets 
 
 1-780 1790 1800 
 
 1870 
 
 Inches -T 
 
 70 
 
 60 
 
 40 
 
 RAINFALL 
 
 ROTHESAY 
 
 L i i it., i n 1 1 1 i i i 
 
 1780 1790 l8oO 10 20 30 40 50 
 
 FIG. 63, Sunspots and rainfall. 
 
 60 1070 
 
 can always be observed. As these curves undoubtedly 
 connect the state of the sun with one physical terrestrial 
 phenomenon, and also with another half-physical, half- 
 meteorological appearance, there would be no inherent 
 improbability in the existence of a relation between sun- 
 spots and weather, 
 
 Such a relation would be on quite a different footing 
 to any quasi-astrological idea of a connection between the 
 sun, moon, or stars, and weather-changes. 
 
ANNUAL AND SEC'ULAR VARIATIONS. 321 
 
 Many investigators think that they can trace some 
 kind ot connection between the amount of rainfall and 
 sun-spots ; others see a connection between the years of 
 maximum sun-spots and the frequency of cyclones in the 
 Indian Ocean ; while some find that marine casualties 
 and commercial panics or crises appear to follow a cycle 
 closely corresponding to that of sun-spots 
 
 One great difficulty in deciding whether there is a 
 real periodicity in rain or storm statistics arises from the 
 very irregular curves with which we have to deal. All 
 the curves, on which it is sought to base the supposed 
 connection between sun-spots and weather, have been so 
 far smoothed, that it is difficult to say what the result- 
 ing curve really signifies and how far true deductions can 
 be made from it. This will be better realized by an 
 example. -^On the upper halt* of Fig. 63 we have, there- 
 fore, plotted the annual amount of rainfall at Rothesay, in 
 Scotland, for eighty years, over the curve of sun-spot 
 extent in the lower portion of the diagram.^ Both curves 
 are purely the result of observation and have not been 
 smoothed in any way. ^The reader can, therefore, draw 
 his own conclusion as to how far there is a real or fanciful 
 connection between the two curves. In some points there 
 is undoubted similarity ; in others, an absolute contrariety. 
 In the rainfall curve, if we take the absolute mathematical 
 definition of a maximum when any value is greater 
 than either the preceding or succeeding ones there is 
 a maximum of rain nearly every other year ; but if we 
 consider the broader sweeps of the curve, we may find 
 more resemblance. For instance, the maxima about 1805, 
 1816, 1828, 1837, 1848, 1860, and 1871 agree passably 
 
 Y 
 
322 WEATHER. 
 
 in both curves ; on the other hand, the absolutely greatest 
 rainfall in the eighty years was in 1811, a year of minimum 
 spotted area; while another very large rain maximum 
 also occurred near a time of minimum spots in 1841. 
 Then again, the greatest minimum but one of rain, in 
 1870, occurred one year before a maximum of spots and 
 only two years before the second largest maximum of 
 rain. These latter cases, of course, throw doubt on 
 whether we are justified in finding any periodicity at all 
 in the rainfall curve. Any attempt to smooth or alter 
 these curves by arithmetical or algebraical processes can 
 only lead to illusive results ; we must base our opinion 
 of the supposed connection between the two curves on 
 our knowledge of other undoubted irregular periodicities. 
 
 In our curve of thermograms, Fig. 60, we pointed out 
 that because there was no obvious trace of diurnal varia- 
 tion of heat on many days, we were not, there fore, justified 
 in saying that there was no such thing as a diurnal 
 heating influence of the sun. All that could be said was 
 that his power had been overridden by more powerful 
 influences. In the same way, the fact that there are 
 heavy rainfalls which have no relation to the extent of 
 sun-spotted area, does not of itself prove that there is no 
 connection between the spots and weather. If we could 
 be certain from any other considerations that there was a 
 real connection between the two phenomena, all that we 
 should be justified in saying was, that whatever influences 
 the spots had on weather, there were other influences 
 which might be much more powerful. 
 
 Another great difficulty which we have to face in 
 forming our judgment of the possible connection between 
 
ANNITAL AND SECULAR VARIATIONS. 323 
 
 the state of tlie sun and weather, arises from the 
 impossibility of laying down any absolute criterion of 
 what is a rainy year. Kain may be produced by so 
 many different causes, and the difference of amount 
 which is measured in places near one another is so great, 
 that we are left a great deal to our own estimate of values 
 or probability. Thunderstorms are the great disturbance 
 of rainfall statistics. Under those circumstances as much 
 as two inches of rain may fall in one place, and but a few 
 drops in another only a few miles distant. Yearly totals 
 show the same discrepancies. 
 
 For example, in the year 1872 a year of sun-spot 
 maximum Buchan has plotted the rainfall within the 
 limited area of Scotland. He finds that while near 
 Aberdeen the rainfall was seventy-five per cent, above 
 the average, the amount at Gape Wrath, about one 
 hundred miles distant, was below the average. Then, of 
 course, the difficulty is, why should we take the returns 
 of one station more than another to compare with sun- 
 spots, when the latter affect the whole world simul- 
 taneously ? 
 
 Kain in the abstract is a mere entity we must say 
 what kind of rain it was which fell. Was it cyclone rain, 
 or secondary rain, or that associated with thunderstorms ? 
 The true criterion of periodicity requires not only an 
 amount of rain which corresponds with the state of the 
 sun's surface, but also rainfall under the same conditions 
 of atmospheric pressure. We must not compare the rain 
 of cyclones with the rain of thunderstorms, unless we can 
 show that they may both be produced by increased 
 intensity of the weather generally. This is, however, 
 sometimes the case. 
 
324 WEATHER. 
 
 There is another point which must be remembered in 
 the discussion of questions as to the connection between 
 the sun and the weather. We have shown that weather 
 in the temperate zone is the product of the passage of 
 cyclones, anticyclones, etc., so that we cannot properly 
 talk of the influence of any physical cause on weather in 
 the abstract. We must thiuk how the physical cause 
 would act on the general circulation of the atmosphere. 
 When we discussed the daily influence of the sun on 
 weather, we showed how heat modified in a different 
 manner the ascending currents of a cyclone or the 
 descending ones of an anticyclone. 
 
 In the same way, if the condition of the sun's surface 
 does affect weather, the action must take place through 
 the medium of cyclones and anticyclones. We must, in 
 fact, show that in years of sun-spot maxima and minima, 
 the circulation of the atmosphere is either more intense 
 generally, or that the formation of cyclones, etc., is then 
 in some manner modified. 
 
 This view of the true nature of solar action explains 
 some anomalies which the advocates of the sun-spot 
 theory have been unable to explain. They find that in 
 j^ome places the maxima of spots are associated with the 
 minima of rain. If we try to connect rainfall and sun- 
 spots in the abstract, we are helpless to explain the 
 discrepancy. But if, on the contrary, we realize that 
 an alteration in the solar heat may modify the formation 
 of cyclones, then we can at once explain the apparent 
 contradiction of results. For instance, in the year 1872, 
 to which we have already alluded, the general position of 
 cyclone centres over North- Western Europe was con- 
 
ANNUAL AND SECULAR VARIATIONS. 325 
 
 siderably displaced. Instead of lying to the west of 
 Scotland, the centre of cyclone activity appeared to lie 
 between England and Norway. This, of course, made 
 England wetter, and the north-west of Scotland drier 
 than usual ; but it will take many years before we are 
 justified in saying that this displacement was due to the 
 influence of solar spots. 
 
 It is, no doubt, a very tempting ideal to look at the 
 sun as the prime mover of the atmosphere, and to en- 
 deavour to follow variations in the heat or energy of his 
 action into their final products as wind or rain. But 
 when we consider what the real nature of weather is, as 
 revealed to us by means of synoptic charts, we see at 
 once that, though undoubtedly an alteration in the sun's 
 power would sooner or later be reflected in his results, 
 any attempt to deduce one from the other directly must 
 lead to disastrous failure. 
 
 RELATION TO FORECASTING. 
 
 Though opinions will doubtless differ as to whether 
 we are justified in asserting that there is any connec- 
 tion between sun-spots and weather, there is no uncer- 
 tainty as to what the value of that knowledge would be 
 to a forecaster. 
 
 The author believes himself that there are signs of 
 some real relation between the extent of spots on the 
 sun's surface and the rainfall curve at Bothesay, but 
 how should we fare if we tried to forecast the rainfall 
 for any particular year? The most cursory glance at 
 the two curves of sun-spots and rainfall will show that, 
 
326 WEATHER. 
 
 if we were to attempt to forecast rainfall on the assump- 
 tion that the amount would follow the sun-spot curve, we 
 should get just the same unsatisfactory results as if we 
 attempted to forecast the temperature at different hours 
 by reference to the mean diurnal curve of heat. Every 
 meteorological element depends for its value on the 
 balance of several nearly equal forces, so that an attempt 
 to forecast the resulting value by means of the variations 
 of one of these forces can only lead to failure. 
 
 So far for the use of the knowledge even of a certain 
 cyclic period in forecasting the character of a year as a 
 whole ; and it is still more impossible to use any abstract 
 periodicity in forecasting the weather for any particular 
 day. We shall see in a future chapter that all weather 
 prevision depends on the estimate which an experienced 
 forecaster can make as to the probable path of any 
 cyclone, or as to the formation of a new one. How much 
 would the abstract knowledge that it was a maximum or 
 minimum sun-spot year help him to form such a judg- 
 ment ? Obviously nothing. 
 
 On the whole, then, we may say that though there 
 are certainly very strong grounds for the belief that there 
 is some real connection between the state of the sun's 
 surface and terrestrial weather, still, from the nature of 
 atmospheric circulation, we are unable to utilize this 
 fact in forecasting weather, either for any season or for 
 any day. 
 
( 327 
 
 CHAPTER XIIL 
 TYPES AND SPELLS OF WEATHER. 
 INTRODUCTORY. 
 
 IN the foregoing chapters we have devoted our atten- 
 tion more to the nature of the causes which produce 
 weather at any moment than to the sequence of weather 
 for several consecutive days. We have, in fact, rather 
 described the nature of the individual disturbances which 
 form, as it were, the units of weather, than the manner 
 in which these components move or follow one another. 
 The word "weather" is used by meteorologists in a twofold 
 sense. When they talk of the weather at a moment, 
 they use the word in a restricted signification, referring to 
 the appearance of the sky, or to the occurrence of rain, 
 snow, etc. When they talk of weather for a longer 
 period, as, for instance, a wet week, or a cold month, 
 they use the word in a more extended sense, and include 
 the sequence of every meteorological element for the 
 time in question. 
 
 We have already mentioned that in the temperate 
 zone the units of weather, such as cyclones or anticyclones, 
 are perpetually moving or altering their shape, and 
 thereby producing changes of weather ; or to put it 
 
328 WEATHER. 
 
 more formally, weather in the temperate zone is the pro- 
 duct of the passage of cyclones, anticyclones, or of the 
 minor forms of isobars. 
 
 We have also pointed out that all forecasting depends 
 on the limited power which we possess of knowing before- 
 hand what the path of any disturbance is likely to be, 
 or what new changes in the distribution of pressure will 
 probably take place. For instance, if we see a cyclone 
 approaching our own country at eight o'clock in the 
 morning, how can we tell in what direction it will move, 
 or if it is likely to grow more or less intense ? If we 
 see an anticyclone, are there any signs by which we can 
 know whether it is going to remain stationary, or to 
 break up and disappear ? 
 
 When we have examined a very large number of 
 synoptic charts we soon see that, though no two are alike, 
 there is much in common so far as their sequence is con- 
 cerned. Though a cyclone may move in any direction, 
 and almost with any velocity, nothing is a matter of 
 accident, but certain types of motion are associated with 
 certain types of general pressure distribution. 
 
 Our purpose in this chapter is to explain the nature 
 of these changes, by giving in some detail the four great 
 types of weather which occur in Western Europe, with 
 shorter notices of those in the United States and in the 
 tropics. In doing so we will bear in mind the twofold 
 object of all scientific meteorology the explanation of 
 past weather by reference to the motion of cyclones, etc., 
 and the classification of typical changes with reference to 
 future forecasts. 
 
 Long verbal descriptions of complicated weather- 
 
TYPES AND SPELLS OF WEATHER. 329 
 
 maps are not only tedious, but unintelligible to all 
 except those who have made synoptic charts their special 
 study. As our object is to convey an idea of the nature 
 of weather-changes to those who have no previous know- 
 ledge of the subject, we shall, therefore, rather trust to 
 copious illustrations of carefully selected specimens, and 
 the reader must look at them and supply his own 
 descriptions. By this means he will learn the character 
 of atmospheric changes and the ways of cyclones by eye, 
 rather than by reference to any written formula. He 
 will see the rapidity with which these changes take 
 place, and acquire that knowledge of the nature of 
 weather which will enable him to form a just conception 
 of the great problems of forecasting. 
 
 We shall assume that he has so far mastered the 
 preceding chapters that, when we talk of a cyclone, he 
 knows that it is equivalent ta bad weather warmth in 
 front, cold in rear, wind according to intensity ; and that 
 when we say an anticyclone covers any country, that 
 means generally fine weather always light wind, but 
 blue sky, mist, heat, or cold, according to the circum- 
 stances of latitude or season. Also that the direction 
 of wind is given at once by naming an isobaric shape or 
 any portion of it. 
 
 Our illustrative charts, mostly on a uniform scale 
 and projection, embrace an area that extends from the 
 Kocky Mountains to Moscow, and from the equator to 
 Greenland. In all, pressures of 29*9 ins. (760 mm.), and 
 all above, are marked by full isobars, while those below 
 are dotted, so that the reader sees at a glance the broad 
 elative distribution of high and low pressure* 
 
330 WEATHER. 
 
 DISTRIBUTION OF PRESSURE OVER THE GLOBE. 
 
 Over the above area the distribution of atmospheric 
 pressure presents certain constant features, namely 
 
 1. An equatorial belt of nearly uniform low pressure. 
 
 2. A tropical belt of high pressure rising at intervals 
 into great irregular elevations or anticyclones. 
 
 3. A temperate and arctic region of generally low 
 pressure, but in which occasionally areas of high pressure 
 appear for a considerable period. 
 
 WEATHER IN THE DOLDRUMS. 
 
 The equatorial belt constantly covers the Sahara and 
 the Amazon valley, and always narrows over the Atlantic 
 at about 30 west longitude, where it often does not 
 reach higher than 10 north latitude. The shape and 
 depth of this area are tolerably constant. 
 
 This is the "doldrums" of the Atlantic navigators. 
 Our charts only show the north side of this area; the 
 south side is formed by an anticyclone, which always 
 lies over the South Atlantic. The doldrums, therefore, 
 form a sort of long hollow, or col, between two anti- 
 cyclones, and though on the one side the north-east trade 
 blows moderately, and the south-east trade on the other, 
 still in the centre there must be calm. This is well 
 shown in Fig. 68, where we see the symbol of calm 
 between the two trades. These sultry doldrums are 
 much dreaded by sailors, for in them "a ship may lie 
 for weeks on the hot smooth water, under a cloudless 
 sky, with pitch oozing from her decks ; a region ol un- 
 
TYPES AND SPELLS OF WEATHEE. 331 
 
 bearable calm, broken occasionally by violent squalls, 
 torrential rain, and fearful lightning and thunder." The 
 general appearance of the sky is a steamy haze, some- 
 times growing into a uniform gloom, with or without 
 heavy rain; at other times gathering into small ill- 
 defined patches of soft cumulus, or the forms of cloud 
 given in Fig. 13, a, b, and c. After dark there is always 
 a great development of sheet lightning till about two in 
 the morning. As the position of this area only varies 
 very slowly in its annual course a little more north or a 
 little more south, there is nothing to change the weather, 
 which therefore remains of a uniform character. 
 
 WEATHER IN THE TRADE-WINDS. 
 
 The tropical belt comprises a region of high pressure, 
 rising at variable intervals into great anticyclones. These 
 anticyclones are usually the longest in an east and west 
 direction, and often rise into two or more heads. Their 
 position is generally variable, with the exception of one, 
 which is always found over the central Atlantic. This 
 anticyclone forms a very important factor of the weather 
 both of Western Europe and of the United States, and 
 will be constantly referred to as "the Atlantic anticy- 
 clone/' The extension south and west of this anticyclone 
 is tolerably constant, while north and east it is variable, 
 sometimes rising as far as 60 north, and stretching over 
 Great Britain and Continental Europe. 
 
 The wind blows round this as in all anticyclones. 
 The north-east and east winds on the southern side of the 
 Atlantic anticyclone constitute the celebrated "trade- 
 
332 WEATHER. 
 
 winds." An inspection of Figs. 68-71 will both show 
 their true nature and correct some popular fallacies as 
 to their position and constancy. It is obvious, from the 
 nature of anticyclone winds, that north or north-east 
 winds must stretch far north on the easterly edge, which 
 accounts for the north-east trade being often picked up 
 off the coast of Portugal. But on the westerly edge of 
 the anticyclone the wind must be more south-east or 
 south, and in practice is lighter and more variable. 
 The centre must be, and is, calm, so that the wind-maps 
 which appear in physical atlases with the north-east 
 trade described as a belt of wind parallel to the equator 
 are most delusive. The degree of constancy in the direc- 
 tion and force of the trades is best gathered from an 
 inspection of the charts. We then see at once that the 
 position of the edges of the anticyclone is perpetually 
 changing, and that the gradients are very variable; so 
 that, as a matter of course, both the direction and 
 strength of the trades vary very considerably. 
 
 The weather in the trades is usually fine, and the 
 sky more or less covered with a peculiar small detached 
 cumulus, often called " trade cumulus." This is a small 
 isolated cloud bending backwards from the flat base, as 
 in Fig. 11, a, in our chapter on Clouds, which often 
 degenerates into the small lens-shaped mass as in Fig. 
 13, e. Sometimes a thin, hard, broken strato-cumulus 
 covers the sky with such regularity that, when seen in 
 perspective near the horizon, we look at a series of bars, 
 like the leaves of a Venetian blind; but if the gradients 
 are all steep, squalls and showers from cumulus cloud 
 are of frequent occurrence in the trade -wind regions. 
 
TYPES AND SPELLS OF WEATHER. 333 
 
 Cyclones are rarely, if ever, formed to the south of 
 this Atlantic anticyclone ; sometimes, however, they have 
 their origin on its south-west side, when they work 
 round the anticyclone, first towards the north-west, and 
 then towards north-east. These are the West India 
 hurricanes. 
 
 The north side of the anticyclone is the birthplace 
 of innumerable cyclones of every size and intensity, which 
 invariably move towards some point of east. These are 
 the cyclone-storms which affect Europe. 
 
 Cyclones are also occasionally formed on the south- 
 east side near Madeira ; these either work very slowly 
 round the high pressure to the south-west, or else leave 
 the anticyclone and go east over the Straits of Gibraltar. 
 In winter-time, another anticyclone generally lies over 
 Mexico, and the col between this and the Atlantic anti- 
 cyclone forms the most prominent feature in the meteor- 
 ology of the United States sea-board. 
 
 WEATHER IN TEMPERATE ZONE. 
 
 The temperate and arctic region extends from the 
 tropical high pressure to the pole. Though ordinarily 
 low, the pressure is perpetually fluctuating by reason of 
 the incessant passage of cyclones ; yet occasionally per- 
 sistent areas of high pressure appear in certain portions 
 of it. 
 
 As a necessary consequence of this, the weather in 
 this zone must be changeable, with variable winds. 
 
 From this brief survey, we see at once the broad 
 features of the climates of the world the persistent 
 
334} WEATHER. 
 
 equatorial calms and rains, the regular trades of the 
 tropics, and the variable wind and weather of the tempe- 
 rate zone. 
 
 We will now proceed to examine the weather of the 
 temperate zone in some detail. 
 
 TYPES OF PRESSURE IN TEMPERATE ZONE. 
 
 In spite of the great variability of the temperate 
 zone, there are with reference to Western Europe at 
 least four constant types of weather which coincide 
 with four distinct types of pressure-distribution. 
 
 1. The southerly, in which an anticyclone lies to the 
 east or south-east of Great Britain, while cyclones coming 
 in from the Atlantic either beat up against it or pass 
 towards north-east. 
 
 2. The westerly, in which the tropical belt of anti- 
 cyclones is found to the south of Great Britain, and the 
 cyclones which are formed in the central Atlantic pass 
 towards east or north-east. 
 
 3. The northerly, in which the Atlantic anticyclone 
 stretches far to the west and north-west of Great Britain, 
 roughly covering the Atlantic Ocean. In this case, 
 cyclones spring up on the north or east side, and either 
 work round the anticyclone to the south-east, or leave 
 it and travel rapidly towards the east. 
 
 4. The easterly, in which an apparently non-tropical 
 anticyclone (or one disconnected with the tropical high- 
 pressure belt) appears in the north-east of Europe, rarely 
 extending beyond the coast-line, while the Atlantic anti- 
 cyclone is occasionally totally absent from the Bay of 
 
TYPES AND SPELLS OF WEATHER. 335 
 
 Biscay. The cyclones then either come in from the 
 Atlantic and pass south-east between the Scandinavian 
 and Atlantic anticyclones, or else, their progress being 
 impeded, they are arrested or deflected by the anticyclone 
 in the north-east of Europe. Sometimes they are formed 
 to the south of the Scandinavian anticyclone, and advance 
 slowly towards the east, or sometimes even towards the 
 west. 
 
 These types are so named because the prevailing wind 
 in each is from south, west, north, and east respectively. 
 The connection of these European groups with those of 
 the United States will be considered under the details of 
 each type. 
 
 Notice will now be directed to the details of these 
 types first to their main character and seasonal modifi- 
 cations, together with the indications of intensity, and 
 then to any signs of persistence or change of type when 
 possible. 
 
 But however much we study details, the above general 
 view of the distribution of pressure in the earth's surface 
 must never be forgotten, as without that we lose the only 
 clue to the ceaseless and complicated changes with which 
 we have to deal. 
 
 SOUTHERLY TYPE. 
 
 In this type the Atlantic anticyclone extends very 
 little to the northward ; another of the tropical belt of 
 anticyclones covers Mexico or the southern states of the 
 American Union ; while a third area of high pressure 
 covers Northern and Eastern Europe. 
 
336 WEATHER. 
 
 The North Atlantic is occupied by a persistent area 
 of low pressure in which cyclones are constantly being 
 formed ; these beat up against the high European pressure, 
 and either die out or are repelled. 
 
 Sometimes, especially in summer, small cyclones 
 arising in the easterly side of the area of depression pa*s 
 rapi'ily near the British coasts in a north or north-east 
 direction. In either case it is somewhat rare for the 
 centre of a cyclone to reach the coast-line of Europe, so 
 that generally Great Britain is under the influence of 
 the rim or edge of either a cyclone or anticyclone. 
 
 At other times the Atlantic low pressure extends over 
 Great Britain, driving the high pressure eastwards, with- 
 out forming any definite cyclone. In this ease, the indi- 
 cations are for tolerably fine weather and little wind, 
 with a very low barometer a condition which often 
 excites remark. 
 
 This type of weather occurs at all seasons of the year, 
 but it is most common and persistent in winter ; in fact, 
 the warmth or otherwise- of the winter principally depends 
 on the number of days of this type. 
 
 No definite sequence of weather to the United States 
 is connected with the occurrence of this type in Europe. 
 While the Mexican anticyclone is tolerably persistent, 
 cyclones which form in the Hudson's Bay Territory 
 usually pass into the Atlantic and are lost there ; but at 
 the same time another totally different class of cyclone 
 forms in the col which lies between the Atlantic and 
 Mexican anticyclones, and moves along the northern edge 
 of the former till they reach Europe. The centres, of 
 course, never touch the American continent, but the 
 
TYPES AND SPELLS OF WEATHER. 
 
 337 
 
 gales associated with the western side of these cyclones 
 often do much damage to the United States coast. 
 
 The above will be more easily understood by reference 
 to an actual example. In Figs. 64-67, we give charts over 
 a large area, for the four days November 10-13, 1877, at 
 7.35 a.m. Washington. None of these show the ZODC of 
 
 FIG. 64. Southerly type of weather. 
 
 equatorial low pressure, but in all the tropical belt of 
 anticyclones and the temperate and Arctic zone of low 
 pressure are very obvious. In all we find three persistent 
 anticyclones, one over the lower Mississippi valley, another 
 in mid- Atlantic, and a third over Moscow. The North 
 Atlantic and Hudson's Bay Territory are covered by low 
 
338 
 
 WEATHER. 
 
 pressure, and this area is the theatre of the formation of 
 an incessant series of new cyclones, whose history we are 
 now going to trace. 
 
 But first let us consider the southern edges of the 
 tropical anticyclones. The east winds under the American 
 high pressure are the trade-winds of Cuba and the Central 
 
 FIG. 65. Southerly type of weather. 
 
 American republics as shown by the small arrows ; the 
 Atlantic anticyclone gives the regular trades of that 
 ocean, and the anticyclone, whose edge we see in the 
 chart over Moscow, really extends over the whole of 
 Siberia, and gives the north -east monsoon to the Indian 
 Ocean. This all shows in a very striking manner the 
 
TYPES AND SPELLS OF WEATHER. 
 
 339 
 
 dependence of weather in different parts of the world on 
 each other, and also the true nature of the problems 
 which the meteorologist has to solve. The cyclone which 
 covered Great Britain on November 10, 1877, had its origin 
 in the Atlantic anticyclone which dominates the trade- 
 winds. Its eastward path was deflected by the Asiatic 
 
 ioo So 
 
 FIG. G6. Southerly type of weather. 
 
 anticyclone which caused the north-east monsoon in 
 Calcutta, and its intensity was increased by a depression 
 which passed into the Atlantic from the Hudson's Bay. 
 At the same time the actual force of the wind was 
 determined at every station by the exposure ; every hill 
 drew a little more or less rain, every tidal river brought 
 
340 
 
 WEATHER. 
 
 up local showers. It is this combination of the very 
 large with the very small which constitutes one of the 
 great difficulties of meteorology, and all the skill of the 
 meteorologist is required to assign to each influence its 
 proper place and value. He cannot explain the weather 
 on any day without casting his eyes over the whole 
 
 FIG. 67. Southerly type of weather. 
 
 northern hemisphere and round the little hills and valleys 
 which bound his own horizon. 
 
 Keturning now to our cyclones north of the tropical 
 belt of anticyclones. On November 10 a well-defined 
 cyclone lay between Scotland and Iceland, a V-depression 
 lay in the col between the Atlantic and American anti- 
 
TYPES AND SPELLS OF WEATHER. 341 
 
 cycloDes, while another cyclone covered the Hudson's 
 Bay. Arrows show the general direction of the wind in 
 the leading capitals and cities, and partially the varying 
 velocity. 
 
 By next day, the llth, the position and shape of the 
 European cyclone had scarcely changed, but the depth 
 had increased no less than six-tenths of an inch (15 mm.), 
 while the position of the isobar of 30'0 inch remained 
 the same over Europe. The Atlantic V and the Hudson's 
 Bay cyclone have disappeared and apparently been 
 .merged into the great depression which now fills the 
 whole North Atlantic. The nature of this change should 
 be carefully considered, as it is most typical of Atlantic 
 weather, and shows the nature of what the meteorologist 
 has got to deal with, and the impossibility of ever 
 arriving at any calculation as to cyclone paths. If a 
 cyclone would only keep a tolerably regular shape, and 
 move in even a moderately definite path, weather fore- 
 casting would be one of the most certain and definite of 
 sciences. But when, as here, two or three cyclones 
 gather themselves up into a new formation within twenty- 
 four hours, there is nothing definite to trace. We cannot 
 say how the Hudson's Bay cyclone has moved into the 
 Atlantic, even if it is correct to say that it had done so 
 at all. However, such is the way of cyclones, and our 
 object here is to explain it all as best we can. We often 
 see a precisely analogous action when watching the flow 
 of a river. The impulse of two or three small eddies 
 seems to form one big one in a new place. 
 
 The effect of these changes on Western Europe would 
 be to cause a rapid fall of the barometer from surge, 
 
342 WEATHER. 
 
 not from advance of a cyclone and to increase the 
 steepness of the gradients with the general intensity of 
 the weather. 
 
 The irregular bends in the isobar of 30'0 ins. (763 
 mm.) over Europe should be noted, for they are due to 
 small secondaries, and indicate rain without wind in their 
 respective districts. 
 
 Below the cyclone-region, the Atlantic and American 
 anticyclones are joined by an arm of high pressure, while 
 a very pronounced depression appears over the Bermudas. 
 
 On the following day, November 12 (Fig. 66), though 
 the position of the centre and depth of the European 
 cyclone are still unchanged, the area of low pressure has 
 extended over the whole of Europe, which is now covered 
 by a mass of secondaries ; and the isobar of 3OO ins. 
 (763 mm.) has been pushed a little eastwards. Observe 
 that the line of weakness, across which the cyclone en- 
 deavours . to pass, is the col between the Atlantic and 
 Siberian anticyclones. 
 
 The loop in the isobars which lay over Bermuda on 
 the previous day has now moved to the north-east and 
 developed into a moderate cyclone, while a third de- 
 pression appears over Hudson's Bay. 
 
 Now look at the last chart (Fig. 67) for November 
 13, and try to say how it is related to the previous figure. 
 The European cyclone is now represented by an irregular 
 depression over Iceland, whose lowest point is 06 in. 
 (15 mm.) above the level of the previous day, but the 
 general sweep of the isobars unquestionably connect this 
 with another depression in mid-Atlantic. The latter 
 certainly represents the cyclone which lay over that 
 
TYPES AND SPELLS OF WEATHER. 343 
 
 region in the preceding chart, much diminished in in- 
 tensity, and partially coalesced with the Hudson's Bay 
 depression. The European secondaries of the previous 
 day are now represented by a well-marked deflection of 
 the isobars over the Gulf of Lyons. We can describe all 
 this, but how can we trace the history of each individual 
 depression ? 
 
 While the weather in the Atlantic has diminished in 
 intensity, the low pressure over Southern Europe has 
 extended into Africa ; but in spite of all these changes, 
 the position of the isobar of 3OO ins. (763 mm.) remains 
 very stationary over the eastern shores of the Baltic. 
 
 Now, though different totally in detail, these changes 
 are exactly analogous to the fusion of various cyclones 
 into new configurations which occurred in the previous 
 days, and similar changes would continue as long as this 
 type of weather lasted. We might describe the whole 
 roughly by saying that, while the anticyclones remained 
 stationary, the generally low area of the North Atlantic 
 was the theatre of the incessant formation and breaking 
 up of cyclones. 
 
 We do not purpose going into the details of the 
 weather-sequence during this type in any one place or 
 country, but the broad features in Western Europe to a 
 solitary observer are very simple. 
 
 As atmospheric pressure falls, temperature rises, and 
 the sky grows dirtier till drizzling rain sets in. The 
 wind, from some point of south, having backed slightly, 
 rises in velocity till the barometer has reached its lowest 
 point. As soon as pressure begins to increase, the wind 
 veers a little and gradually falls, the air becomes cooler, 
 
314 WEATHER. 
 
 and the sky begins to clear ; but the clouds rarely become 
 hard, or form well-defined cumulus. By next day, per- 
 haps, the same sequence is repeated, varying only in 
 intensity, but not in general character, and this alteration 
 perpetually lasts for weeks at a time. 
 
 The temperature of this type is always high, partly 
 because of the prevailing southerly winds, and, as the 
 cyclones die out, the slight degree of cold which follows 
 is very noticeable. Sometimes a portion of the Russian 
 anticyclone reaches Great Britain, and in winter white 
 frost of short duration would ensue. The air is always 
 damp, principally from the action of southerly winds, and 
 for the same reason the sky is usually dull or overcast. 
 The wind is remarkable for its steadiness, both in direction 
 and the way of blowing. This results from the large 
 scale on which the cyclonic action takes place. 
 
 So far for the explanation of weather after it has 
 passed, but we may now consider how this example illus- 
 trates the nature of forecasting. Beginning in the west, 
 the United States forecaster has two classes of cyclones 
 to deal with. No rules can be laid down in the abstract 
 by which, given a cyclone to-day, he can calculate where 
 it will be to-morrow. But by experience he knows that 
 the cyclones which form near Bermuda run a totally 
 different course from those which form over Hudson's 
 Bay, and he can generally form a very fair estimate of 
 their probable motion. 
 
 In Great Britain it is evident that when a persistent 
 spell of this type is recognized as having set in, the 
 general character of the weather and direction of the 
 wind are at once indicated. The forecaster knows that 
 
TYPES AND SPELLS OF WEATHER. 345 
 
 the cyclones which press in from the Atlantic will never 
 get past, so that his country will always be under the 
 influence of the front only of the depressions. All that 
 is necessary for storm-warnings is to watch for signs of 
 the intensity becoming so great as to give rise to a gale. 
 The example we have just given in Figs. 64 and 65 is 
 very characteristic of a gale coming on entirely from in- 
 crease of intensity, without any motion of a cyclone. This 
 shows the value of any indications of increasing or decreas- 
 ing intensity which can be derived from any source. 
 
 An inspection of the illustrative charts will show that 
 the area involved is so large that it is hopeless to trace 
 the cyclones as a whole, but that usually within the area 
 of the British telegraphic reports, and always somewhere, 
 there are localities to the east or north-east where the 
 pressure is steady. Over the Atlantic great variations 
 occur, and the forecaster has, therefore, first to try and 
 discover the area of steady pressure, and then to keep a 
 sharp look-out for any rapid fall of the barometer over 
 the west coast of Ireland, which would produce steep 
 gradients and their associated gales. When once a frag- 
 ment of a ring of steep gradients is formed, its progress 
 eastwards must be traced by telegraph, and watch must 
 be kept that there is no giving way of pressure over 
 Scandinavia. Since the rate of progress of the steep 
 gradients is usually slow and pretty regular, and since, 
 as has" beenj shown above, the direction of the wind with 
 the general character of the weather is subject to little 
 uncertainty, gales 'of this type are practically forecast 
 .with almost greater success than any other class. 
 
 The forecaster in Central Europe is not so fortunate. 
 
346 WEATHER. 
 
 The nature of the changes there are so complex and sex 
 ill defined that he can scarcely follow them after they 
 have happened, so that he can do little more than forecast 
 generally unsettled weather while the barometer is falling 
 and secondaries are forming in sympathy with the great 
 Atlantic cyclones. After the mercury has begun to rise, 
 improving weather is certainly indicated. 
 
 The Kussian forecaster has a totally different task. 
 He recognizes the type, and knows that as long as his 
 anticyclone lasts there is no fear of bad weather. We 
 have shown that there is always some isobar-, in this case 
 3OO ins. (763 mm.), which remains nearly stationary, and 
 he has to find this out in each case, and watch for any 
 symptoms of a serious change. 
 
 Thus we see, as the foundation of all synoptic fore- 
 casting, that the official in charge of the central bureau 
 must learn by experience the ways of cyclones in his own 
 country, and decide each case on its own merits accord- 
 ing to the best of his judgment. 
 
 The property of any type of weather to continue for 
 any length of time is called the " persistence " of that 
 type. Many phases of weather are due to this principle, 
 and for forecasting it is very important to recognize any 
 signs of this continuance ; but, as the indications for this 
 type are the same as for any other, we will describe the 
 details of persistence later on. 
 
 Then as to signs of change. This type may merge 
 insensibly either into the westerly on one side, or the 
 easterly on the other, the latter change being usually the 
 more abrupt ; but it is not possible to give any detailed 
 description of symptoms of change. 
 
TYPES AND SPELLS OF WEATHER. 34:7 
 
 WESTERLY TYPE. 
 
 In this type the permanent belt of anticyclones does 
 not extend very far north, and pressure decreases steadily 
 from the tropics towards the north. Under these circum- 
 stances, cyclones are developed on the north side of the 
 Atlantic anticyclone, which roll quickly eastwards along 
 the high-pressure belt and usually die out after they 
 have become detached from the Atlantic anticyclone in 
 their eastward course. Their intensity, and consequently 
 the weather they produce, may vary almost indefinitely. 
 When the cyclones are formed so far south that their 
 centres cross Great Britain, and are of moderate size, the 
 intensity is usually great, and severe well-defined storms, 
 with sharp shifts of wind, are experienced. These occur 
 most frequently in spring and autumn, and are the most 
 destructive storms which traverse Great Britain. 
 
 In another modification, while the pressure is low to 
 the north, and the isobars run nearly due east and west, 
 the whole of the arctic area of low pressure surges south- 
 ward, with an exceedingly ill-defined cyclone, bringing a 
 rim of steep gradients along the edge of the Atlantic 
 anticyclone, and across Great Britain, in a manner 
 analogous to the phase of southerly type before ex- 
 plained. The indications then are for rain and westerly 
 gales, with very little shift of wind. This phase belongs 
 almost exclusively to the winter months. 
 
 But the commonest modification at every season, and 
 that which forms about seventy per cent, of European 
 weather, is when the intensity is moderate, and the 
 cyclone paths are so far to the north of the British 
 
348 WEATHER. 
 
 Islands that the wind merely backs a point or two from 
 the south-west as the cyclone approaches, and veers a 
 point or two towards the west as the cyclone passes, the 
 general direction of the wind being between south-west 
 and west, without rising to the strength of a gale, while 
 rain is moderate in quantity. 
 
 Sometimes in summer a prolongation of the Atlantic 
 anticyclone covers the southern portion of Great Britain, 
 and distant cyclones of small energy just influence the 
 northern countries of Europe. Then the intensity is 
 too small to develop rain, and only produces cloud in 
 the middle of the day, so that fine, dry weather is indi- 
 cated, which when very prolonged may give rise to 
 drought. This is by far the commonest of all weather 
 types in temperate regions, and occurs at every season of 
 the year. 
 
 The existence of this type in Europe is sometimes 
 associated with a similar phase of weather in the United 
 States. That is to say, pressure being high over Mexico, 
 cyclones form over the Kocky Mountains, and then pass 
 along the line of the Lakes into the Atlantic. To this 
 class belong almost exclusively the cyclones which pass 
 from the United States, over the Atlantic, into Europe. 
 At other times, a persistent anticyclone may cover the 
 American continent, and the whole of the European 
 system of cyclones is born and developed in mid- Atlantic. 
 
 Before we give more details, it may be well to 
 exemplify some of the leading features of this type. In 
 Pigs. 68-71 we therefore give charts over the North 
 Atlantic and Europe for the four days, February 26 to 
 March 1, 1865. These may be taken to represent a fair 
 
TYPES AND SPELLS OF WEATHER. 
 
 34D 
 
 100 
 
 specimen of ordinary broken weather in Europe, without 
 sufficient intensity to give steep gradients and severe 
 gales. In all, the Atlantic anticyclone was flanked on 
 the west by another over the American continent, and 
 
 60 40 20 20 *0 GO 
 
 80 
 
 100 80 60 40 20 
 
 FIG. 68. Westerly type of weather. 
 
 40 
 
 on the east by another over Central Asia. This last 
 only appears in three of the charts. We have therefore 
 to deal with the trade-wind region south of the Atlantic 
 anticyclone ; the cols on either side of it ; and the slope 
 of decreasing pressure which extends towards the Pole. 
 
 60 
 
350 
 
 WEATHER. 
 
 We shall take the equatorial region first, because we 
 want to show the nature of weather-changes in that part 
 of the world, but not to have to recur to the subject 
 again till the end of this chapter. On all four days the 
 
 20 
 
 20 
 
 60 
 
 40 
 
 20 
 
 100 
 
 60 40 20 i 
 
 FIG. 69. Westerly type of weather,, 
 
 40 
 
 broad features of tropical pressure distribution are the 
 same ; that is to say, the type of weather is essen- 
 tially constant. But in details no two days are alike, 
 for a series of bends in the isobars denote a succession 
 of weather changes, some of whicli eventually have an 
 
TYPES AND SPELLS OF WEATHER. 
 
 351 
 
 influence on Europe. On the first day, February 26 
 (Fig. 68), the isobar of 29*9 ins., (760 mm.) only shows 
 one bend northwards, while the north-east and south-east 
 trades are separated by a calm near the equator. By 
 
 100 80 60 40 20 20 40 
 
 60 
 
 ic!v" 
 
 ''-^f.. \ 
 
 / I ) 
 
 
 
 20 
 
 100 
 
 60 40 20 20 
 
 FIG. 70. Westerly type of weather. 
 
 40 
 
 next day this bend had become more pronounced, and 
 moved a little more to the north-east. This latter motion 
 is very interesting, for the prevailing wind is north- 
 easterly, but the direction of the wind has not conformed 
 to the bend of the isobars in the manner which might 
 have been expected. 
 
S52 
 
 WEATHER. 
 
 On the third day, February 28 {Fig. 70), very great 
 changes have occurred. Under the col which lies near 
 Bermuda, a second bend has made its appearance so a& 
 to greatly modify the trade-wind region in the West 
 Indies; by next day (Fig. 71) this bend has developed 
 into a well-defined cyclone of very moderate intensity,. 
 
 FIG. 71. Westerly type of weather. 
 
 which moved towards the north-east, and eventually 
 affected the coasts of Great Britain. We thus see that 
 the details of pressure-distribution are perpetually changing 
 in this region, but never more than a certain amount. 
 We can therefore easily understand why the weather 
 which is always experienced in these latitudes is described 
 
TYPES AND SPELLS OF WEATHER. 353 
 
 as generally easterly, variable in strength, with the 
 weather fine or showery according to circumstances, but 
 never following the cyclonic sequence of the temperate 
 zone. This modified alternation of weather is called the 
 fluctuation of its type, as opposed to a change of type, 
 which would involve a totally different distribution of 
 pressure. 
 
 From this digression on the trade-winds we must now 
 return to the cyclone-traversed region of the temperate 
 zone and the cols of the more tropical parts of the world. 
 On February 26 (Fig. 68), we find a fragment of a large 
 cyclone over Norway, a V over Great Britain, some 
 complex secondaries over the Mediterranean, and an 
 anticyclone over the United States. Note, however, 
 three innocent-looking bends on the north-west edge of 
 the Atlantic anticyclone. 
 
 By next day the Norwegian cyclone and the British 
 V have fused or merged into an irregular cyclone which 
 covers Scandinavia ; while the Mediterranean secondaries 
 have also formed a new cyclone, and a corner of the 
 Asiatic anticyclone just appears near the Black Sea. 
 Further west, the three bends in the isobars which looked 
 so harmless the preceding day are now reduced to two, 
 but have gained intensity. One lies to the south of 
 Iceland, and the wedge which precedes it determines the 
 weather for this day in Great Britain. The other, which 
 is less intense, lies south of Newfoundland ; but the 
 American anticyclone has somewhat retreated. 
 
 By next day, February 28 (Fig. 70), the Norwegian 
 cyclone had nearly died out, while the Atlantic cyclone, 
 with its associated wedge, had travelled eastwards and 
 
 2 A 
 
354 WEATHER. 
 
 much increased in intensity. In connection with this, a 
 mass of secondaries had developed over Germany and 
 Central Europe in the col which lay between the Atlantic 
 and Asiatic anticyclones. Similar changes are most cha- 
 racteristic of European weather during the persistence 
 of this type, and a knowledge of them is of the utmost 
 importance in forecasting. The cyclone which has come 
 in from the Atlantic is 'moving and will continue to 
 move towards the north-east, and so far it might be said 
 that it did not affect the forecasters in Central Europe ; 
 but when we know that the passage of the depression 
 will develop secondaries and bad weather, it is evident 
 that the indirect influence of the Atlantic cyclone is 
 very great. In every part of the world we may say that 
 the passage of a cyclone in the temperate zone will 
 develop secondaries in the tropical col over which it 
 passes. We may also use this as an illustration of the 
 fact that the tracking of existing cyclones plays but a 
 small part in forecasting, as compared with the larger 
 question of detecting influences which will make new 
 cyclones or destroy old ones. 
 
 The col nearly over Bermuda had developed a well- 
 marked inflection near the West Indies. 
 
 Lastly, by the morning of March 1 (Fig. 71) all these 
 changes had somewhat developed. The British cyclone 
 had begun to fill up, and the European secondaries had 
 much diminished in intensity. This is an example of 
 what we have already mentioned in the abstract that a 
 cyclone which is filling up is decreasing in intensity, and 
 vice versa. In mid- Atlantic, the bend in the isobars near 
 Bermuda, as before mentioned, has developed into a small 
 
TYPES AND SPELLS OF WEATHER. 355 
 
 cyclone, which lies between the Atlantic and American 
 anticyclones. The latter has moved a little towards the 
 east. 
 
 We may give the general features of British weather 
 for these four days as a sample of the type, and the 
 reader may fill up those in any other country for himself. 
 On the morning of the 26th, the weather in Great Britain 
 was wet and broken from the influence of the V. Next 
 day the weather was beautifully fine from the wedge ; 
 the third day, wet and stormy this time from a true 
 cyclone and, finally, cold and fine from the rear of the 
 same cyclone on the fourth day. Similar alternations 
 of weather would go on, with endless modifications, so 
 long as the type persisted. From this we see the con- 
 trast which the westerly type presents to the southerly 
 one. In the latter, Great Britain was constantly exposed 
 to the influence of the fronts only of cyclones ; in the 
 former, both fronts and rears develop their characteristic 
 weathers. We gave the details of the changes of tem- 
 perature which occurred over the whole of Europe 
 during the first three days in our chapter on Heat and 
 old. 
 
 In this example the United States was constantly 
 under the influence of a persistent anticyclone, and, so 
 far as it goes, this shows the nature of a type of weather 
 in that country. 
 
 We can now easily understand the following particu- 
 lars of the characteristic weather of this type. 
 
 The general temperature of this type is about the 
 average of the season a little warmer in front of the 
 cyclones, and a little colcjer in rear. In winter, however, 
 
35(5 WEATHER. 
 
 a great prevalence of this type gives an open season, as 
 the high wind prevents frost, unless the cyclones are so 
 far north that the influence of the Atlantic anticyclone 
 is felt. 
 
 In summer, on the contrary, if the type be intense, 
 the temperature is below the average, from the excess of 
 cloud hiding the sun. 
 
 Another important consideration, as regards tempera- 
 ture, depends on the position of the normal cyclone-path. 
 The difference of temperature just north and south of a 
 cyclone-centre is very marked, so that when cyclones 
 pass further south than usual, the temperature of the 
 region lying between the usual and actual paths is greatly 
 lowered. 
 
 To this type also belongs a peculiar class of warm, 
 cloudy anticyclones, which seem to be associated with 
 cyclones passing to the far north, but which have not 
 yet been investigated. 
 
 As regards damp, wind, and weather, the most notice- 
 able feature of this type is the changeableness of all 
 these elements. This must be so, because the rapidly 
 moving cyclones bring up alternately the damp, rainy, 
 southerly winds and the dry, cold, northerly currents of 
 their fronts and rears respectively. 
 
 The telegraphic forecaster, instead of thinking how 
 cyclones are going to die out, as in the southerly types, 
 has to consider along what paths they will move. No 
 generalities are of much assistance ; his opinion must be 
 formed by his own judgment, and from experience of 
 cyclone-paths in his own country. For instance, in Great 
 Britain, he can often tell whether the centre will skirt 
 
TYPES AND SPELLS OF WEATHER. 357 
 
 the north-west coasts of Scotland, or else traverse England 
 on its way to Denmark. 
 
 Dr. J. van Bebber has classified the cyclone-paths of 
 Central Europe for the use of the Deutsche Seewarte, 
 while in the United States they know that the great 
 majority of cyclone-paths pass along the line of the 
 Lakes and St. Lawrence valley. But, in spite of any 
 classification, we must never forget that a cyclone may 
 travel in nearly any direction, and for that reason the 
 knowledge of the most usual paths is of less use in fore- 
 casting than in explaining the climatic peculiarities of a 
 country. 
 
 NORTHERLY TYPE. 
 
 The special feature of this type is the presence of a 
 large anticyclone over Greenland and the arctic portion 
 of the Atlantic, which either joins the Atlantic anti- 
 cyclone or is only separated from it by a col. On the 
 east side of this, over Europe and Russia, lies a persistent 
 area of low pressure, which is the theatre of the forma- 
 tion of an incessant series of cyclones, while innumerable 
 secondaries are formed over Great Britain and France. 
 The cyclones either move eastwards, or else, if they stand 
 still, surge up and down and alter their shape in a very 
 peculiar manner. 
 
 This is, in fact, the exact converse of the southerly 
 type. In that, Europe was persistently under the in- 
 fluence of southerly winds and cyclone-fronts ; in this, it 
 is as steadily under the influence of northerly winds and 
 cyclone-rears. 
 
358 
 
 WEATHER. 
 
 This type occurs chiefly in the winter, spring, and 
 summer ; it is very rare in the autumn months. 
 
 On the American side of the Atlantic, this distribution 
 of pressure exercises a profound influence on the general 
 character of the weather. Instead of the cyclones finding 
 an easy path into the Atlantic, their eastward progress 
 
 DO 40 
 
 O 2O 
 
 FIG. 72. Northerly type of weather. 
 
 is checked by the areas of high pressure, and in some 
 instances their direction is even reversed. 
 
 For instance, in Figs. 72-75 we give reductions from 
 the United States maps of the northern hemisphere, for 
 March 22 to 25, 1878, at 0.43 p.m. Greenwich, or 7.35 
 a.m. Washington time. In all the Atlantic high pressure 
 
TYPES AND SPELLS OF WEATHER. 
 
 359 
 
 will be found stretching far north, till it nearly meets 
 another anticyclone lying over Greenland ; and in all, 
 relatively low pressure will be found both over Northern 
 Europe and the western states of the American Union. 
 
 On March 22 (Fig. 72), each of these low areas con- 
 tains a cyclone, one over Finland, giving northerly winds 
 
 
 FIG. 73. Northerly type of weather. 
 
 and cloudy weather over Great Britain and the greater 
 part of Europe ; the other about three hundred miles 
 west of Newfoundland. An independent cyclone lies 
 near Florida, and a col separates the Atlantic and Green- 
 land anticyclones. 
 
 By next day (Fig. 73), though the centre of the 
 
360 
 
 WEATHER. 
 
 Finland cyclone has hardly changed its position, the 
 area has extended westwards, and the weather over 
 Western Europe becomes rather worse. 
 
 Note particularly that the barometer has fallen about 
 three-tenths of an inch in some parts of England, but 
 owing to a surge, and not to the passage of a cyclone. 
 
 FIG. 74. Northerly type of weather. 
 
 On the other side of the Atlantic, the Newfoundland 
 cyclone has moved westwards, joined the Florida cyclone, 
 and so extended its area as to cover the whole of the 
 northern states. This is the reverse of any we have 
 seen before. The Atlantic anticyclone has enlarged, and 
 projects further north. 
 
TYPES AND SPELLS OF WEATHEK. 
 
 361 
 
 By midday of the 24th (Fig. 74), the Finland cyclone 
 has lost any definite shape, while another centre has 
 formed over the Carpathians, and a complicated system 
 of secondaries over Western Europe. The whole is most 
 typical of this kind of weather. 
 
 We referred to this chart in our chapter on Squalls, 
 
 FIG. 75. Northerly type of weather. 
 
 for out of the complex bends in the isobars which we see 
 over England and France developed a V-shaped de- 
 pression of great intensity, a squall in which capsized the 
 British man-of-war Eurydice almost within sight of port. 
 
 The American cyclone has moved towards the south- 
 west, and is now centred over the New England states. 
 
362 WEATHER. 
 
 It has also slightly diminished in size, but increased in 
 intensity, probably under the action of the anticyclone 
 which lies in the north-west. 
 
 Lastly, on the chart for March 25 (Fig. 75), we 
 see that the two centres of the European cyclone have 
 moved as if they were revolving round each other, or 
 round a common centre, while the whole level has risen, 
 and the secondaries have much diminished in com- 
 plexity. 
 
 With these changes, and the rise of the barometer, 
 the weather over Great Britain and Western Europe has 
 much improved, but the wind retains its prevailing 
 northerly set. 
 
 Our illustration certainly represents weather-changes 
 of exceptional complexity, but still it shows all the more 
 forcibly the impossibility of applying numerical calcu- 
 lations either to the motions, the winds, or any other 
 phenomena of a cyclone. 
 
 This is equally evident when we look on the other 
 side of the Atlantic. The cyclone there has reversed 
 its direction and now gone towards the north-east. Besides 
 this, the intensity has still further increased so as to give 
 worse weather over Canada, New Brunswick, and Nova 
 Scotia, while one secondary projects towards Bermuda, 
 and another in the direction of Iceland. 
 
 So long as this type continues the sequence of weather 
 at any station is tolerably simple in Great Britain. As 
 the barometer falls, the wind veers towards the north- 
 east, with a hard, cloudy sky ; wind and rain according 
 to the intensity, with an increase of temperature ; and 
 then the sky clears, the wind backs by north towards the 
 
TYPES AND SPELLS OF WEATHER. 363 
 
 north-west, and the air gets colder as the mercury begins 
 to rise. 
 
 But during the whole continuance of this type, the 
 general northerly set of the wind and the peculiarly 
 hard sky are never lost, and numerous secondaries will 
 give rise to many puzzling contradictions between the 
 movement of the barometer and the severity of the 
 weather. 
 
 From this it is manifest that the general temperature 
 of the type must be below the average, and the air must 
 be also dry from the prevalence of northerly winds. 
 
 During the persistence of this sequence of weather, 
 all European forecasters have to solve a problem exactly 
 the converse of that which was presented to them by the 
 southerly type. Then they looked westwards for the 
 daily arrival of cyclones, and eastwards for any symptoms 
 of a change of type. Now they look eastwards for a 
 daily formation of new depressions, and westwards for 
 any signs of decreasing pressure over Ireland which would 
 be the forerunner of a different type of atmospheric 
 circulation. 
 
 EASTERLY TYPE. 
 
 In this type the sequence of weather and cyclone- 
 motion turns round the presence of a persistent anti- 
 cyclone over Scandinavia, which profoundly modified the 
 motion of depressions which come in from the Atlantic. 
 The Atlantic anticyclone is, of course, always there ; but 
 a col, which is formed between it and the Scandinavian 
 high pressure, crosses Europe and impresses a very 
 
36 4< WEATHER. 
 
 definite character on the weather-changes. When cyclones 
 coming in from the Atlantic meet this col, they are 
 either arrested in their course, and remain brooding over 
 the Bay of Biscay, or else they pass through the col in 
 a south-easterly direction. In rare cases cyclones are 
 formed on the southern side of the Scandinavian anti- 
 cyclone, with their centres over Southern Europe or the 
 Mediterranean Sea, and these often move towards some 
 point of west. Nothing can show more clearly than this 
 the value of type-groups in determining the probable 
 course of any cyclone. In the abstract, a cyclone may 
 go in any direction, and in all the European classes we 
 have so far examined they always move towards some 
 point of east ; but in this type of pressure-distribution 
 only we may sometimes look for depressions which travel 
 westwards. 
 
 This type occurs at all seasons of the year, though 
 it is most frequent in winter and spring, and most rare 
 in autumn. In Great Britain it often persists for two 
 or three weeks consecutively, and gives rise to destructive 
 easterly gales. Nearly one-half of the wrecks on the 
 British coast are due to gales of this class. No direct 
 connection can be traced between the occurrence of this 
 type in Europe and any particular phase of weather in 
 the United States or Canada. 
 
 But before we go into details, we may illustrate the 
 nature of this type by an actual example. In Figs. 
 76-79, we give large charts of a considerable portion of 
 the northern hemisphere for the four days, February 
 25-28, 1875, at about 8 a.m., Greenwich. In all, an area 
 of high pressure rests over Scandinavia, while the Atlantic 
 
TYPES AND SPELLS OF WEATHER. 
 
 365 
 
 anticyclone reaches so far north as to suggest some 
 features of the northerly type. The col of low pressure 
 below these two anticyclones is the theatre of cyclone 
 activity, and we will now describe how the weather in 
 Western Europe was affected by these changes. On the 
 morning of February 25 (Fig. 76), we find the Scandi- 
 
 60 40 
 
 FIG. 76. Easterly type of weather. 
 
 navian anticyclone almost meeting a wedge of high 
 pressure stretching northwards from the Atlantic anti- 
 cyclone to Greenland. The pressure for several days 
 previous had belonged to the northerly type, with an 
 anticyclone over Greenland, which had now drifted east- 
 wards and joined the Scandinavian anticyclone. To the 
 
366 
 
 WEATHER. 
 
 south of this at least three cyclones are found : one over 
 the Azores, another at the entrance to the English 
 Channel, the third over Italy. These must all be treated 
 as belonging to the same system, as they are all formed 
 in the same pit of low pressure. The weather, of course, 
 is bad all over France, Germany, and Italy. The American 
 
 FIG. 77. Easterly type of weather. 
 
 reports are meagre, but point to the existence of a cyclone 
 in Lower Canada. 
 
 By next day (Fig. 77), the Scandinavian anticyclone 
 has increased in height, while the Atlantic one has 
 retreated nearly to its usual position. The Italian 
 cyclone has moved a little to the north-east, while that 
 
TYPES AND SPELLS OF WEATHER. 
 
 367 
 
 in the Bay of Biscay has apparently moved a very little 
 to the south-west, and so far absorbed the Azores depres- 
 sion that the latter has become degraded into a secondary. 
 Here we have the same fusion of cyclones which we have 
 seen in all the other types, combined with the stationary 
 character which is so peculiar to this class of weather. 
 
 FIG. 78. Easterly type of weather. 
 
 Across the Atlantic an intense secondary has formed over 
 New Brunswick, while another shallow one has pushed 
 itself into the col between the Scandinavian and Atlantic 
 cyclones. 
 
 On ^February 27 (Fig. 78) these changes have made 
 further progress. Though the general position of the 
 
368 
 
 WEATHER. 
 
 European area of low pressure has not materially altered, 
 the cyclones which lie within it have decreased in com- 
 plexity, though a new depression has formed in a col 
 between the Azores and the Canaries. The two American 
 secondaries have fused into one large primary, and a 
 large col covers the Central Atlantic. 
 
 FIG. 79. Easterly type of weather. 
 
 Lastly, on the 28th (Fig. 79), while the Scandinavian 
 anticyclone has diminished in height and area, the Atlantic 
 anticyclone, on the contrary, has increased no less than 
 04 inch (10 mm.) in height, and much increased in size. 
 The size and intensity of the European low pressure has 
 diminished, but its components are more complex; so 
 
TYPES AND SPELLS OF WEATHER. 369 
 
 that while weather has improved over Great Britain, it 
 is worse in many parts of France and Italy. Across the 
 Atlantic, all we can say is that where one large cyclone 
 was yesterday, there are now two secondaries : one intense 
 over Nova Scotia, another slight in the Atlantic col. 
 This is one of the numerous cases where it is impossible 
 to trace the exact history of pressure-changes. 
 
 The general character of the weather in Great Britain 
 during the persistence of this type is very well marked. 
 The sky is usually black, and, even if there is a certain 
 amount of blue overhead, the horizon has a peculiar 
 black, misty look, popularly known as an "eastern haze." 
 This is quite different from the misty horizon of a calm 
 day in the westerly type, and is associated with the 
 peculiar bitter feel of an east wind. A well-known 
 saying is 
 
 " When the wind is in the east, 
 It's good for neither man nor beast ; " 
 
 and this is certainly no exaggeration. 
 
 This is the most striking illustration we can have of 
 the general principle that no instrumental records can 
 take the place of verbal description. We might find two 
 north-east winds recorded automatically, of exactly the 
 same velocity and temperature one on the northern side 
 of a cyclone of the westerly type, the other at the edge 
 of the Scandinavian anticyclone in the easterly type. 
 Head mechanically, they might be taken to be identical, 
 while practically they are very different. Unfortunately 
 we can give no explanation of the malignant nature of 
 true east winds. 
 
 The temperature is generally low, but more variable 
 
 2 B 
 
370 WEATHER. 
 
 than during the northerly type. This is because the 
 cyclone-centres sometimes get so far east as to bring up 
 a breath of southerly wind, which is speedily driven back 
 by a new irruption of pressure from Scandinavia. 
 
 The wind is always from some point of east, with less 
 tendency to back towards the north than during the 
 continuance of the northerly type, and generally keeping 
 between north-east and south-east. The contrast between 
 this and the westerly type will be strikingly evident if 
 we look back at Figs. 71-73, and note that they refer to 
 the same three days of the year, February 26-28, as 
 our last three (Figs. 77-79). By selecting these dates on 
 different years, all diurnal and seasonal variations are 
 equalized, and the entire difference of wind and weather 
 is solely due to difference of type. 
 
 Forecasting during the persistence of this type pre- 
 sents the greatest difficulties, especially in Western 
 Europe. Though the general character of the weather- 
 sequence may be sufficiently obvious, still there is the 
 utmost uncertainty as to the paths of cyclones. When 
 these come in from the Atlantic, we have no means of 
 saying whether they will pass through the col in a south- 
 easterly direction, or whether they will be deflected to a 
 north-easterly course. In addition to this, the motion 
 of cyclones, in whatever direction, is so irregular that the 
 forecasters doomed to frequent failure. 
 
 The signs of persistence are chiefly such as may be 
 derived from watching the position of the Scandinavian 
 anticyclones, and the continuance of low pressure to 
 the west of Ireland. The signs of change, on the con- 
 trary, turn round any diminution of pressure in Sweden, 
 
TYPES AND SPELLS OF WEATHER. 371 
 
 or the appearance of high pressure far north in the 
 Atlantic. 
 
 The four great types of weather which we have now 
 sketched are capable of being divided more minutely 
 into sub-types ; but these would vary so much for different 
 countries, that they cannot be detailed in this work. All 
 that we can do here is to note the universality of the 
 principle, and the properties of weather which the exist- 
 ence of types explains so readily. 
 
 Although we have already mentioned the great prin- 
 ciples of weather-changes which are designated by the 
 terms "intensity," "fluctuation," "persistence," "recur- 
 rence," and "dependence" of type more or less inci- 
 dentally, it may be well to add a few remarks here on 
 all of them, beginning with 
 
 INTENSITY. 
 
 We have already explained the term " intensity " as 
 applied to single cyclones, and shown both how it is 
 measured by the gradients and how it influences the 
 weather,, 
 
 But " intensity of type " denotes the character of a 
 sequence of weather to which the epithet of " broken " 
 would be applied. Broken weather is found by synoptic 
 charts to be the product either of small quick-moving 
 cyclones which only exist for a very short time, or of 
 numerous secondaries ; in contradistinction to the weather 
 produced by large low-gradient cyclones, moving slowly 
 and lasting for some days, which would be associated 
 with more settled weather. 
 
372 WEATHER. 
 
 The relation between these two kinds of intensity is 
 analogous to that between long, single, heavy gusts, and 
 numerous short puffs of wind; both are symptoms of 
 great atmospheric disturbance, though of a different kind 
 in each case. 
 
 FLUCTUATION. 
 
 The word " fluctuation " is applied to that limited 
 alternation of a general distribution of pressure which 
 occurs every day all over the world. In the tropics, 
 where pressure-distribution is unchanged for months, 
 fluctuation is chiefly confined to small modifications of 
 intensity, which make the weather a little better or a 
 little worse, according to circumstances. In semi-tropical 
 countries fluctuation is much larger, usually from the 
 formation of secondaries, though the general type does 
 not materially change. In the temperate zone, on the 
 contrary, we have not only enormous fluctuation of type, 
 but also complete alteration of the type itself. The 
 classification of phenomena called fluctuation is of the 
 greatest value in handling questions of weather-sequence, 
 as it enables us to separate that which is incidental from 
 that which is essential to any type. 
 
 PERSISTENCE. 
 
 The word " persistence " describes that prominent 
 feature of remaining pretty stationary which charac- 
 terizes all pressure-distribution over large areas. This 
 is always concurrent with a persistence of appropriate 
 
TYPES AND SPELLS OF WEATHER. 373 
 
 weather, and in this property of types we find the explana- 
 tion of many phenomena of weather and of many popular 
 prognostics. 
 
 For instance, in Great Britain, an interval of cold 
 weather in winter may be produced by the persistent 
 influence of either the northerly or easterly type ; or, if 
 only for two or three days, from the wedge-shaped area 
 of high pressure between two cyclones. So also a drought 
 may be induced either by a persistent anticyclone, or 
 else by cyclone-centres far north, when the intensity is 
 slight, while long-continued rain may accompany almost 
 any persistent type if the gradients be steep. 
 
 Then, as to weather-prognostics. It is a well-known 
 saying, that "When grouse come down into the farm- 
 yards it is a sign of snow." The birds are driven down 
 in search of food by the excess of snow already existing 
 on the moors, and so far the prognostic would refer to the 
 past rather than to the future ; but, by the principle of 
 persistence, the type which has already given so much 
 snow may be expected to continue for some time, and 
 therefore more snow may be expected. 
 
 In Germany there is a proverb, " Fresh snow, fresh 
 cold," which holds good for the same reason. 
 
 Similarly, the prognostics, " When a river rises with- 
 out any rain having fallen, bad weather may be expected," 
 or " Irregular tides are signs of rain," have a significance 
 for the future, though both are caused by past bad weather 
 at a distance; for the persistent type will almost cer- 
 tainly, sooner or later, bring more bad weather over the 
 place of observation. 
 
 On the same principle, the prognostic, " Breakers in 
 
874 WEATHER. 
 
 shore without wind are a sign of storm," holds on the east 
 coast as well as on the west, but for a different reason. 
 
 On the west coast, the breakers have sometimes run 
 on ahead of the cyclone which raised them ; but on the 
 east coast this does not occur, as, practically, all cyclones 
 move towards some point of east. 
 
 Nevertheless, though the storm which raised the 
 waves has never affected the place where they occur, 
 still it is extremely probable that another of the same 
 series will do so ; therefore the prognostic is good, 
 though less certain than on the west coast. 
 
 It is also manifest that the principle of persistence 
 has an important bearing on forecasts. Unfortunately, 
 though such types are common, it is not yet possible to 
 define any certain indications of change from one to 
 another. One sign of persistence may, however, be men- 
 tioned which rarely fails. 
 
 Sometimes a type apparently fails for a day or two, 
 bat then is re-established with great intensity. When 
 this occurs, its continuance for a considerable time may 
 safely be predicted. For instance, with the easterly 
 type a small cyclone frequently passes rather far to the 
 east, and the wind shifts to the south-west with increased 
 warmth; but when this dies out the easterly type is 
 re-established in full force. In these cases the appearance 
 of the weather is sometimes very characteristic, for, though 
 the wind is west, the look is that of an east wind, and 
 so obvious is this that the people say " that the east 
 wind is not gone yet." 
 
NIVEK. 
 
 Ni^ 
 
 TYPES AND SPELLS OF WEATHER. 875 
 
 BECURRENCE. 
 
 We have already explained the tendency of certain 
 kinds of weather to recur about the same date every year 
 so fully in our chapter on Seasonal Variations, that it is 
 unnecessary here to do more than allude to that great 
 principle of meteorology. We shall, however, better 
 understand now how the recurrence of weather is the 
 secondary product of the recurrence of a certain type of 
 pressure-distribution; and that to be a true periodicity 
 of cold, for instance, it is not only cold in the abstract, 
 but cold of the same type which must recur about the 
 same date in most years. 
 
 DEPENDENCE. 
 
 By "dependence" of type or weather is meant the 
 supposed connection between the occurrence of any par- 
 ticular type at one season of the year, and the consequent 
 occurrence of it or of another type at another season. 
 
 For instance, there is a common saying in Great 
 Britain, that if easterly winds prevail about the time 
 of the spring equinox, then a great preponderance of 
 easterly winds may be expected during the summer. 
 Put into the language of synoptic charts and types, this 
 means if the easterly type happens to prevail about the 
 23rd of March, then there will be a tendency of that 
 type to occur more often than usual in the course of the 
 summer. 
 
 Again, in most temperate countries, hot summers are 
 popularly supposed to be followed by cold winters, and 
 
376 WEATHER. 
 
 the latter are thought to depend in some way on the 
 former. This is much more difficult to express in synoptic 
 language, for heat and cold are not always produced by 
 the same causes, and, unless the same type of summer 
 is followed by the same type of winter, the apparent 
 relation of the two seasons is illusory. 
 
 The same conception of the dependence of one season 
 on the other is found in the tropics. H. F. Blanford 
 has found that in India there is an apparent dependence 
 or sequence of the summer wet season on the preceding 
 winter rains. 
 
 At present we can do little more than note such a 
 relationship of seasons, and cannot say whether there is 
 even such a dependence at all. The older weather-lore 
 seems to have been founded partly on observation, partly 
 on an intuitive belief in the general balance of nature. 
 In the main, the course of nature is constant; if the 
 summer is hotter than usual, a cold winter is required to 
 restore equilibrium, and so on for any other phenomenon 
 of weather. 
 
 The middle stage of meteorological investigation 
 seeks to find proof of such relation by comparing statistics 
 of rain at different seasons. Here, of course, the great 
 difficulty is to be certain whether all the rains which we 
 compare are really of the same type. 
 
 The latest phase of thought would look for some 
 connection or sequence between the forms and intensities 
 of atmospheric eddies. All we can do is to note the facts 
 for future research ; and to remark that at the present 
 time no use can be made of dependence of type in 
 practical weather-forecasting. 
 
TYPES AND SPELLS OF WEATHER. 377 
 
 CHANGE OF TYPE. 
 
 So far we have supposed well-defined specimens of. 
 each type, but in practice we meet with many transitional 
 forms. Thus the southerly type may merge by insensible 
 gradations into either the easterly or westerly, but in no 
 case can it grow into the northerly. Similarly the 
 westerly type may approximate on either side towards 
 the southerly or northerly, but never jump suddenly into 
 the easterly. In like manner the northerly and easterly 
 types can only merge into those next to themselves on 
 either side, but never into their opposites. This is 
 obvious when we reflect that the types are determined, by 
 the surrounding anticyclones, and that a slight shift of 
 one of these latter may modify the type very materially 
 on either side, while a change to an opposite type would 
 involve a total rearrangement of pressure over the whole 
 northern hemisphere. 
 
 In a few cases we have been able to point out signs of 
 an impending change of type, but unfortunately the 
 forecaster is often confronted by very sudden altera- 
 tions in the whole distribution of pressure over the 
 northern hemisphere. Future research may perhaps 
 some day lead to the detection of more certain symptoms 
 of change, though at present we can say but little. 
 
 NORTH-EAST MONSOON. 
 
 But perhaps the nature of European types will be 
 more readily comprehended if we give some illustrations 
 of the Indian monsoons. These will be very valuable 
 
378 
 
 WEATHER. 
 
 both as showing weather-features of a totally different 
 character from any which we have hitherto examined, and 
 as explaining the connection between the fluctuations of 
 weather in the tropics and the more variable changes of 
 the temperate regions. In Figs. 80 and 81 we therefore 
 give isobaric and isothermal charts for January 4 and 5, 
 
 -p.m. 4.35 6.jj 
 
 FIG. 80. North-east monsoon; great cold. 
 
 1878, at about 6-30 p.m., Calcutta time that is, during 
 the season of the north-east monsoon in the Indian Ocean. 
 These charts commence in longitude 40 east of Green- 
 wich, where our Atlantic maps left off, and so continue 
 on the same projection our survey of the world 60 further 
 east. 
 
TYPES AND SPELLS OF WEATHER. 
 
 370 
 
 On both days we find an anticyclone, exceeding 31*0 
 ins. in height, resting over Tartary, to the east of Lake 
 Aral. North of this, pressure slopes away towards the 
 Arctic Ocean ; southwards the pressure falls away to 
 the equator. In fact, this anticyclone is probably the 
 counterpart of the Atlantic anticyclone ; while the low 
 
 FIG. 81. North-east monsoon j great cold. 
 
 pressure over Southern India corresponds to the trade- 
 wind slope, which we also saw in the Atlantic. The most 
 noticeable feature in both these charts is the persistence 
 of the central Asiatic anticyclone and of the southern 
 slope of low pressure, while the northern elope is more 
 variable ; just as we saw in the Atlantic. 
 
380 WEATHER. 
 
 The wind circulates round the anticyclone in the 
 usual manner; but note, however, that the winds over 
 Lower Bengal are from north-west, not from north-east, 
 as might have been expected. This is due to a small 
 permanent depression near the mouth of the Ganges, that 
 cannot be shown on so small a map. As this general 
 distribution of pressure lasts all through the winter 
 months, we see that the north-east monsoon is the exact 
 representative of the trade-winds of the Atlantic; only 
 that the result of Asia being a land area is that the 
 easterly winds have a far greater extension northwards 
 than over the ocean. 
 
 We have already referred to the temperature-charts 
 in our chapter on Heat and Cold. All that we need do 
 here is to call attention to the great cold 30 Fahr. 
 near the centre of the anticyclone. 
 
 The sky was blue and clear at almost every station 
 in India on both the days in question. What most con- 
 cerns us here is to note the limited amount of fluctuation 
 over India, and the somewhat irregular nature of the 
 winds, relative to the isobars, in that country. For 
 instance, in Fig. 80 the isobar of 30*0 in. is to the north 
 of Calcutta, while on the following day (Fig. 81) it is a 
 short distance to the south of that city. This, of course, 
 would have been associated with a rise of the barometer, 
 though the general character of the monsoon would not 
 have been affected. This is, in fact, precisely analogous 
 to the fluctuation of the isobars which we saw over the 
 Atlantic to the south of the great permanent anticyclone. 
 Temperature varies in a similar manner, for the isotherm 
 of 60 has been somewhat deflected on the second day by 
 the increasing pressure. 
 
TYPES AND SPELLS OF WEATHER. 381 
 
 As these charts are very fair specimens of any others 
 during the persistence of this monsoon, we see that the 
 task of the Indian forecaster would be comparatively 
 simple. For, though a limited fluctuation of the general 
 distribution of pressure takes place from day to day, the 
 amount never exceeds a moderate quantity, and still less 
 is the whole character of the weather ever altered in the 
 manner which we have seen in more northern latitudes. 
 
 THE SOUTH-WEST MONSOON. 
 
 The general character of the south-west monsoon will 
 be best illustrated by giving first a typical sample of two 
 consecutive days, and then by making some remarks on 
 the whole system of Indian weather. 
 
 In Figs. 82 and 83 we therefore give charts for June 
 17 and 18, 1881, over India and Central Asia, at about 
 6*30 p.m., Calcutta time. These may be considered as 
 typical of the distribution of pressure in those countries 
 during the summer months, just after the burst of the 
 south-west monsoon of the Indian Ocean. In Bengal they 
 relate to the time when the hot season has just begun to 
 give place to the rains. 
 
 In both maps we see an oval isobar enclosing pressure 
 less than 29'4 ins. round Lahore, in the Punjab ; and in 
 both an isotherm of 100 Fahr. (38 C.), nearly con- 
 terminous with that isobar. This depression is usually 
 more distorted by secondaries than on these two days. 
 
 The winds blow, on the whole, round the lowest 
 pressure in the usual manner, being from west or south- 
 west to south, and from north-east or east on the northern 
 
382 
 
 WEATHER. 
 
 side of the low pressure. The few wind-arrows which our 
 diagrams admit of will, however, show that the relation of 
 wind-direction to isobars is not so constant as in higher 
 latitudes. For instance, the winds in Lower Bengal are 
 more from the north-west than the general laws of wind- 
 rotation would have indicated. 
 
 p.m. 4.35 
 FIG. 82. South-west monsoon ; great heat. 
 
 The weather was cloudy or overcast at almost every 
 station, with rain at several, and blue sky at only one on 
 either day; but all the rain is either of the secondary 
 or of the non-isobaric type, and cannot be located by 
 looking at the charts. 
 
 It is very interesting to contrast this weather with 
 
TYPES AND SPELLS OF WEATHER. 
 
 383 
 
 that of the north-east monsoon. In the latter there is a 
 difference of one inch of pressure and 100 of temperature 
 between Central Asia and India ; in the former only six- 
 tenths of an inch of pressure and 40 of temperature. In 
 the cold monsoon there is scarcely anything but blue 
 sky ; while with the warm south-west wind the heavens 
 are almost entirely covered by clouds. 
 
 &? 
 
 700 
 
 p.m. 4.35 
 FIG. 83. South- west monsoon; great heat. 
 
 An important but difficult question will immediately 
 present itself as to the relation which the low summer 
 pressure over India bears to the equatorial bert of low 
 pressure over the rest of the world. 
 
 In these last two maps we no longer find any arrange- 
 
384 WEATHER. 
 
 ment similar to that which occurs in the Atlantic. The 
 persistent pit of low pressure over Scinde and the North- 
 Western Provinces of British India is probably the 
 representative of the equatorial belt of low pressure 
 which constantly covers the Atlantic about 10 north 
 latitude. Here, however, the lowest point is nearly in 
 latitude 30 north ; but we know that there is no lower 
 pressure between it and the equator. 
 
 The greatest difference is in the absence of an anti- 
 cyclone north of the equatorial low-pressure belt. In 
 Fig. 82 there is a very narrow arm of high pressure 
 between the two isobars of 28*8, and just the fragment of 
 an anticyclone stretching over from the north-east of 
 Siberia. This latter is very persistent over that region at 
 this season of the year, but the difficult point is to deter- 
 mine the relation of the Indian low pressure to the low 
 pressure which usually lies over European Eussia in the 
 summer months. This we are unable to give. But we 
 may notice that a somewhat similar phenomenon appears 
 on a smaller scale over Mexico in the summer. Then the 
 Atlantic anticyclone, and another one in the Pacific, about 
 the same latitude, form a col over Mexico ; pressure at 
 the same time is persistently low over the United States 
 and also over Central America. Then the distribution of 
 pressure over the whole American continent has some 
 analogies to that over Asia at the same season of the year. 
 
 Many meteorologists have contended that this circular 
 persistent depression over Upper India should be con- 
 sidered as a stationary cyclone ; but the author's researches 
 have conclusively proved that if we take cloud-forms as 
 a test, none of the true monsoon rains partake of the 
 
TYPES AND SPELLS OF WEATHER 385 
 
 character of a primary cyclone-front roost of the rain 
 falls from cumuloform clouds while that in front of a 
 Bengal or any other cyclone seems to grow out of the air. 
 This Indian depression seems to be somewhat analogous 
 to the pit of low pressure which covers the North Atlantic 
 during the winter months. Neither are cyclonic, but both 
 are the theatre of atmospheric disturbance. The former 
 only breeds thunderstorms and secondaries ; the latter, 
 well-developed primary cyclones. 
 
 We have already described, in our chapter on Non- 
 isobaric Rains, the remarkable character and unknown 
 origin of the rain in this south-west monsoon ; and how, 
 without any marked change in the shape or position of 
 the isobars, the dry, hot south-west breezes are suddenly 
 converted into wet and stormy winds. There can be 
 little doubt that the source of this change is to be 
 found in the upper currents which feed the south-west 
 surface-winds, but the subject is too obscure to be 
 discussed in this work. 
 
 What mostly concerns us here is the nature of the 
 limited fluctuation of isobars and weather over India ; for 
 this is typical of the origin of the modified day to day 
 weather-changes in the tropics as opposed to extreme 
 changes of the temperate zone. There is little fluctuation 
 in the two charts we have given in Figs. 82 and 83 ; but 
 the bend in the isobar of 29*6 below Calcutta is less pro- 
 nounced on the second than on the first day. This would 
 be associated with a slight improvement in the weather 
 on the second day. 
 
 Sometimes, however, larger changes take place during 
 the continuance of this monsoon. Once or twice during 
 
 2o 
 
386 WEATHER. 
 
 the rainy season June to October the pit of low pressure 
 near Lahore stretches further east, and becomes less 
 pronounced, and less distorted by secondaries. H. F. 
 Blanford has shown that this fluctuation is associated 
 with that period of drier weather which is known as a 
 " break in the rains." 
 
 We may correct here a popular error that during the 
 rainy season in any part of the tropics it rains all or even 
 every day. Sometimes, no doubt, rain may fall twenty- 
 nine out of thirty days, or for forty-eight hours without 
 cessation ; but there are always periods of less intensity 
 and of more intermittent showers. 
 
 Returning to India in particular, as the season gets 
 on, small secondary cyclones occasionally form over the 
 Bay of Bengal, and, advancing nearly northwards, strike 
 land on the Orissa coast, and continue their course undis- 
 turbed by mountain or valley till they reach the great 
 chain of the Himalayas. In this these secondaries contrast 
 in a marked degree with the great primary cyclones which 
 form on the Bay of Bengal at the change of the monsoon. 
 These latter almost invariably break up when the centre 
 reaches the land. The main characteristic of the 
 secondaries is light wind with torrential rain ; as much as 
 fifteen inches of rain has been collected within twenty- 
 four hours during the passage of one of these small 
 depressions. 
 
 Occasionally, during the month of May, primary 
 cyclones of considerable intensity develop over the Bay 
 of Bengal, and move towards some point of the west, 
 usually to north-west ; and again in October, as the 
 south-west monsoon gives way to that from the north- 
 
TYPES AND SPELLS OF WEATHER. 387 
 
 east, cyclones of very great intensity form in the same 
 Bay, and these too are propagated towards the west or 
 north-west. In either case the coasts exposed to their 
 influence experience very bad weather, with rain and 
 wind of hurricane force in the October cyclones. 
 
 From this very brief sketch of monsoon weather we 
 may, however, learn that the persistent seasonal weather 
 in the tropics is exactly analogous to the persistent types 
 of weather which appear periodically in temperate regions. 
 Both are primarily caused by the distribution of pressure, 
 but the changes which only occur once a year in the 
 tropics take place at short and at irregular intervals in 
 higher latitudes, while between the two extremes we find 
 an intermediate series of recurrent types which are 
 neither so regular as in the tropics nor so uncertain as in 
 Western Europe. In both the differences of weather from 
 day to day are due to the fluctuation of the type, or to 
 small alterations either in the shape or intensity of the 
 depressions which form in the low-pressure areas. 
 
 We also see problems of forecasting totally different 
 from any which present themselves in the European or 
 American offices, and had we been able to give a greater 
 number of examples, we should have found the truth of 
 the general principle that no mechanical rules can be 
 laid down as to the probable path of a cyclone, or the 
 fluctuation of a type, but that the forecaster who knows 
 the ways of the barometric movements in his own country 
 can usually form a very good opinion as to their future 
 progress. 
 
 It may be advantageous here to pause a moment and 
 survey the general aspect of the great problem of weather 
 
388 WEATHER. 
 
 as it is now presented to us. We have seen over a large 
 portion of the world the same seven forms of isobars con- 
 stantly reproduced, though with details greatly modified, 
 not only by the type, but still further by the size and 
 intensity, by the time of day, the season of the year, and 
 also by local causes. 
 
 But though these sources of variation prevent our 
 writing down on a chart more than the general character 
 of the weather under any isobars, their classification, 
 grouping, and co-ordination each in its proper place, 
 enable us to distinguish the essential from the more 
 accidental features of weather. 
 
 And when we come to watch the ceaseless changes of 
 isobars, we see that sometimes cyclones disappear so 
 quickly that within twenty-four hours no trace of their 
 existence is to be found, while at other times the same 
 cyclone may exist for weeks together. Then also we see 
 that a cyclone may either remain stationary, or move in 
 almost any direction with a very wide range of velocity ; 
 and we learn the still more curious phenomenon of that 
 fusion of one or more cyclones into a single system which 
 so often makes it impossible to track the paths of 
 depressions. 
 
 We also see very clearly how the old idea that a 
 cyclone is necessarily a destructive storm is no longer 
 tenable ; and that we must adapt ourselves to the concep- 
 tion that a cyclone is an eddy of very variable intensity, 
 always rainy and always surrounded by a very definite 
 rotation of air, though the force of the wind may vary 
 from a zephyr to a hurricane. When we talk of cyclonic 
 weather we must use descriptive epithets such as mode- 
 
TYPES AND SPELLS OF WEATHER. 389 
 
 rate, intense, etc., to denote the general force of the 
 wind. 
 
 It is also manifest from the great scale on which 
 changes of pressure-distribution take place, that there is 
 some greater cause at work behind them than any local 
 developments of heat or rain. This cause is undoubtedly 
 the general circulation of the atmosphere from the hot 
 equator to the cold Poles ; though doubtless temperature 
 and precipitation have a modifying effect on the greater 
 changes. If the earth were surrounded by a vapourless 
 atmosphere, cyclones and anticyclones would undoubtedly 
 be formed, though not the same as those with which we 
 are so familiar. 
 
 Now that we know what weather is, we may consider 
 how far it can be forecast more or less in advance. 
 
390 WEATHER 
 
 CHAPTER XIV. 
 
 FORECASTING FOB SOLITARY OBSERVERS. 
 NATURE OF THE PROBLEM. 
 
 A COMPREHENSIVE view of weather- science divides itself 
 into three problems one direct and two inverse. The 
 direct problem of weather is to explain by mechanical 
 causes the origin and nature of all the complicated 
 phenomena of wind and weather which present themselves 
 to our senses, and the nature of tue sequence of weather- 
 changes. This we have already partially done in the 
 preceding pages. The inverse problem of meteorology is, 
 given a portion of a sequence of the weather, to tell what 
 is going to follow. The morning is fine, but now cirrus 
 begins to form, and the mercury has begun to fall what 
 weather is coming ? Last night a cyclone lay over 
 Ireland, this morning it covers Wales what will the 
 weather be over Great Britain for the rest of the day ? 
 
 These two illustrations point at once to a natural 
 subdivision of the questions of forecasting: the best 
 that a single observer can do, who has his eyes to look at 
 the appearance of the sky and any instruments at his 
 
FORECASTING FOR SOLITARY OBSERVERS. 391 
 
 disposal ; and the best that a meteorologist can do, who is 
 seated in a central bureau, with abundant telegraphic 
 intelligence for many miles round the country for which 
 he has to issue forecasts, so as to enable him to construct 
 synoptic charts at such intervals as he may think neces- 
 sary. The latter doubtless represents the highest develop- 
 ment of which forecasting is capable ; but the former can 
 never be superseded for use among sailors, fishermen, and 
 shepherds. For this reason we will discuss them in 
 separate chapters, and we will take the problem of a 
 solitary observer first, as it is the older and the more 
 generally useful. We shall only attempt to give general 
 principles, and not to go into all the details for any one 
 country. 
 
 PROGNOSTICS. 
 
 We have already gone very fully into the subject of 
 prognostics, and pointed out both the reasons for their 
 success as well as for their failure. When we come to 
 look at all that has been done, we see that, on the whole, 
 we have not been able to develop the practical utility of 
 prognostics very materially, though we have been able to 
 place the whole branch of the subject on a scientific 
 basis. 
 
 The most valuable addition of recent times to weather- 
 lore is undoubtedly in the methodical observation of 
 cirrus clouds. The recognition of cirrus as a sign of rain 
 is as old as meteorology, but the deductions which can be 
 made from the direction of the motion of the upper 
 clouds are quite of modern date. No absolute test can 
 
392 WEATHER. 
 
 be given for the discrimination of fine weather from 
 dangerous cirrus beyond the general surroundings and 
 experience of the observer ; but Ley has shown the 
 importance of noting by eye the velocity of the cirrus, 
 because rapid-moving cirrus is a much worse sign of the 
 weather than slowly moving cloud. This is probably one 
 of the most important advances which has been made. 
 
 THE BAROMETER. 
 
 We propose rather in this chapter to deal with the 
 value of the indications which the barometer can afford 
 to a solitary observer, and especially to explain why the 
 indications of that instrument so often fail. 
 
 Why do we sometimes have rain with a rising or 
 steady barometer, and why is the weather sometimes fine 
 with a falling barometer? Then, again, why do we 
 sometimes experience a heavy gale with only a slight 
 fall of the mercury, while at other times the barometer 
 will fall very low without any unusual amount of wind ? 
 
 These apparent anomalies in the indications of the 
 barometer occur all over the world, and those in each 
 country must be explained by reference to the meteorology 
 of the place. Though we shall draw our illustrations 
 from Great Britain only, the principles which we shall 
 lay down are of universal application. In no branch of 
 the subject shall we find synoptic charts more indis- 
 pensable, for without them no explanation could ever 
 have been afforded of irregular barometric fluctuations. 
 
FOEECASTING FOR SOLITARY OBSERVERS. 303 
 
 GENERAL INDICATIONS. 
 
 The preceding chapters will have sufficiently ex- 
 plained the reasons for what we may call the generally 
 correct indications of the barometer. We can now readily 
 understand why the rapid rise in rear of a cyclone in- 
 dicates unsettled weather, and the gradual rise of an 
 incipient anticyclone settled fine weather ; also why the 
 steady barometer of a persistent anticyclone indicates 
 dry seasonable weather, and the rapid fall of an oncoming 
 cyclone presages storm and rain. All these indications 
 of the barometer can be detected by intermittent obser- 
 vations, or, in fact, by merely looking occasionally at the 
 instrument. 
 
 The author has, however, discovered that we can 
 sometimes utilize the greater refinements of self- registered 
 barographs to deduce some knowledge of the future force 
 of the wind from flexures in the recorded curves. These 
 deductions are of the more value now that efficient baro- 
 graphs are so cheap as to be within the reach of 
 everybody. 
 
 AUTHOR'S KULES FOR INFERRING FROM A BAROGKAM 
 WHETHER A GALE IS GOING TO INCREASE OR 
 DECREASE. 
 
 The principle on which the author's rules are founded 
 depend on what is called the " direction of curvature " of 
 a curve. In the lower portion of Fig. 84, the portion of 
 the trace near the letter A has its hollow turned upwards, 
 and is called convex, relative to the base line. A little 
 
394 
 
 WEATHER. 
 
 further down, near the figures 14, the curve is hollowed 
 downwards, and would be called concave. The other half 
 of the curve is convex almost throughout. From this we 
 see that both convexity and concavity are independent of 
 
 30 I 
 
 Ins 
 
 30 1 
 
 299 
 297 
 295 
 293 
 29 1 
 ?A a 
 
 A NOV 14-r 1875 
 
 Jns 
 
 301 
 
 299 
 297 
 295 
 293 
 291 
 ?A-a 
 
 s ^ 
 
 \ / 
 
 \ / 
 
 v / 
 
 ^^-^^ ^/ 
 
 
 FIG. 84. Gradients, and flexure of barogram. 
 
 whether the mercury is rising or falling, and also of the 
 rapidity of their rise or fall. 
 
 If the barometer change at a uniform rate, either 
 upwards or downwards, it is evident that the resulting 
 trace will be a straight line, either rising or falling, and 
 it does not the least matter how rapid the rise or fall is. 
 If, however, the rate of fall changes with diminishing 
 pressure, then the curve will become convex or concave, 
 
FORECASTING FOE SOLITARY OBSERVERS. 
 
 395 
 
 according as the rate increases or decreases. For instance, 
 suppose, as in Fig. 85, F, that the barometer fell two- 
 tenths of an inch between one and two o'clock, and 
 another two-tenths between two and three o'clock, the 
 resulting barographic trace would be a straight descending 
 line, like s; if in the second hour the mercury fell three- 
 tenths of an inch instead of only two-tenths, the resulting 
 trace would be a convex, like #; while if it only fell 
 
 2 3 Hours I 23, 
 
 In. 
 
 \ 
 
 FIG. 85. Illustrating the origin of convex and concave barograms. 
 
 one- tenth in the second hour, the trace would be concave, 
 as a. 
 
 If we define the barometric rate as the number of 
 hundredths of an inch which the mercury moves, either 
 up or down, per hour, the above may be put in this form. 
 
 With a falling barometer, the trace is convex for an 
 increasing rate, concave for a decreasing one. A glance 
 at Fig. 85, R, will show that for a rising barometer the 
 converse is the case ; for when the rise is greater the 
 second than the first hour, the trace is concave, as in A ; 
 but when less, then convex, as at x ; and this result may 
 be stated as follows. With a rising barometer, the trace 
 
396 WEATHER. 
 
 is convex for a decreasing rate, concave for an increasing 
 one. This is the reverse of what happens with a falling 
 barometer. Now, the simplest and commonest case of 
 barometric change occurs when the centre of a cyclone 
 drifts past a station ; the fall of the barometer is then 
 proportional to the steepness of the gradients. When 
 steeper gradients approach, the barogram will become 
 convex ; when slighter gradients arrive, the curve will be 
 concave. The converse holds good for a rising baro- 
 meter : when steeper gradients approach, the cur\ 7 e is 
 concave ; when slighter, then convex. 
 
 Now, as the force of the wind is proportional to the 
 steepness of the gradients, we find that the direction of 
 curvature of a barogram tells us whether a gale is going 
 to get worse or otherwise, because we can tell if the 
 gradients are becoming steeper or otherwise. We must 
 be very careful to remember that, though a rapid rate of 
 fall is in a general way a worse sign of weather than a 
 moderate one, the indications deduced from the curvature 
 of a barographic trace depend on the variation of the 
 rate, and not on the rate itself. For instance, in Fig. 84, 
 the top part of which gives the isobars over Great Britain 
 on November 14, 1875, at 8 a.m., the crossed line denotes 
 the direction of the cyclone, and an unsymmetrical 
 arrangement of the steepest gradients with reference 
 to the centre is very obvious. 
 
 To get the barographic section of a cyclone, or to find 
 out what curve the propagation of the depression would 
 leave on a recording instrument, we have to draw a line 
 across any portion of the plan, as shown on a synoptic 
 chart, parallel to the path of the cyclone, and then, by 
 
FORECASTING FOR SOLITARY OBSERVERS. 397 
 
 measuring the distance in time between any two con- 
 secutive isobars, we arrive at the flexures of the trace. 
 For the sake of simplicity, we will suppose, in the first 
 instance, that we are stationed exactly on the line of the 
 path of the cyclone, so that the centre will pass over us. 
 By this we make the line of section of the cyclone 
 coincide with the line of gradients, which is not the case 
 in any other portion of the depression. 
 
 In the lower part of Fig. 84 we give such a section 
 of the cyclone, sketched in the upper portion, along the 
 line A B. The position of A and B are obverted in the 
 section, so as to read from left to right like an ordinary 
 barogram. Then we see that as the cyclone approached 
 the gradients got steeper, so that the rate of barometric 
 fall increased, and therefore the trace was convex ; during 
 this period the gale got worse. After a time, as the ring 
 of steep gradients passed, and the slighter gradients 
 in front of the centre approached, the rate of fall of 
 the mercury decreased and the trace became concave, 
 though still going downwards. The gale moderated 
 somewhat during this time. The passage of the centre 
 marked the turn of the barometer ; but as the distance 
 between each consecutive isobar increased regularly after 
 29*5 inch, the resulting barogram was convex after 
 that level. The actual curve for the day, as given 
 at Stonyhurst, which lay almost in the line of the 
 centre, differs only slightly from this. Thus we see that 
 the normal barographic trace in a cyclone is simply the 
 reflection of the typical shape of isobars in that kind of 
 depression, and that, moreover, to a single observer the 
 direction of curvature that is, the convexity or concavity 
 
398 WEATHER. 
 
 of a barogram enables him to tell whether more or less 
 steep gradients are approaching, and therefore whether a 
 gale is going to get better or worse. There is, however, 
 one limitation which considerably detracts from the value 
 of this deduction. If the line of section of the cyclone 
 which passes over the observer is not square to the 
 isobars, the relative distance between any two consecutive 
 isobars is no longer a measure of the gradients. For 
 instance, if the cyclone in Fig. 84 had passed over an 
 observer anywhere on the line c G, his trace from o to E 
 would have been concave, because c D is a shorter line 
 than D E. But all the time he is getting into a region 
 of steeper gradients, as measured square to the isobars, 
 and therefore the criterion of increased gradients fails. 
 But if a concave need not be an absolute test of de- 
 creasing gradients, a convex trace can never fail to indi- 
 cate steeper gradients with a falling barometer. This 
 may be readily seen by considering the nature of con- 
 centric lines. 
 
 Conversely, with a rising barometer, we see, in Fig. 84, 
 that from E to G the barogram will be concave, though the 
 gradients are decreasing ; but under no possible conditions 
 could a convex trace fail to indicate a decreasing gradient. 
 The author's rule is, then, as follows : Assuming that the 
 force of a gale is proportional to the gradients, a convex 
 barogram is always bad with a falling, and good with a 
 rising barometer ; a concave trace is sometimes a good 
 sign with a falling, and not always a bad indication with 
 a rising barometer. 
 
 This rule, of course, involves the supposition that the 
 motion of the barometer is solely due to the propagation 
 
FORECASTING FOB SOLITARY OBSERVERS. 399 
 
 of isobars over the observer, but in practice much more 
 complicated changes sometimes occur. 
 
 For instance, in a very common class of gale belong- 
 ing to what we have described as the southerly type of 
 weather, a cyclone, after arriving near the British coasts, 
 remains stationary, but increases, maybe, half an inch in 
 depth. The fall of the barometer which then occurs at 
 any station is no longer of the same kind as that which 
 we have just examined, and the flexure of the trace is 
 determined by other considerations. The direction of 
 curvature would then depend on any variation of the rate 
 of deepening, not on the motion of the cyclone. 
 
 For instance, suppose a stationary cyclone which 
 began to deepen from increasing intensity if the rate of 
 deepening was constant, the trace would be a straight 
 descending line ; if the rate increased, the curve would 
 be convex ; if it decreased, concave. 
 
 But, as we know that the deepening of a cyclone 
 means increased intensity, we may look on a decrease of 
 that rate as a favourable sign, and therefore the indica- 
 tions of the relation of curvature to weather would remain 
 good. The complications which arise from a deepening 
 or shallowing moving cyclone need not be discussed here, 
 but it is important to notice the two distinct causes of 
 barometric change the passage of a moving cyclone, and 
 the deepening of a stationary one. 
 
 APPARENT FAILURES OF THE BAROMETER. 
 
 So far we have dealt with what may be called the 
 regular movements of the barometer, that is to say, move- 
 
400 LEATHER. 
 
 merits which are associated with or followed by the 
 weather which was anticipated. But we must now explain 
 certain cases in which the weather and the barometer do 
 not seem to be connected in the ordinary manner, and 
 show how, in spite of apparent anomalies, the same 
 general principles of meteorology hold throughout. 
 
 ClRRUS BEFORE THE BAROMETER. 
 
 In the regular course of isobaric movements, there is 
 one case in which cloud, and sometimes rain, forms before 
 the barometer begins to fall, though almost immediately 
 the mercury turns downwards and falls fast with in- 
 creasing rain. Thf,s happens just in front of the crest of 
 a wedge, and it is for this reason that in the diagram of 
 wind and weather in wedges which we gave in Fig. 7, we 
 placed the word " halo " partly in front of the line of the 
 crest. This is quite common in Great Britain, and it 
 often causes comment that the cirrus begins to form 
 before the barometer indicates the approach of rain. 
 Here, in fact, the sky speaks first, but not so soon as the 
 isobars. If any morning a British forecaster saw a wedge 
 lying over Ireland, and blue sky was reported from the 
 east of England, he could safely forecast that cirrus would 
 appear in the course of the day, before the barometer 
 began to fall. 
 
 On the other hand, the author has discovered that, in 
 the tropics especially, the ominous sunsets which precede 
 a hurricane are developed often twenty-four hours before 
 any appreciable depression is formed anywhere ; and of 
 course squalls and secondaries have threatening skies as 
 
FORECASTING FOR SOLITARY OBSERVERS. 401 
 
 their forerunners without any definite barometric indica- 
 tions. We may lay it down as a general rule that when 
 the sky threatens while the barometer says nothing, 
 something bad is coming ; but whether thunder, squall, 
 or gale depends on circumstances. 
 
 BAIN WITH A KISING BAROMETER AND AN EAST WIND. 
 
 Kain with a rising barometer and an east wind is so 
 common in England that Admiral Fitzroy engraved it on 
 the scales of his barometers as an exception to the general 
 rule that the mercury fell for rain. No explanation was, 
 however, attempted, and, in fact, could not have been 
 given, in the then state of meteorology. The author has 
 made a large number of unpublished observations on the 
 subject, and he finds a singular uniformity in the isobaric 
 conditions under which this apparent anomaly appears. 
 
 In every case which he has examined, the rain with 
 a rising barometer was associated with a peculiar phase 
 of the northerly type of weather. This, as we explained 
 in the last chapter, is a type or spell of weather in which 
 pressure remains constantly high to the north and north- 
 west of Great Britain, while cyclones form over France or 
 Germany. 
 
 The character of this phase will be best understood 
 by means of an actual example. In Fig. 86 we give a 
 copy of a barogram in London on April 20, 1877, and 
 underneath the appropriate portion we have marked the 
 time during which rain fell. 
 
 Now, at first sight this might seem opposed to all we 
 have said before as to the nature of cyclone-barogranas. 
 
 2 D 
 
402 
 
 WEATHER. 
 
 Instead of a well-marked fall of the mercury, with rain 
 near the lowest portion, we see a remarkably uniform 
 
 Inch. 
 
 JO.O 
 
 ^9-5 
 
 20. 
 
 6. a.m. Noon. 6. p.m. Mic 
 
 
 
 
 
 ^^^**^~~ 
 
 
 
 
 29-4-77 
 
 London 
 
 R a 
 E. 
 
 Eei^M0 
 
 / 71 
 
 Wind. 
 
 FIG. 86. Rain with rising barometer and east wind. 
 
 trace, in which the diurnal variation of the barometer is 
 very clearly marked. A slight general rise, however, 
 
 PIG. 87. Charts to illustrate rain with rising barometer and east wind. 
 
 occurred in the afternoon, and rain fell from about 3 to 
 
FORECASTING FOR SOLITARY OBSERVERS. 403 
 
 9 p.m., while the wind remained with little change from 
 north-east. 
 
 In Fig. 87 we give charts at 8 a.m. and 6 p.m. on that 
 day, so that the changes of pressure as shown by the 
 isobars may be readily apprehended. In both charts the 
 edge of an anticyclone covers the north of Scotland, but 
 the ill-defined area of low pressure which lay over France 
 and the English Channel in the morning had by the 
 afternoon gathered itself into a well-defined secondary 
 over the north of France. At the same time, partly by 
 & slight general upward surge or increase of pressure for 
 the lowest isobar in the second chart is 29*8 instead of 
 29 '7 and partly by the advance of the isobar 29*9 nearer 
 to London, the pressure rose in that city, as shown in the 
 barogram, while the rain was due to the formation of 
 the secondary. In this, as in all other similar instances, 
 the advance of pressure from the north-west appears to 
 develop small secondaries, just as a big advancing wave 
 makes small eddies in front of itself. 
 
 These secondaries give rain with a rising barometer 
 east wind. 
 
 BAIN WITH RISING BAROMETER AND WEST WIND. 
 
 In the ordinary course of depressions the barometer 
 falls before rain, because the centre of the cyclone con- 
 tains the rain-area; and if all cyclones moved along 
 a pretty regular path, and did not alter much either in 
 depth or extent, then we should never fail to forecast rain 
 correctly whenever we saw the mercury began to fall, and 
 iine weather soon after the barometer began to rise. But 
 
404 WEATHER. 
 
 sometimes, before either the centre or trough of a cyclone- 
 has reached a station, the depression begins to fill up so 
 rapidly that the barometer actually rises, though in front 
 of a cyclone. Then we get rain with a rising barometer, 
 but the sky retains the appearance due to the front of 
 a cyclone. These are among the cases when synoptic 
 charts enable us to explain what would be hopeless with- 
 out them, and to see the truth of the statement that 
 weather depends on the position of the observer in a 
 cyclone, and not on the height or motion of his barometer. 
 The next example will illustrate this point very clearly r 
 and also the complications which arise when a secondary 
 forms in rear of a cyclone. 
 
 The following sequence of weather was observed by 
 the author at the beginning of September, 1883, about 
 sixteen miles west of the town of Leeds, and 520 feet 
 above the sea-level. His journal, as written at the time, 
 runs thus : 
 
 "September 1, 1883. Early, blue sky, misty, heavy 
 dew, wind south ; by noon, sky threatening, halo of 46 
 diameter, visible from 12 to 2 p.m., then overcast. At 
 4.45 p.m. light rain began; wind to south-east, almost 
 calm. By 8 p.m. rain heavy ; wind up and more to east 
 Barometer fell fast all day. Kemark : Very slow 
 coming on. 
 
 " September 2. Early, a gale ; 8 a.m., uniform nimbus^ 
 wind south-east, moderate; the same all day, rain off 
 and on, often rather heavy, but the wind falling light 
 towards night. At 8 a.m. it was seen that the barometer 
 had been falling all night, and was then very low. The 
 mercury continued to fall all day till 6 p.m., when it 
 
FORECASTING FOR SOLITARY OBSERVERS. 405 
 
 turned without a squall ; though about 6.45 there was 
 a passing shower. After this there was not the look of 
 the rear of a cyclone. Bemark : Eain with a south-east 
 wind lasts long ; twenty-eight hours. 
 
 " September 3. Warm, wet, and stormy ; soft ; south- 
 south-west to south-west, fresh to strong. About 5.30 a.m. 
 a squall ; 6.30, heavy shower, wind round. All day dirty, 
 misty, driving showers, though barometer rising fast. 
 About 5 p.m. rain off, but soft stratus, not cumulus of 
 cyclone-rear; night overcast. Kemarks: Weather like 
 front, not rear of a cyclone, and much worse than the two 
 previous days, when the barometer was falling." 
 
 By next day, the sky was bright, and covered with 
 cumulus and occasional showers, as is usual in rear of a 
 cyclone, while the wind was round to the north-west. 
 What we have, then, to explain is, first, twenty-eight 
 hours' rain with a falling barometer, and then twenty- 
 three hours' rain, and worse weather after the mercury 
 began to rise ; also, not only the increased severity of the 
 weather, but why the sky did not assume its ordinary 
 appearance after the centre of the cyclone had apparently 
 
 In Figs. 88 and 89 we give the 6 p.m. chart for Sep- 
 tember 2, and also that for 8 a.m. the next day, on a 
 large scale ; the coast lines are omitted for the sake of 
 clearness, but the position of the letters w, p, D, for 
 Wick, Penzance, and Dover, will sufficiently indicate the 
 scale of the chart. The spot marked L shows the station 
 where the observations were made. 
 
 At 6 p.m., September 2nd, the centre of a cyclone of 
 considerable intensity lay near Loughborough, but the 
 
406 
 
 WEATHER, 
 
 isobar of 28*7 ins. had the form of an ellipse, whose 
 longer axis lay from about Barrow to near Cambridge. 
 This gives the direction of the trough of the cyclone. 
 
 2-Q-Sj 
 
 6. p. m. 
 
 29.0 
 
 .w 
 
 FIG. 88. Bad weather with rising barometer. 
 
 which in this case forms an acute angle of 60 with the 
 direction of the path of the cyclone. 
 
 If we assume that the cyclone travelled at a uniform 
 rate till 8 a.m. next morning, it must have been at least 
 
FOKECASTING FOR SOLITARY OBSERVERS. 
 
 407 
 
 three hours before the trough of the cyclone passed near 
 Leeds, that is to say, before the point on the trough 
 marked 0*67 in Fig. 88 reached L. The barogram at L, 
 
 FIG. 89. Bad weather with rising barometer. 
 
 however, began to rise at 6 p.m., and the explanation of 
 this apparent anomaly is as follows : 
 
 At 6 pm. on the 2nd (Fig. 88), the lowest barometer 
 marked 28*66 ins., while by 8 a.m. next morning 
 
408 WEATHEE. 
 
 (Fig. 89) the lowest pressure was only about 29'0 ins., 
 so that the cyclone had filled up by 0*34 inch during 
 those fourteen hours, or at the rate of 0*024 inch per hour. 
 
 It would, therefore, appear that the barometer rose for 
 at least three hours near Leeds, while the centre of the 
 cyclone was still approaching, because the rise of the 
 mercury due to the filling up of the cyclone was greater 
 than the fall of the barometer owing to the approach of 
 the cyclone- trough. 
 
 The actual figures in this instance were as follows : 
 If there had been no filling up, the mercury should have 
 fallen 0*03 inch during the three hours 6 to 9 p.m. This 
 is got at as follows : The difference of pressure between 
 L, 28*7 ins., and the point on the trough marked 0*67 
 is 0*03 inch, and the distance between the two points is 
 fifty miles. 
 
 This would be traversed in three hours, because the 
 cyclone-centre moved two hundred and thirty miles to 
 the point marked 8 a.m., 3rd (Fig. 88), in the fourteen 
 hours which elapsed between the times for which the 
 charts are constructed. 
 
 If there had been no advance, but only a filling up of 
 the cyclone, the mercury would have risen 0'07 inch per 
 hour. Therefore the balance of rise over fall should have 
 been 0*04 inch, and this was the amount actually ob- 
 served. At 8 a.m., September 3rd (Fig. 89), we see that 
 the centre of the primary cyclone was north-north-east of 
 Leeds, and about two hundred miles distant. The centre 
 was also moving north, so that the motion of the barometer 
 would be upwards from the action of the primary. There 
 is, however, a marked irregularity in the lie of the isobars 
 
FORECASTING FOR SOLITARY OBSERVERS. 40 ( J 
 
 over the Irish Channel, which points to the existence of 
 a secondary in that neighbourhood. The chart at 6 p.m. 
 the same day showed that the secondary which lay over 
 the Irish Channel at 8 a.m. had become more pronounced, 
 and had then its lowest portion near Liverpool. Con- 
 sequently Leeds and its neighbourhood were still under 
 the influence of the front of this secondary, though the 
 mercury had risen about O2 inch, partly owing to the 
 progress of the primary, and partly also to the cyclone 
 gradually filling up. 
 
 By next day the charts showed that the primary had 
 moved still further to the north, and that the secondary 
 was lying over the North Sea, so as to form a sort of V-- 
 shaped depression to the south of the primary cyclone. 
 
 The explanation of the apparently anomalous weather 
 is then very simple. The first twenty-eight hours of rain 
 with a falling barometer were due to the front of a 
 primary cyclone which was moving very slowly ; and so 
 far this represents the usual sequence of weather in such 
 cases. The first three of the twenty-three hours of rain 
 with worse weather after the barometer began to rise 
 were also due to the cyclone-front, though the mercury 
 rose from filling up. The remaining twenty hours of 
 rain and the characteristic sky of the front of a cyclone 
 were due to the formation of a secondary in rear of the 
 primary ; so that though the barometer was rising, owing 
 to the passage and filling up of the primary, still Leeds 
 was during the whole of that day exposed to the influence 
 of the front of the secondary with its characteristic dirty 
 weather. The wind was stronger after the glass began 
 to rise, because the gradients were steeper in rear of the 
 
410 WEATHER. 
 
 primary than they had been in most portions of its front. 
 By the fourth day the secondary had passed away, and 
 then the typical weather of the rear of a cyclone was 
 experienced. 
 
 KAIN WITH STEADY BAROMETER. 
 
 So far for rain with a rising barometer; now we 
 must consider precipitation with a steady barometer. To 
 Englishmen this is more perplexing than rain with a 
 rising mercury. 
 
 In the latter case, we see at once that there is some 
 disturbance going on ; but in the former we often have a 
 steady downpour for several hours, with an absolutely 
 steady barometer. Kain of this class is much more 
 common in continental Europe than in Great Britain, 
 except in one very rare case, which will be mentioned 
 hereafter. 
 
 The rain is always either non-isobaric, or of that kind 
 which is associated with secondaries and not with primary 
 cyclones. For this reason, the rain is never accompanied 
 by a gale of wind, though there are often angry gusts at 
 the beginning and end of the rainfall. 
 
 In Fig. 90 we give a photographic engraving of the 
 author's barographic trace in London, on July 1 and 2, 
 1877. This, being absolutely untouched by hand, gives 
 the minute irregularities of pressure in a manner which 
 no hand-copied diagram can ever do. The horizontal 
 lines represent differences of half an inch of pressure, the 
 thickest one marking the level of 29*5 inches. The 
 horizontal lines are drawn at six-hour intervals. 
 
FORECASTING FOR SOLITARY OBSERVERS. 411 
 
 At first sight, there might seem to be little sign of 
 any disturbance, for the actual 
 changes of barometric level are 
 insignificant, and the diurnal 
 variation is more obvious than 
 usual in Great Britain. If, 
 however, we look carefully at 
 the trace, we shall find that just 
 before 6 a.m. on July 1 there is a 
 very small dip of the barometer, 
 and that then the trace is al- 
 most quite straight till about 
 4.30 p.m., when there is another 
 small dip; after which the 
 regular diurnal variation is 
 absolutely undisturbed. In 
 London rain commenced at the 
 first dip, and continued without 
 intermission till the second, 
 after which the sky cleared. 
 
 The charts for that day, 
 which unfortunately the num- 
 ber of illustrations at our dis- 
 posal does not admit of repro- 
 ducing, show that this was all 
 caused by the formation and 
 passage of a small secondary 
 over the north of France and 
 the English Channel ; and both 
 the rain and the barographic trace are most characteristic 
 of this class of depression. A case of this sort shows, 
 
412 WEATHER. 
 
 more than any other, the superior value of a continuous 
 trace over an intermittent barograph ; for, though the 
 latter permits of the tabulation of hourly values for 
 the determination of diurnal variations, they entirely lose 
 all chance of following the more minute alterations of 
 pressure, which are often accompanied by great changes 
 of weather. The most interesting point about secondaries 
 is the contrast between the intensity of the weather which 
 they induce and the apparently small disturbance of 
 pressure. In primary cyclones the gradients are to a 
 ertain extent a good measure of the intensity. In 
 secondaries, on the contrary, the rainfall has no relation 
 whatever to the barometric disturbance. This, of course, 
 makes it very difficult for the forecaster. All he can say 
 when he sees a secondary is rain ; but he can give no 
 estimate of the quantity of precipitation, as he can of the 
 force of the wind in a primary cyclone. 
 
 Earely in Great Britain, frequently in continental 
 Europe, habitually in the tropics, we have purely non- 
 isobaric rains, totally unconnected with any secondary. 
 These are often indicated on the barographic trace by a 
 sudden sharp rise of the type we illustrated in our chapter 
 on Thunderstorms. This is probably a purely local effect 
 of a heavy downpour pressing the air down by its own 
 weight. 
 
 The other case of rain this kind often with a gale of 
 wind with an apparently steady barometer, only occurs 
 in very unsettled weather. In our chapter on Weather- 
 Types, we gave several examples of nearly stationary 
 cyclones, which increased much in depth, while some of 
 the adjacent anticyclones increased in height. As a 
 
FORECASTING FOR SOLITARY OBSERVERS. 
 
 necessary consequence, there must be some station where 
 no change of pressure would be observed ; but on one 
 side pressure would decrease, while it increased on the 
 other; so by this means very steep gradients might come 
 to lie over the station. The wind would rise to a gale, 
 while the weather would conform to the shape of the 
 isobars, but the mercury would remain stationary; we 
 might, in fact, say that the station was " nodal " as 
 regards the fluctuations of surrounding pressure. 
 
 It would be an extreme case when no change of 
 pressure took place, and could only happen at a limited 
 number of places. But under the same conditions there 
 will always be a number of stations where only a moderate 
 fall of the barometer takes place, but a gale out of all pro- 
 portion to the apparent depression is experienced. This 
 illustrates the important difference between the fall of 
 the barometer clue to the passage of a well-defined 
 cyclone, and that due to the rearrangement of the distri- 
 bution of pressure round the station. As an example, we 
 may turn to Fig. 93 in the next chapter, where we give 
 two charts of North- Western Europe, on February 6, 
 1883, at 8 a.m. and 6 p.m. respectively. The position of 
 the isobar of 30'4 ins. is practically the same in both 
 maps ; but between the morning and evening observations* 
 pressure has fallen O4 inch in the west of Ireland, and 
 risen 0*2 inch over Sweden. The shape of the isobars has 
 not altered much, so that gradients have become steep, 
 with little change of wind-direction. Thus many stations, 
 near the nodal isobar, will experience an increase of wind 
 with either a rising, stationary, or slightly falling barometer. 
 For instance, at Aberdeen, marked A, the wind-arrow 
 
414 WEATHER. 
 
 shows that the wind had risen from a fresh breeze to a 
 moderate gale ; while the motion of the isobars does not 
 indicate a fall of more than Ol inch in the ten hours 
 which elapsed between the two sets of observations. 
 
 FINE WEATHER WITH Low OR FALLING BAROMETER. 
 
 From the above, in which the weather is out of all 
 proportion to the depression of the mercury, we readily 
 pass to the converse case, in which the fall of the 
 barometer is quite disproportioned to the severity of the 
 weather which is afterwards experienced. In the North 
 of Europe, during the winter months, and when the 
 westerly type of weather prevails, the barometer will 
 sometimes fall half an inch or more, and often below 
 28*5 ins., while no strong winds follow, and the general 
 appearance of the sky is bright, with perhaps a little 
 cumulus cloud. This also is readily explained by reference 
 to our large Atlantic charts. In them we saw that when 
 the Atlantic is covered by a persistent area of low pressure, 
 the depth of the lowest point often suddenly decreases 
 nearly an inch, and that the gradients near the centre 
 are very slight. In some phases of that type of weather, 
 the area of low pressure stretches over Europe, and the 
 minimum of this area rises up and down exactly as when 
 the centre lies over the ocean. 
 
 If, then, Great Britain, for instance, lay within that 
 area, pressure might decrease a whole inch, and neither 
 storm nor rain be experienced. The great fall of pressure 
 would, of course, develop steep gradients, somewhere to 
 the west of those islands ; but as the depression was not 
 
FORECASTING FOR SOLITARY OBSERVERS. 415 
 
 caused by the drifting past of a cyclone, neither wind nor 
 rain would follow in England. The centre of these great 
 depressions, which are not true cyclones, is usually asso- 
 ciated with cool, bright weather and cumulus cloud, and 
 therefore weather of that description would probably be 
 experienced. From a case of this sort, we learn how to 
 avoid the popular errors that the violence of a gale is 
 always proportional to the fall of the barometer, and that 
 a very low barometer is necessarily associated with very 
 bad weather. 
 
 COMPLICATIONS ON BOARD SHIP. 
 
 All the examples which we have now given in this 
 chapter will sufficiently explain the nature of forecasting 
 by means of a single barometer and observations on the 
 appearance of the sky, as also the true nature of the 
 apparent exceptions to the ordinary relationship between 
 weather and the movements of the mercury in a barometer 
 tube. 
 
 Our space, unfortunately, does not permit us . to 
 describe the still greater difficulties which occur when 
 the observations are taken on board a moving ship ; then, 
 of course, we have not only the motion of cyclones, but 
 also that of the ship to take into account, and it is 
 manifest that many of the rules which we have laid down 
 for land-stations would require considerable modification. 
 The same limitation of space also compels us to omit the 
 notice of the theory of handling ships in the small cyclones 
 which occur in tropical countries under the names of 
 hurricanes, typhoons, etc., but the author hopes to make 
 this branch of meteorology the subject of another work. 
 
416 WEATHER. 
 
 CHAPTER XV. 
 
 FORECASTING BY SYNOPTIC CHARTS. 
 STATEMENT OF THE PROBLEM. 
 
 BY synoptic forecasting we mean that branch of weather- 
 prevision which is carried on by means of synoptic charts. 
 The forecaster in a central bureau is in telegraphic com- 
 munication with observers for many hundred miles round. 
 From their reports he constructs synoptic charts at such 
 intervals as seem necessary. To the indications which 
 he derives from the appearance of these maps, he adds 
 all his own accumulated experience of the nature of the 
 meteorology, and the motion of depressions in his own 
 country ; and also such knowledge of the recurrent 
 periods of different kinds of weather as he may be 
 acquainted with. From all that he forms his own 
 judgment as to what changes are likely to take place, and 
 issues his forecast accordingly. 
 
 From the nature of things there can never be many 
 forecasters. The rapid nature of meteorological changes 
 makes the employment of the electric telegraph absolutely 
 necessary, and the great expense which is thereby in- 
 
FORECASTING BY SYNOPTIC CHARTS. 417 
 
 curred, compared with the uncommercial nature of the 
 results, practically relegates forecasting to the functions 
 of a Government office. 
 
 From the preceding chapters we now know what 
 weather is. Instead of dealing with abstractions called 
 wind, rain, cloud, heat, etc., we have gradually been led 
 up to the idea that all meteorological phenomena are 
 the products of the motion and circulation of a moist 
 atmosphere. Now we know that when we talk about 
 forecasting weather, we mean that we are going to say 
 how or where certain aerial eddies will move, or when 
 new ones are likely to form; also whether any cyclone 
 will be violent or gentle. 
 
 AIDS TO FORECASTING. 
 
 In this chapter we propose to make some additional 
 remarks on the whole aspect of the subject. We shall 
 enumerate several aids to forecasting which can be 
 obtained from various sources, and point out both the 
 present difficulties and the future possibilities of weather- 
 prevision. Finally, we shall give some examples of 
 successful and unsuccessful forecasts in different countries, 
 and an account of the various percentages of success 
 which the different offices have achieved. In an inter- 
 national work we shall better illustrate the general 
 principles of the subject by exemplifying forecasts in 
 different countries than by trying to give any one in 
 detail. A tolerably full account of the nature of forecast- 
 ing, and of the details of the methods and machinery for 
 issuing storm-warnings in Great Britain, will be found in 
 
 2 E 
 
418 WEATHER. 
 
 the author's work, " Principles of Forecasting by Means 
 of Weather Charts," issued by the authority of the 
 Council of the Meteorological Office. 
 
 UNEQUAL BAROMETRIC CHANGES. 
 
 We have already fully explained the use of the recog- 
 nition of weather-types in every country, during which 
 sequence the motion of depressions follow either a certain 
 general direction or maintain a certain general position ; 
 but in variable climates we often find tracts of weather 
 which can be assigned to no particular type. The fore- 
 caster is then at a great disadvantage, for he has little to 
 guide him a& to the future. 
 
 The very idea of weather-type involves the knowledge 
 that the sequence of changes will follow in a certain 
 groove, so that when no type is obvious, there is little 
 basis on which to frame a forecast. In most cases the 
 forecaster has to rely on the difference of barometric rate 
 in various districts. If he sees that the barometer is 
 falling much more rapidly in one district than in any 
 other even if no definite depression is formed he knows 
 that steeper gradients must thereby be formed, so that 
 the wind must increase, and whatever weather is due to 
 the existing shape of isobars will get worse. 
 
 Conversely, if he finds pressure increasing in a district 
 of low barometer, he knows that gradients will decrease, 
 and that both wind and weather will moderate. The 
 details vary indefinitely, and no rule can be laid down 
 even for a single country ; everything must be left to the 
 judgment and experience of the forecaster. 
 
FORECASTING BY SYNOPTIC CHARTS. 419 
 
 CYCLONE-PATHS. 
 
 The paths of cyclones, and the nature of the influences 
 which deflect or otherwise alter them, are so important 
 that we propose to devote some paragraphs to their con- 
 sideration, of course with a special reference to the bear- 
 ing which they have on forecasting. 
 
 When the paths of the rare but violent cyclones of 
 the tropics, which are known as hurricanes, typhoons, or 
 cyclones, are plotted on a chart, we find that, though 
 there is a general similarity in their tracks, there is still 
 so much difference that we cannot attempt to lay down 
 any absolute law of their motion. 
 
 For instance, the West India hurricanes usually 
 begin with a westward course, and then gradually bend 
 round till they end by moving towards the east or north- 
 east. But in some instances they continue in a westerly 
 direction, and traverse the southern portion of the 
 American Union, instead of curving round across the 
 Atlantic. 
 
 For this reason, if a ship was handled on the supposition 
 that the hurricane would always go the same course, she 
 would be exposed to very great danger. 
 
 In the temperate zone, where cyclone-paths are still 
 more irregular, any attempt to lay down any hard and 
 fast rule for the tracks of depressions could only lead to 
 disastrous failure of any forecasts which were based on 
 that system; but though the numerous causes which 
 have been found to modify the paths of cyclones cannot 
 be allowed for in estimating the probable future path of 
 
420 WEATHER. 
 
 any actual depression, still many points, which have been 
 noted, are so interesting that we shall mention some of 
 them more in detail. 
 
 TENDENCY TO FOLLOW CEKTAIN TRACKS. 
 
 During the persistence of any type, two or three 
 successive cyclones have a remarkable tendency to follow 
 the same course. This, of course, is the natural product 
 of the fact that the path of a cyclone is determined by 
 the type of pressure in which it is formed. Sometimes 
 this path is entirely dictated by surrounding pressure ; 
 but at other times local configuration of the land exercises 
 a most powerful directive influence. 
 
 For instance, in Great Britain, during the westerly 
 type, when the depressions are so far south as to cross 
 that island, the centres have a decided tendency to 
 traverse either the line of the Caledonian Canal in Scotland, 
 or the low-lying ground which separates the valleys of 
 the Forth and Clyde. Both of these courses coincide 
 with what we may call lines of least resistance, for these 
 are the two easiest lines by which it is possible to cross 
 the mountainous districts of Scotland. Another well- 
 marked tendency of cyclone-centres is to hug the sea- 
 shore, rather than to strike inland. When a cyclone 
 comes up the English Channel, it often skirts the south 
 coast of England, and then moves more northward along 
 the east coast, rather than pass directly to the north-east 
 across the land. In like manner, large cyclones which 
 come in from the Atlantic, when they meet the coast of 
 Norway, often hug the coast for several days, instead of 
 
FORECASTING BY SYNOPTIC CHARTS. 421 
 
 going straight to the north-east. In the United States 
 the great majority of cyclones traverse the line of the 
 great lakes, and then either follow the valley of the St. 
 Lawrence or strike across the New England States into 
 the Atlantic. 
 
 Great chains of mountains also influence very power- 
 fully the paths of cyclones. 
 
 In Europe, the chain of the Alps almost forms a natural 
 boundary between the weather of the Mediterranean and 
 that of the northern portion of the continent. As a rule, 
 that great inland sea has a totally different atmospheric 
 circulation from that which affects the rest of Europe. 
 
 This will be very obvious if we turn again to the large 
 charts which we gave in our chapter on Weather-Types. 
 
 Sometimes we can trace a cyclone in the Mediterranean 
 trying to cross the Alps, and being broken up in the 
 attempt. We can readily understand that if a mountain 
 chain, 12,000 feet high, sliced off the lower half of such 
 a shallow and complex vortex as a cyclone, the whole 
 system might very easily be destroyed. Exceptional 
 cases, however, do occur in which large cyclones cross 
 the great barrier of the Alps. 
 
 In India, too, the still loftier chain of the Himalayas 
 imposes an even greater influence on the meteorology of 
 that country, as a glance at the charts which we have 
 already given of the monsoon districts will abundantly 
 show. 
 
 STORMS CROSSING THE ATLANTIC. 
 
 But the cyclones whose motions have created by far 
 the greatest interest in Europe are those which sometimes 
 
422 WEATHER. 
 
 come across the Atlantic. The public have been fasci- 
 nated by the idea that a storm could be telegraphed from 
 New York, and its arrival on the coasts of Europe foretold 
 three or four days in advance. If cyclones only moved 
 with tolerably uniform velocities and in tolerable uniform 
 paths, and the intensity remained constant, then, indeed, 
 it would often be possible to obtain timely warning from 
 the United States or Canada. Although the diagrams 
 which we have already given of Atlantic weather would 
 sufficiently show the real character of Atlantic cyclones, 
 still the nature of the paths of these depressions will be 
 more clearly understood if we give the tracks of all the 
 depressions which appeared in the Atlantic during a 
 single month. This will do as a sample of any other 
 month or season. In Fig. 91 we therefore give a chart 
 of all the cyclones which could be traced for more than 
 two days in the United States, the Atlantic, and Europe 
 during the month of July, 1879. During that month 
 there were seven well-defined cyclone-tracks within the 
 above-mentioned area. 
 
 These paths are plotted on our chart, and the 
 position of the centre of each cyclone on every day is 
 clearly marked. 
 
 Now, the first glance will at once satisfy us as to the 
 broad idea that cyclones usually move in a certain 
 general direction. 
 
 The whole of the paths lie along a comparatively 
 narrow belt of the ocean ; but when we come to look into 
 the details, we shall find that the smaller variations of 
 motion effectually preclude the use of this knowledge in 
 forecasting. 
 
FORECASTING BY SYNOPTIC CHARTS. 
 
 423 
 
 Of the seven cyclones, four Nos. I., II., V., and VII. 
 were formed in mid-Atlantic, and then pursued a more 
 or less irregular course towards Europe. Observe how 
 the curious loop to the northwards, which the path of 
 No. I. makes at the beginning of the month, is almost 
 exactly reproduced at the end of that time by cyclone 
 
 FIG. 91. Cyclones crossing the Atlantic. 
 
 No. III. Cyclones IV. and VI. were formed over the 
 United States ; both passed into the Atlantic, but neither 
 reached the coasts of Europe. 
 
 Cyclone No. III. also had its origin in the American 
 Union, though, unlike the two others, it not only survived its 
 journey across the Atlantic, but, after traversing Europe, 
 
424 WEATHER. 
 
 passed into Siberia. Our chart follows its history for the 
 ten days from July 9 to 28 ; but let us try to see how we 
 should fare if we attempted to issue forecasts on the 
 supposition that the depression would move either in a 
 uniform direction or with a uniform velocity. From the 
 9th to the llth the cyclone moved towards the north- 
 east with a considerable velocity ; the next two days it 
 turned to the south-east with diminished speed, and left 
 the shores of the United States with a south-easterly 
 trajectory. The day of leaving the velocity increased; 
 but by next morning the direction changed again to the 
 north-east, and the velocity gradually diminished for the 
 next seven days, by which time the depression had 
 reached the coast of Ireland, after being eight days in 
 transit from Nova Scotia. A crack steamer would have 
 done the distance in five days. From that day, the 21st, 
 the speed increased again, and the cyclone turned still 
 more towards the north. Then, with gradually decreasing 
 velocity, the path bent round to the south, and afterwards 
 turned once more to the northwards, with increased 
 speed, till the 28th of July, when we lose sight of the 
 depression in the frozen marshes of Siberia. 
 
 This example will abundantly prove that we can form 
 no estimate of the future path or velocity of a cyclone- 
 centre by any observations on its earlier motion. In this 
 case the direction and velocity of the depression when it 
 left the American shore gave no clue either to its path 
 across the ocean, or its meanderings after reaching the 
 continent of Europe. 
 
 There is another point which we must remember in 
 the discussion of this question we track cyclones, but 
 
Of TH 
 
 TJNJVEK8ITY )] 
 
 CAUI 
 
 FORECASTING BY SYNOPTIC CHARTS. 425 
 
 not necessarily storms. The size and intensity of this 
 cyclone varied every day of its life. Some days the 
 intensity was so great that the wind rose to the force 
 of a gale in places ; other days the gradients were never 
 developed of sufficient steepness to give rise to more 
 than a breeze. No general rule can be laid down that 
 will apply to the life-history of a cyclone ; we must 
 watch from day to clay for symptoms of increasing or 
 decreasing intensity. 
 
 From all this we can also estimate the value of the 
 idea that a swift Atlantic mail-steamer could arrive 
 before a storm, and so give notice of approaching danger. 
 
 The cyclone which we have just traced travelled rather 
 slower than usual; we often find depressions cross the 
 Atlantic in four days. However, in this case, the cyclone 
 came across at just about the speed of the fastest steamers. 
 The first two days the cyclone would have been passing 
 the vessel ; on all the other six days, the steamer would 
 have been catching up the cyclone. The ocean route 
 from the mouth of the St. Lawrence to Cork is almost 
 exactly along the track of tins cyclone. A steamer 
 would, therefore, have experienced little wind, but a 
 uniformly low barometer during her voyage. Any report 
 which she alone could give would be useless to a fore- 
 caster in London or Paris; but if several boats were 
 arriving, and they all telegraphed up their observations 
 at 8 a.m. on the three or four preceding days, then the 
 combination of their results would certainly enable the 
 forecaster to deduce some useful indications. 
 
 In all British forecasting a certain amount of un- 
 certainty must always remain as to the future path of a 
 
426 WEATHER. 
 
 cyclone, even when we see a well-defined depression 
 lying off the coasts of Ireland ; how much greater must 
 the uncertainty be when we attempt to forecast the path 
 of a cyclone four days ahead, and from a distance of three 
 thousand miles? If the forecaster cannot hit England 
 straight when he aims from Ireland, will he be likely to 
 hit her at all if he shoots from New York ? The number 
 of cyclones which actually cross the Atlantic from shore 
 to shore appears to vary from about eight to twenty in any 
 year. In many cases it is difficult to say whether it is 
 the same cyclone which we trace, from the peculiar manner 
 in which two depressions may fuse into a single new one. 
 On the whole, then, we see that the crude notion of 
 forecasting European storms from the United States 
 contains some elements of truth, but that still, from the 
 nature of cyclone-motion, the idea can never be used in 
 practical forecasting. 
 
 PATH AS INDICATED BY THE STRONGEST WIND AND 
 HIGHEST ADJACENT PRESSURE. 
 
 A good deal of work has been done, both in England 
 and Germany, on the question of how far the path of a 
 cyclone can be determined by the general direction or 
 force of the surrounding wind, and the investigators have 
 found that generally the propagation of the cyclone is in 
 the same direction as the strongest surface-wind in the 
 neighbourhood. There are, of course, a good many 
 exceptions ; and it is impossible in our present state of 
 knowledge to say whether the strongest wind indicates 
 the general direction of the generating current in which 
 
FORECASTING BY SYNOPTIC CHARTS. 427 
 
 the cyclone is only an eddy, or whether the strongest 
 wind is the product of the combination of surface rotation 
 and propagation, being nearly in the same direction at 
 one particular point. 
 
 All our charts have shown that a cyclone usually tries 
 to keep an area of high pressure on its right-hand side ; 
 and this, too, has a good deal to do with the strongest 
 wind being found at right angles to the centre, and 
 therefore nearly in the same direction as the motion of 
 the whole depression. 
 
 INFLUENCE OF SURROUNDING TEMPERATURE. 
 
 We now come to the far more difficult but important 
 question as to the influence of surrounding temperature 
 on the propagation of cyclones, and as to whether the 
 development of heat on the right-hand side of a cyclone 
 is the cause or product of cyclone-motion. 
 
 Putting all theoretical considerations aside, the facts of 
 the case, as far as Europe is concerned, are as follows : A 
 cyclone nearly always has the highest temperature on the 
 right-hand side of the path ; and for the same distribution 
 of pressure, there is a considerable difference in the path 
 of depressions at different seasons of the year, when the 
 general slope of heat from the equator to the pole is not 
 the same. 
 
 Dr. J. Y. Bebber has discovered the following relations 
 special for Germany and Central Europe: "If the dis- 
 tribution of air-pressure and temperature in the neighbour- 
 hood of a depression are directed to the same sense, then 
 the propagation of the depression is nearly perpendicular 
 
428 WEATHER. 
 
 to the pressure and temperature-gradient. If the air- 
 pressure and temperature in the neighbourhood of a 
 depression are distributed in an opposite sense, and if the 
 differences are nearly equal, so is the motion of the 
 depression checked, or even arrested (stationary depres- 
 sion), whereby the depression takes a long, more or less 
 distorted form, of which the longer axis lies perpendicular 
 to both the air-pressure and temperature-gradient. If, 
 with the same distribution as before, either the air- 
 pressure or temperature-gradient overweighs on one side 
 of the depression, so will the direction of the path be 
 determined by the predominating element. If air-pressure 
 and temperature are not, indeed, opposite, but also not 
 distributed in the same sense round the depression, so 
 will the depression strike out a resultant direction." He 
 also thinks that pressure is the more important deter- 
 mination of cyclone-motion in winter, and temperature 
 the predominant influence in summer. 
 
 The conception of temperature and pressure gradients 
 being distributed in the same or opposite senses, appears 
 to be as follows : If the highest pressure and highest 
 temperature are either both to the north,, or both to the 
 south of a cyclone, they are said to be in the same sense, 
 and the depression will move at right angles to both. 
 But suppose pressure was highest to north, and tempera- 
 ture to south; then these two elements would be dis- 
 tributed in the opposite sense, and the cyclone would 
 probably be arrested in its usual eastward course. 
 
 These observations are more suitable to Germany 
 than to Great Britain, as some of the expressions are 
 hardly applicable in the latter country, and in England 
 
FORECASTING BY SYNOPTIC CHARTS. 429 
 
 local variation is so great, and the area of observation so 
 small, that the distribution of surrounding temperature 
 can scarcely be used in practical forecasting. But in all 
 continental Europe we have one practical rule that if 
 pressure is high to the north or north-north-east of a 
 cyclone, and temperature also higher on that side than to 
 the south, then the propagation of the depression will 
 probably be towards some point of west, instead of 
 towards the east as usual. For instance, suppose we 
 found some morning a cyclone over Central Europe, with 
 an anticyclone over the North Sea, the natural presump- 
 tion would be that the depression would move always 
 very slowly in this type towards Russia ; but if, as in 
 Figs. 95 and 96, we found the highest temperatures in 
 the Baltic, and not in Austria, and especially if the 
 temperature seems to rise to the north or north-west 
 of the centre, then we might forecast that the depres- 
 sion would move, as in this instance, westwards towards 
 Great Britain. 
 
 The question how far the cyclone affects temperature, 
 and how far the latter directs the former, will be best 
 explained as follows: Let us call the general slope of 
 temperature from land to sea, which varies according to 
 the time of year, the "seasonal gradient of heat," and 
 the patch of heat on the right of a cyclone " cyclone 
 heat ; " then we may say that, while the seasonal gradient 
 has a directive influence on the path of the depression, 
 the cyclone heat is the product of the moving whirl 
 itself. The conclusive proof that the heat-patch on the 
 right front of a depression belongs to the cyclone directly, 
 and not indirectly, through the disturbance of radiation, 
 
430 WEATHER. 
 
 is found in that peculiar quality that no thermometer 
 can appreciate, but which is readily recognized by our 
 more delicate sensations. In a typical east-going cyclone 
 the neuralgic, pain-producing heat comes with the south- 
 east wind on the right front of the depression ; but when 
 a cyclone goes west, the then right front has a north-west 
 wind and the same distressing quality of heat. 
 
 FORECASTING DEPENDS ON NO THEORY. 
 
 We can now readily understand from all the fore- 
 going remarks that forecasting depends neither on any 
 theory nor on any calculation. The whole science, from 
 beginning to end, rests solely on observation. 
 
 The shapes of isobars, and the relation of wind and 
 weather to them, are matters of experience only. We 
 find that certain kinds of weather are associated with 
 different portions of each fundamental form of isobars 
 and we classify accordingly. We give each shape of 
 isobars a conventional name, but that does not bind us 
 to any theory of atmospheric circulation. In like manner, 
 we see that no averages or mean values are of any avail 
 in forecasting weather. Cyclones may usually take a 
 certain path, but they need not do so ; the greater 
 portion of the rainfall of any country may come with a 
 south-west wind, but that does not prevent many fine 
 days with the wind from that quarter. On an average, in 
 England, three days out of four may be cloudy, and the 
 forecaster who always announced a cloudy day would have 
 seventy-five per cent, of success. Still, in an anticyclonic 
 period his calculations would totally fail ; he could never 
 
FORECASTING BY SYNOPTIC CHARTS. 431 
 
 say what kind of cloud would appear, and such a system 
 would have no claim to be called forecasting in the 
 modern sense of the word. It is impossible to suppose 
 that we have yet nearly reached the highest perfection 
 of which forecasting is capable, but still we know enough 
 of the nature of the subject to say with certainty that 
 calculation will never enter much into the science of 
 weather-prevision. Natural aptitude and the experience 
 of many years' study are the qualifications of a successful 
 forecaster. "In fact, meteorology is not an exact, but 
 an observational science, like geology or medicine ; and 
 just as, however accurately the symptoms or treatment of 
 any malady may be described, the skill to recognize and 
 the judgment to treat must rest on the ability of the 
 physician, so in meteorology, however carefully the 
 relation of weather to isobars may be defined and 
 the nature of their changes described, the judgment 
 which experience alone can give, to enable a warning to 
 be issued, must ever depend on the professional skill of 
 the forecaster." 
 
 DETAIL POSSIBLE. 
 
 It may not be out of place to introduce here a few 
 remarks as to the amount of detail which it appears 
 possible to give to daily forecasts. Under various head- 
 ings, we have already discussed the influence of local 
 obstacles in modifying the appearance or intensity of 
 any kind of weather, and also the powerful diurnal 
 variations of every element in all parts of the world. When 
 to these we add the tendency of cyclones to form secon- 
 
432 WEATHER. 
 
 daries, so small as not to show in an ordinary synoptic 
 chart, then we may easily understand that it is the general 
 character only of weather which a forecaster can ever 
 safely predict. The general character is the quality of 
 weather which we have taken such pains to show is 
 constant in each portion of every shape of isobars, and 
 that never changes under any local or diurnal variation. 
 
 If we live in any place which commands a view over 
 any large tract of country, and we think how often we 
 see both cloud and rain which only affect & very small 
 portion of our horizon, we can readily understand that, 
 even if it were possible to issue minute forecasts, every 
 few square miles of country would require a separate 
 warning. 
 
 How FAR IN ADVANCE CAN FORECASTS BE ISSUED? 
 
 We may also consider how far in advance forecasts 
 can safely be issued. The numerous charts which we 
 have already given will show the reader the amount of 
 change which twelve or twenty-four hours may develop 
 in the distribution of pressure. Sometimes we have been 
 able to trace the changes in either of these intervals 
 quite easily; at other times it has been difficult to say 
 how the first set of isobars has grown into the second. 
 In the United States the observations are taken three 
 times a day, and this appears to be sufficiently frequent 
 for all practical purposes. In most European countries, 
 reports are not sent up more than twice a day ; but with 
 this interval, cyclones sometimes form so suddenly that 
 they are not forecast in time to give any warning. We 
 
FOKECASTING BY SYNOPTIC CHARTS. 433 
 
 shall give an example of such a case further on in this 
 chapter. 
 
 Thus, from eight to twelve hours seems to be the 
 furthest time for which forecasts can be issued in ad- 
 vance, and even then many local details cannot be given. 
 Some meteorologists are of opinion that a good deal of 
 forecasting will be done in the future, with the assistance 
 of a complete knowledge of recurrent periods of heat, 
 cold, rain, or storm ; and we lean strongly to that view, 
 if these periods are used in the manner so fully 
 explained in our chapter on Seasonal and Cyclical 
 Periodicities. 
 
 TIME OF PREPARATION. 
 
 A few particulars of the time necessary for collecting 
 and examining the materials for synoptic charts will 
 perhaps enable the public better to understand the prac- 
 tical conditions of the problem of weather-forecasting 
 and storm-warnings. 
 
 In Great Britain, the morning observations are taken 
 at 8 a.m. Even with all the rapid organization of the 
 British Post-office, the majority of the reports do not 
 arrive till between 9 a.m. and 10 a.m. As fast as they 
 arrive, the information is entered on a chart, and a 
 synoptic chart is constructed. If necessary, telegraphic 
 intelligence of storms is immediately sent to the coasts, 
 and in every case information as to the state of the 
 weather, and a forecast for twenty-four hours ahead, is 
 sent to the press. 
 
 In practice, storm- warnings can rarely be despatched 
 
 2 F 
 
434- . WEATHER. 
 
 before 11 a.m. ; that is to say, three hours after the 
 observations have been taken. If we allow at least 
 another hour before the public can have access to the 
 information, we see at once that the day is so far gone 
 that the forecast can have little practical importance for 
 the majority. 
 
 The greatest value is when a storm has just begun 
 to show over Valentia at 8 a.m. ; then the English coasts 
 can be warned in time. Still, in the three or four hours 
 which must elapse before the storm can be warned, the 
 cyclone will have advanced, perhaps, as much as a hundred 
 and twenty miles, so that, before a telegram can reach 
 the western shores of England, the gale will either have 
 commenced, or the appearance of the sky will have given 
 unmistakable warning. 
 
 The whole theory of storm-warnings by means of 
 the electric telegraph is based on the supposition that 
 the message travels faster along the wire than the storm 
 along the earth's surface. But, as the practical organi- 
 zation of collection and distribution of intelligence takes 
 at least three hours, the storm must either move slowly 
 or over a considerable intervening district before any 
 set of stations can be successfully warned. The forecasts 
 which are issued from reports taken at 6 p.m. are of 
 more use. 
 
 The organization of the press enables the public to 
 obtain the office forecast much more quickly than by any 
 other means. The British reports are taken at 6 p.m., 
 while the United States Signal Office obtain their latest 
 about 11 p.m. These five hours are an unquestionable 
 gain. In Great Britain there are, however, difficulties 
 
FORECASTING BY SYNOPTIC CHARTS. 435 
 
 in the way of transmission of intelligence during the 
 night. 
 
 Except in the large towns, the majority of telegraphic 
 stations are closed till 8 a.m., so that while the evening 
 forecasts do not reach them till past eight o'clock in the 
 morning, the information for that morning arrives about 
 three hours later. Thus we see the practical difficulties 
 in the way of forecasting. There is no doubt that in 
 time some of them will be successfully overcome. 
 
 WHEN MOST SUCCESSFUL. 
 
 A few remarks on the circumstances under which the 
 most successful forecasts can be issued will also much 
 help a general apprehension of the subject. We will 
 confine our observations to Great Britain only. It is 
 very obvious that the more striking the weather-changes, 
 the more have we something definite to forecast. When 
 we have a well-formed cyclone, which traverses a well- 
 defined path, we have strongly marked sequences of wind 
 and weather, and any error in the forecast will only arise 
 from some slight difference between the expected and 
 the actual track. But when we have what we have seen 
 is the more usual state of things in Great Britain ill- 
 defined depressions which move irregularly, and one or 
 more of which fuse into a fresh cyclone with a new centre 
 then we have no definite sequence of weather to deal 
 with, but a change which is produced by the weather 
 at each station gradually conforming to the varying 
 shapes of isobars. The best that can be done then is to 
 forecast generally broken weather, and more or less rain 
 
436 WEATHER. 
 
 generally; but no attempt can be made to foretell any 
 definite series of wind-shifts, as in a true cyclone. 
 
 Experience has shown that in Great Britain no serious 
 gale has ever been experienced, unless there is more 
 than half an inch of difference of barometric pressure 
 between some two stations. Synoptic charts will always 
 detect even much smaller differences; so that, though 
 some uncertainty will always remain as to the direction 
 of the wind, the force will generally be at least approxi- 
 mately forecast correctly, except in the case of a very 
 sudden and unexpected fall of the barometer. 
 
 Very different, however, is the case of rain. Secon- 
 daries and non-isobaric rains are the forecaster's bugbear ; 
 they form so quickly, show so little on a synoptic chart, 
 and move so irregularly, that rain in general terms is all 
 that the forecaster can usually say. In summer, when 
 he sees the characteristic loops in the isobars which con- 
 stitute secondaries, he can safely predict thunder and 
 rain ; but he cannot attempt to localize either of these 
 phenomena. 
 
 Sometimes, too, secondaries are so small that they do 
 not show at all on a synoptic chart, which is constructed 
 on reports received from stationsoften a hundred and fifty 
 or two hundred miles apart. The whole loop of a 
 secondary need not be nearly so large ; and then a depres- 
 sion of that class might lie between two stations, and yet 
 be indicated at neither. The weather, however, would be 
 profoundly modified, and the forecasts would probably be 
 erroneous. 
 
 There is also always the important difference between 
 wind and rain, that the former is always in the main 
 
FORECASTING BY SYNOPTIC CHARTS. 437 
 
 determined by the steepness of the gradients, while the 
 amount of precipitation bears no relation to any known 
 meteorological element. 
 
 In many shapes of isobars we know that there will be 
 rainfall, but whether much or little, we cannot tell at 
 present. 
 
 From these considerations we need not be surprised to 
 find that in all offices, except in Japan, wind is better 
 forecast than rain. 
 
 SOURCES OF FAILURE. 
 
 From the conditions of successful forecasts, we can 
 readily turn to those of unsuccessful predictions. Besides 
 the uncertainty of rainfall due to the action of secondaries, 
 there are four principal sources of failure : the sudden 
 formation of an intense cyclone ; the sudden dying out of 
 an existing cyclone; the motion of a cyclone in an un- 
 expected path ; and, lastly, an error in the judgment of 
 the forecaster. 
 
 In the first case, of the sudden formation of a new 
 cyclone, the whole forecast is necessarily totally upset, 
 and the weather which is experienced is worse than had 
 been anticipated. 
 
 The converse occurs when an intense cyclone suddenly 
 dies out. Then the weather is much better than was 
 expected ; but neither in this case nor in the preceding 
 one can settled weather be expected. 
 
 When a cyclone takes an unusual path, the general 
 character of the weather will remain bad, but the direc- 
 tion of the wind and the details in different districts will 
 
438 WEATHER. 
 
 be wrongly forecast. We have already given instances 
 of cyclones which move in no well-defined path, and more 
 complicated cases often occur. Sometimes the path will 
 describe a complete circle of no very great diameter ; but 
 the commonest case in Western Europe is when the path 
 of a cyclone takes the form of the letter V. For instance, 
 a cyclone comes in from the Atlantic from about due 
 west, and after it has gone as far as England, it moves 
 back again in a north- westerly direction, as it has not 
 been able to pass the area of high pressure which would 
 then be lying over Northern and Central Europe. In 
 another common case, the cyclone comes down from the 
 north-west on to England, and then passes off in a north- 
 easterly direction towards Norway. In all such cases the 
 forecaster is at a great disadvantage. 
 
 Lastly, the judgment of the forecaster will sometimes 
 err. We have shown that no absolute law of cyclone- 
 motion can be laid down, and that, in fact, the tracking 
 of well-defined depressions forms but a small portion of 
 the forecaster's business. On the larger number of days 
 he has to estimate how, or where, cyclones will form in 
 an ill-defined area of low pressure, or how far an area of 
 low pressure will encroach on another region of high 
 barometer. In this, he must rely on his own opinion and 
 experience alone; that must be fallible sometimes, but 
 better results are obtained by trusting to personal skill 
 than by attempting to use any mechanical rules or 
 maxims. 
 
 Men differ in their aptitude to forecast weather in the 
 same way as physicians differ as to the accuracy of their 
 diagnosis; but just as the best results are obtained by 
 
FORECASTING, BY SYNOPTIC CHARTS. 439 
 
 selecting the doctor whom experience has shown to be the 
 most successful practitioner, so the best forecasts are got 
 by selecting the meteorologist who has been the most 
 successful in that branch of the subject. In the United 
 States Signal Office at the present time, four men take the 
 duty of forecasting in rotation. They have so far all been 
 ground in the same mill, by passing through a two- 
 years' course of the same hard training ; and it is found in 
 practice that the difference between the best and worst is 
 two per cent, in the number of successful forecasts. For 
 instance, if the best man gets ninety per cent., the worst 
 will attain to eighty-eight per cent, of success. 
 
 SOME COUNTRIES EASIER THAN OTHERS. 
 
 From all that we have now explained, it will be very 
 evident that forecasting is much easier in some countries 
 than others. In the tropics, the great seasonal changes 
 come on regularly, and the smaller changes from day to 
 day are insignificant. In the two or three days of any 
 year on which a regular cyclone may form, the pre- 
 monitory symptoms are so obvious that there is no diffi- 
 culty in framing a forecast. 
 
 In temperate regions, those countries will be the best 
 situated which lie to the east of a well-observed land area, 
 because most disturbances in the temperate zone move 
 from the west. 
 
 Thus Germany and Norway are much more favourably 
 located for weather-prevision than either England or 
 France. 
 
 In the vear 1869 twentv-three storms were felt in 
 
440 WEATHER. 
 
 Hamburg, and of these twenty-two had previously passed 
 over some part of Great Britain. In the seven years, 
 1867-1874, 301 warning messages were issued from 
 London to Hamburg; seventy-two per cent, of these 
 warnings were followed by gales, while in only three 
 cases did the storm outrun the message. Then in the 
 United States, the majority of cyclones commence in the 
 Kocky Mountains ; so that with the admirable organization 
 of the Signal Office, timely warning of serious gales can 
 usually be sent to the Eastern States of the Union. 
 
 Great Britain is situated in a region of peculiar diffi- 
 culty. Not only does her insular position preclude any 
 early knowledge of the advent of cyclones, but, from the 
 nature of weather-types, she is more exposed to unsettled 
 weather than any other part of Europe. 
 
 We have seen in our chapter on Weather-Types, that 
 the positions of the great areas of high and low pressure 
 are to a certain extent determined by the areas of land 
 and water. 
 
 When the persistent anticyclone of the southerly type 
 lies over Scandinavia, the Atlantic is covered by low 
 pressure and bad weather; when the great anticyclone 
 covers the Atlantic in the northerly type, then pressure 
 is lowest and weather worst in Scandinavia ; so that, in 
 almost every case, Great Britain is on the boundary 
 between a cyclonic or anticyclonic system, and is there- 
 fore exposed to changeable weather. Just as an outlying 
 rock is exposed to the wash of every sea, so England is 
 exposed to the disturbing influences of every type of 
 European or Atlantic bad weather. 
 
FORECASTING BY SYNOPTIC CHARTS. 441 
 
 EXAMPLES OF ACTUAL FORECASTS. 
 BRITISH. 
 
 After these explanations we will now give some 
 examples of actual forecasts in different countries, com- 
 mencing with Great Britain. The latter are taken, with 
 some important additions, from the author's work on the 
 principles of forecasting before mentioned. We have 
 selected our first example to illustrate a completely suc- 
 cessful forecast which depended on the estimate of the 
 forecaster as to the progress of an ill-defined area of low 
 pressure towards the east. This is one of the commonest 
 c*ases which occur in Great Britain. The chief points 
 which the forecaster had to consider were the direction in 
 which the depression would move, and especially how far 
 east it would pass without being arrested in its progress. 
 Also, whether the gradients would become sufficiently 
 steep to give rise to serious gales. 
 
 But to understand properly the details of the warn- 
 ings, we must first explain the districts into which the 
 United Kingdom is divided for the localization of weather- 
 forecasts. 
 
 In Fig. 92 we give a map of the eleven districts in the 
 British Islands which are separately warned ; and by 
 ineans of this map the subsequent details will be easily 
 followed. 
 
 A glance at the relative size of any one of these 
 districts and the area covered by even a small cyclone, 
 will show at once how much a small change in the cyclone 
 may mar the most carefully drawn deductions of the fore- 
 
442 
 
 WEATHER. 
 
 caster ; the smallest loop in the isobars which we saw so 
 often in our large charts of weather-types would entirely 
 alter the details, though not the general character of the 
 weather which would be experienced. The action of such 
 a secondary might reduce the force of the wind so much 
 that some district would receive a warning which was not 
 
 FIG. 92. British forecasting districts. 
 
 justified by the event, or develop rain where fine weather 
 had been anticipated and forecast. 
 
 In the left-hand portion of Fig. 93 we give the chart 
 from which forecasts had to be issued at 8 a.m., February 6, 
 1883. We see in it at once the commonest features of 
 the southerly type of weather with the pressure high over 
 Scandinavia and low over the west of Ireland, while the 
 isobars run nearly due north and south. Southerly gales 
 have already commenced in the west and north, while fine 
 
FORECASTING BY SYNOPTIC CHARTS. 
 
 weather prevails over the south and east coasts of Great 
 Britain. 
 
 It was also known, by comparison with the previous 
 charts, that while the barometer was rising over Norway, 
 it was falling, but only slowly, over the western coasts of 
 Ireland. Now, from all that we have already explained 
 
 6.2.83 
 
 8. a.m. 
 
 FIG. 93. Successful forecast (British). 
 
 as to the nature of this type, it is evident that there is 
 no fear of the depression crossing England so as to bring 
 tiny great change of wind, but that the gradients will get 
 steeper for southerly winds with bad weather, and that 
 probably the south and east coasts will not be affected. 
 Then, as to storm-warnings, all the north and west (Dis- 
 tricts 0, 1, 6, and 9) were already warned, but as the south 
 of Ireland (District 10) will be affected by the increasing 
 
444 
 
 WEATHER. 
 
 gradients, warnings are now necessary for it also. Hence 
 the following forecasts were issued to the different dis- 
 tricts : 
 
 FORECASTS FOR THE TWENTY-FOUR HOURS ENDING AT NOON ON 
 FEBRUARY 7, 1883. 
 
 Districts. 
 
 Forecasts. 
 
 0. Scotland, N. 
 
 1. Scotland, E. 
 
 2. England, N.E. 
 
 3. England, E. 
 
 4. Midland Counties ... 
 
 5. England, S., and the 
 
 Channel 
 
 6. Scotland, W. 
 
 7. England, N.W. 
 
 8. England, S.W. 
 
 9. Ireland, N. 
 
 10. Ireland, S. ... 
 Warnings 
 
 Southerly strong winds and gales; cloudy 
 generally, with some rain. 
 
 Do. Do. 
 
 South-easterly winds, moderate inland, strong 
 on coast ; fair generally. 
 
 Do. Do. 
 
 Same as No. 5. 
 
 South-easterly and southerly winds, mode- 
 rate or fresh ; fair generally. 
 
 South-easterly and southerly strong winds, 
 perhaps a gale j fair to cloudy, and 
 unsettled. 
 
 Do. Do. 
 
 South-easterly and southerly winds, in. 
 creasing ; cloudy. 
 
 South-easterly and southerly winds, in- 
 creasing to a gale ; cloudy, unsettled ; 
 some rain. 
 
 Do. Do. 
 
 The south cone is still up in Districts 0, 6, 9, 
 and parts of 1 and 7, and has been re- 
 hoisted this morning in District 10. 
 
 By looking at the right-hand portion of the chart 
 {Fig. 93) for 6 p.m. on the same day, we find that the 
 above anticipations have been completely verified. Wind 
 -and rain have increased in the west and north, but in 
 South-east England the weather remains fine. In his 
 journal near Dover, in District 5, on that day, the author 
 finds the following entry :" February 6, 1883. Cold, 
 
FORECASTING BY SYNOPTIC CHARTS. 
 
 445 
 
 dry, very fine and bright ; wind south-east, fresh." Hence 
 the forecasts were a complete success. The weather was 
 cool near Dover because that town was under the influence 
 of the European anticyclone ; but in all the western dis- 
 tricts temperature was very high for the season. In the 
 
 FIG. 94. Failure of forecasts. 
 
 selection of this example we had, however, an additional 
 object, viz. to illustrate what we have laid down relative 
 to the use of periodicities in forecasting. 
 
 We have already mentioned that the period February 
 7-10 is one of recurrent cold weather, whence, if the fore- 
 caster had trusted blindly to periodicities, he would have 
 made a complete failure. On the other hand, had he 
 discovered on this day the commencement of either the 
 northerly or easterly types, the knowledge of the 
 periodicity would have been of great use to him. 
 
446 
 
 WEATHER. 
 
 Our next illustration will be that of a kind which, 
 fortunately, rarely occurs, viz. the sudden formation of 
 a cyclone in an unexpected position, which entirely upsets 
 all forecasting. In the left-hand portion of Fig. 94 we 
 give a chart for 6 p.m., October 23, 1882. There we see 
 the most familiar features of the westerly type of weather, 
 and though the barometer was falling over the Bay of 
 Biscay, and rising over Scotland, there was no reason to 
 expect that the ordinary sequence of that kind of weather 
 would be disturbed that is to say, that west and south- 
 west winds, with rather showery weather, would prevail. 
 Accordingly the following forecasts were issued : 
 
 FORECASTS OF WEATHER FOR OCTOBER 24, 1882, ISSUED AT 8.30 P.M. 
 
 THE PREVIOUS DAY. 
 
 Districts. 
 
 Forecasts. 
 
 0. Scotland, N. 
 
 1. Scotland, E. ... 
 
 2. England, N.E. 
 
 3. England, E. 
 
 4. Midland Counties 
 .5. England, S 
 
 6. Scotland, W. 
 
 7. England, N.W. ... 
 
 8. England, S.W. 
 
 9. Ireland, N. 
 
 10. Ireland, S. ... 
 Warnings 
 
 South-westerly breezes, fresh or moderate ; 
 showery. 
 
 South-westerly breezes ; moderate j some 
 showers, with bright intervals. 
 Do. Do. 
 
 Same as No. 5. 
 
 Same as No. 1. 
 
 Westerly and south-westerly breezes, light 
 to fresh ; fine and cold at first, some local 
 showers later. 
 
 Same as No. 0. 
 
 Same as No. 0. 
 
 South-westerly winds, fresh to strong; 
 showery. 
 
 Wind returning to south-west, and freshen- 
 ing ; weather showery. 
 
 Do. Do. 
 
 None issued. 
 
FOEECASTING BY SYNOPTIC CHAETS. 447 
 
 When we come to look, however, at the right-hand 
 chart in the figure for 8 a.m. the following morning, we 
 find that a small well-defined cyclone had formed during 
 the night over the English Channel, which moved during 
 the day towards north-north-east, and thereby produced 
 continuous rain with complete shifts of the wind through 
 180 in many parts of the country, so that the forecasts 
 issued were a complete failure. 
 
 Present Results. 
 
 It will now be interesting to give some idea of the 
 amount of success which at present attends both every- 
 day weather-forecasts and also storm-warnings, as issued 
 by the British Meteorological Office for every district ; 
 each forecast being considered under the separate headings 
 of " Wind " and " Weather," and the amount of success 
 or failure is divided into four degrees complete success, 
 partial (more than half) success, partial failure, and total 
 failure. In practice it is found that the percentage of 
 any district varies but little from year to year, though, 
 on the whole, there is a slow progressive improvement. 
 The subjoined summary of weather-forecasts for the year 
 ending March 31, 1882, may, therefore, be taken as a fair 
 sample of the results usually attained by the Meteorological 
 Office. 
 
448 
 
 WEATHER. 
 
 SUMMARY OF EESULTS. 
 
 
 Percentages. 
 
 
 
 
 Total 
 
 District. 
 
 
 
 
 
 pprcentage 
 
 
 Complete 
 
 Partial 
 
 Partial 
 
 Total 
 
 of success. 
 
 
 Success. 
 
 success. 
 
 failure. 
 
 failure. 
 
 
 Scotland, 1ST 
 
 39 
 
 42 
 
 14 
 
 5 
 
 81 
 
 Scotland, E 
 
 35 
 
 43 
 
 15 
 
 7 
 
 78 
 
 England, N.E 
 
 32 
 
 46 
 
 17 
 
 5 
 
 78 
 
 England, E. 
 
 33 
 
 44 
 
 17 
 
 6 
 
 77 
 
 Midland Counties ... 
 
 31 
 
 46 
 
 18 
 
 5 
 
 77 
 
 England, S 
 
 35 
 
 46 
 
 14 
 
 5 
 
 81 
 
 Scotland, W 
 
 30 
 
 44 
 
 19 
 
 7 
 
 74 
 
 England, N.W. ... 
 
 32 
 
 44 
 
 17 
 
 7 
 
 76 
 
 England, S.W 
 
 34 
 
 42 
 
 18 
 
 6 
 
 76 
 
 Ireland, N 
 
 36 
 
 44 
 
 14 
 
 6 
 
 80 
 
 Ireland, S 
 
 35 
 
 41 
 
 16 
 
 8 
 
 76 
 
 Summary 
 
 34 
 
 44 
 
 16 
 
 6 
 
 78 
 
 By this it will be seen that the complete or partial 
 successes amount to seventy-eight per cent., varying from 
 seventy-four per cent, in the west of Scotland to eighty- 
 one per cent, in the north of Scotland and south of 
 England. 
 
 Checking Forecasts. 
 
 It might appear at first sight that when a forecast 
 had been issued, it would be the simplest thing possible 
 to check it, and to say whether it had been successful 
 or not. 
 
 In practice, however, it is very different, as will be 
 seen from the following remarks. The difficulty arises 
 from two sources the local variation of wind and rain in 
 the same district, and the difficulty of assigning a 
 
FORECASTING BY" SYNOPTIC CHARTS. 449 
 
 mechanical measure to such elements as a gale of wind 
 or a rainy day. 
 
 For instance, some of the British forecasting districts 
 are about two hundred miles by one hundred, and contain, 
 two or three hundred square miles. Even within this 
 limited area considerable differences of weather may be 
 experienced. It may blow a gale at Dover, and only a 
 fresh breeze in London, though those towns are only 
 seventy miles apart. The difficulties are even greater 
 when we come to treat the British Islands as a whole. 
 
 For instance, suppose we want to test the truth of the 
 popular saying as to the frequency of gales at the 
 equinox, hoAV are we to define what is a gale? Is it 
 enough to prove the saying if a gale has been experienced 
 in only one of the eleven' districts, or must we report a 
 gale from three or four districts at least, before we can 
 say that a storm swept over Great Britain about such or 
 such a date ? It follows from these general considerations 
 that the total success which is credited to any district 
 will always be much better than if the records at any 
 one station had been compared with forecasts issued to 
 the district in which it lay. Every office checks its own 
 forecasts by its own method, so that the relative per- 
 centages of success which we shall give hereafter cannot 
 be strictly compared. They are, however, very good 
 approximations to the truth. 
 
 GERMAN. 
 
 We will pass from the consideration of winter and 
 autumn gales, which move in an easterly direction, to the 
 
 2 G 
 
450 
 
 WEATHER. 
 
 very different state of things which brings thunder and 
 rain to Central Europe during the summer months. We 
 have, therefore, selected an illustration of a partially 
 successful set of forecasts issued by the Deutsche Seewarte 
 at Hamburg on August 13 and 14, 1880. This will be 
 a very typical example of rain with secondaries, and of 
 the apparent independence of rain on the barometer. In 
 Fig. 95 we give a synoptic chart over the greater portion 
 
 JO. I 
 
 FIGS. 95 and 96. Partially successful forecast (Germany). 
 
 of Central Europe at 8 a.m., Hamburg time; and in 
 Fig. 96 a similar chart for the succeeding morning. The 
 broad features of these two days is very simple. An anti- 
 cyclone rests over Great Britain, while a shallow cyclone 
 is moving westwards up the middle valley of the Danube. 
 When we come to look at the movement of the isobars 
 between the first and second day, we find that the position 
 of the line of 29*9 ins. (750 mm.) has scarcely altered, but 
 that the isobar has become looped up into secondaries. 
 
 The result of all this on the weather was to produce 
 rain and thunder with very little wind, and insignificant 
 changes in the reading of the barometer at any station. 
 
FORECASTING BY SYNOPTIC CHARTS. 451 
 
 The following forecasts were issued from the office of 
 the Deutsche Seewarte at Hamburg for these two days : 
 
 "Prospects for the weather of August 14, 1880, in 
 Germany. General. Continuance of the changeable 
 weather, with precipitation, and light to fresh wind in 
 the north, mostly northerly ; in the south, mostly westerly 
 to northerly, with a temperature little changed or else 
 falling. Here and there thunderstorms. 
 
 "Prospects for the weather of August 15, 1880, in 
 Germany. General. Bather warm in the west for the 
 most part bright weather ; in the east overcast weather 
 prevalent, with light wind. Inclination to thunder." 
 
 The cyclone which we see on our first chart lying 
 over Hungary had been moving slowly for two or three 
 previous days up the valley of the Danube river, and the 
 above forecasts are evidently based on the supposition 
 that the motion of the depression will continue in the 
 same direction. The forecaster who is skilled in the 
 meteorology of Germany knows both the kind of cyclone 
 which moves westwards and also the kind that will 
 develop rain and thunder, but he cannot tell exactly 
 where the rain will be heaviest, nor whether the intensity 
 will increase. 
 
 For this latter reason the above forecasts do not 
 sufficiently indicate the very heavy rain and disastrous 
 floods which occurred in Austria and South Germany 
 during the period in question. 
 
 J. Hann (LXXXII. Bunde Accad. d. Wiss. II. Abbth. 
 Nov., 1880) has made the weather of this period the 
 subject of a special memoir. He finds the following dates 
 for the heaviest rainfall: August 11, Siebenburg and 
 
452 WEATHER. 
 
 South-East Hungary; 12th, all Hungary, Schlesien, 
 Nieder Ostereich ; 12th and 15th, Ober Ostereich, east of 
 South Bavaria ; 13th, west of South Bavaria, Bohemia, 
 Saxon Erzgebirge ; 14th, North Tyrol and Pinzgau ; 
 15th, second maximum, Salzkammergut, West Schlesien, 
 JSTorth Bohemia. That is to say, that on the whole the 
 position of greatest rainfall travelled westwards with the 
 primary cyclone. Some, but not all, of the rain was 
 accompanied by thunderstorms. His investigations were 
 mostly from the point of view which connects rain with 
 the motion of the barometer, as observed at any one 
 station. The results which he obtains are most striking 
 illustrations of the principle we have so often alluded to, 
 that the rain of secondaries is out of all proportion to the 
 barometric changes as recroded by a solitary observer; 
 and that the position of heaviest rainfall cannot be given 
 from an inspection of the isobars, as in the case of 
 primary cyclones. 
 
 The conclusion which he arrives at as regards these 
 two points are as follows : 
 
 " The appearance of a barometer minimum in Hungary 
 occasioned enormous and extended precipitation on the 
 west and north-west sides of this barometer depression. 
 A reaction of this precipitation on the position of the 
 centre of the depression is scarcely perceptible. 
 
 " Also the general distribution of pressure (the form 
 of the isobars) shows no relation to the area of the 
 intense precipitation. 
 
 " We find, therefore, through the investigation of the 
 relative lowest barometer reading in its behaviour t& 
 rainfall, that our former conclusions are confirmed. 
 
FORECASTING BY SYNOPTIC CHARTS. 453 
 
 " A relation between barometer change and rainfall is 
 scarcely obvious, and the conclusion is justified that the 
 barometer fall, in the first instance, does not depend upon 
 rainfall, and especially is not perceptibly influenced by 
 the last." 
 
 Seewarte Success. 
 
 The following is the result of the tests of the general 
 (allgemein) weather-forecasts published by the Deutsche 
 Seewarte at Hamburg in 1882. The total percentage of 
 success is credited with half the partial success. 
 
 (Weather ... ... 78 per cent. 
 
 Wind ... ... 75 
 
 Temperature ... ... 78 
 
 ( Success ... ... 69 
 
 fipnpvn l Partial success ... 15 
 
 leial Failure 15 
 
 I Total of success ' 77 
 
 UNITED STATES FORECASTS. 
 
 We will now give some illustration of forecasts and 
 results in the United States and Canada. As an example 
 of successful forecasting, let us look back at Figs. 30-35, 
 in which we gave very detailed charts of the wind and 
 weather in the United States on January 20 and 21, 1873. 
 
 Figs. 31 and 34 give the isobars, wind, and weather 
 on the 21st, 4.35 p.m., Washington time. The cyclone 
 which we found there over the Middle States had 
 travelled in an east-north-east direction since morning, as 
 we see by reference to the preceding chart (Fig. 30). 
 
 The subjoined forecast was evidently based on the 
 
454 WEATHER. 
 
 idea that the cyclone would continue to move in the 
 same direction, so as to pass over the New England States, 
 and that the Eocky Mountains cyclone would develop 
 and advance eastwards. That is to say, that in the New 
 England States the wind would shift to the north and 
 west with a rising barometer, falling thermometer, and 
 clear sky of the rear of a cyclone ; that in the Middle 
 States somewhat similar weather would be experienced ; 
 but that from Tennessee, northwards over Ohio, the wind 
 would shift to the south and east, with a rising temperature 
 and cloudy sky, from the action of the front of the new 
 cyclone. 
 
 In the result, by 11 p.m. the same day (see Figs. 32 
 and 35), the first cyclone moved as expected, and the 
 forecasts were a complete success in the New England 
 and Middle States. 
 
 The new cyclone did not, however, advance as 
 anticipated, and the southern anticyclone increased in 
 size. Hence, from Tennessee, northward over Ohio, though 
 some south and east wind was experienced, the weather 
 remained fine, and the temperature fell from the radiation 
 of the anticyclone, instead of rising for the cyclone. 
 Hence the forecast was only partly verified. Nothing 
 could show more clearly the difficulties of a forecaster 
 than this example. He was unquestionably justified in 
 expecting the advance of the new cyclone, but he was 
 baffled by one of the endless shifts which accompany the 
 growth of cyclones. 
 
 Turn now to the charts for the next day, which we 
 gave in Figs. 42-44, to illustrate the nature of diurnal 
 temperature- variations. There we find by 11 p.m. that 
 
FORECASTING BY SYNOPTIC CHARTS. 455 
 
 day (see Fig. 42) the second cyclone had advanced very 
 much as had been expected, only more slowly. The 
 increase of the anticyclone over the Southern States on 
 the first day was due to that gathering-up pressure which 
 we have seen so often precedes the full development of 
 an incipient cyclone, but could not have been forecast in 
 our present state of knowledge. 
 
 The most unsuccessful portion of the forecast related 
 to the weather in the north-west. The probabilities were 
 given for " winds shifting to northerly and westerly, with 
 rising barometer, falling temperature, and clearing but 
 partly cloudy weather." This was based on the sup- 
 position that the new cyclone would be small and follow 
 nearly the track of the preceding depression. The pre- 
 diction was not justified by the result, but shows very 
 clearly the scope of individual judgment. The follow- 
 ing is an exact copy of the published synopsis and 
 probabilities : 
 
 " Washington D.C. 
 " Tuesday, January 21, 1873, 4-35 p.m. 
 
 " Synopsis. 
 
 "The barometer has continued falling, with rising 
 temperature from Florida to the Middle and New Eng- 
 land States, the lowest being central over the Lower 
 Lake region, where fresh and brisk variable winds and 
 rain and snow are now prevailing. Cloudy weather, rain, 
 and fresh and brisk southerly to easterly winds are now 
 prevailing from North Carolina to New York and New 
 England, excepting light snow over northern part of 
 latter. Generally clear weather from South Carolina to 
 
456 WEATHER. 
 
 Tennessee, and southward to the Gulf. Westerly to 
 northerly winds, cloudy weather, light snow, and falling 
 temperature from Kentucky to the Upper Lakes and 
 Lake Erie. The rivers have fallen at Pittsburgh and 
 Cairo, but reported to have risen over five feet at 
 Cincinnati. 
 
 " Probabilities. 
 
 "For New England, winds shifting to northerly and 
 westerly on Wednesday, with falling temperature, rising 
 barometer, and clearing weather, accompanied by occa- 
 sionally light snow. For South Atlantic and Middle 
 States, rising barometer, fresh to brisk westerly to 
 northerly winds, and clear and clearing weather, with 
 falling temperature over latter, and possibly areas of 
 light snow over northern portion. For Gulf States, 
 falling barometer, somewhat higher temperature, south- 
 easterly and southerly winds, and increasing cloudiness. 
 with possibly threatening weather. From Tennessee, 
 northward over Ohio and southern portions of Michigan 
 and Wisconsin, winds shifting to southerly and easterly, 
 rising temperature, cloudy weather, and possibly light 
 rain. For Northern portions of Michigan and Wisconsin, 
 easterly to northerly winds, cloudy weather, and snow. 
 For the North-west, winds shifting to northerly and 
 westerly, with rising barometer, falling temperature, and 
 clearing but partly cloudy weather. A portion of the 
 afternoon telegraphic reports from Minnesota and Dakota 
 are missing. 
 
 " Facts 11 p.m. (following the above ' Probabilities '). 
 
FORECASTING BY SYNOPTIC CHAKTS. 457 
 
 " Wind and Weather. 
 
 "1. Clear. At Augusta, Mobile, and Montgomery, 
 cairn ; San Diego, wind north-east, light ; Memphis, 
 Nashville, Baltimore, Virginia City, and Washington, 
 wind west, light ; Norfolk, Wilmington, Charleston, and 
 Savannah, wind south-west, fresh; New Orleans, wind 
 south-east, fresh ; Denver, wind north-west, fresh ; Corinne, 
 wind north, fresh. 
 
 " 2. Fair. At Keokut and Saint Louis, calm ; Shreve- 
 port, wind south-east, light ; New York, wind south-west, 
 gentle; Philadelphia, wind west, gentle; Milwaukee, 
 wind north-west, gentle ; Lynchburg, wind south-west, 
 fresh ; Cairo and G-alveston, wind south-east, fresh. 
 
 "3. Cloudy. At Burlington, Chicago, and Oswego, 
 calm; Portland, Oreg., wind north-west, light; Daven- 
 port, wind north-east, light ; St. Paul and Sangeen, wind 
 north-east, gentle ; Louisville, wind south-west, gentle ; 
 Leaven worth, wind south-east, gentle ; Kochester, Toledo, 
 and Indianapolis, wind west, fresh ; Cleveland, wind south- 
 west, fresh ; Cincinnati, Stanley, and Toronto, wind north- 
 west, fresh ; Pittsburgh, wind west, brisk ; Breckenridge, 
 wind north-east, brisk. 
 
 " 4. Eainy. At Omaha, calm ; Boston, wind west, light ; 
 New London, wind south-west, gentle ; Cheyenne, wind 
 north, fresh. 
 
 "5. Snowy. At Montreal, wind north-east, gentle; 
 Portland, Me., Kingston, and Quebec, wind north-east, 
 fresh ; Buffalo, wind north, fresh ; Dover and Detroit, 
 wind north-west, fresh. 
 
458 
 
 WEATHER. 
 
 " General Remarks as to Verifications. 
 
 "The above 'Probabilities' were generally verified, 
 except 'from Tennessee, northward over Ohio and 
 southern portions of Michigan and Wisconsin, winds 
 shifting to southerly and easterly, cloudy weather, and 
 possibly light rain,' and ' for the North-west, clearing but 
 partly cloudy weather,' partly verified ; ' from Tennessee, 
 northward over Ohio and southern portions of Michigan 
 and Wisconsin, rising temperature/ and * for North-west,, 
 westerly winds,' not verified." 
 
 The foregoing example will fully illustrate the great 
 advantage which the New England States possess in the 
 ease with which cyclone -depressions can be traced before 
 they reach the eastern seaboard. To this circumstance, 
 and to the energetic management of the Signal Office, we 
 may fairly attribute the high percentage of success which 
 is achieved in the United States. The following table 
 gives the percentages of success both of weather-forecasts 
 generally, and of special storm- warnings : 
 
 -Year. 
 1872 
 1873 
 1874 
 1875 
 1876 
 1877 
 1878 
 1879 
 1880 
 1881 
 1882 
 
 These results, like those of the British Meteorological 
 
 Weather forecasts. Storm warnings. 
 
 Per cent. Per cent. 
 
 ... 76-8 
 
 70 
 
 77-6 
 
 
 
 ... 84-4 
 
 75 
 
 87-4 
 
 76 
 
 ... 88-3 
 
 77-3 
 
 86-2 
 
 78-9 
 
 ... 88-4 
 
 75'9 
 
 90-7 
 
 79-9 
 
 ... 90-3 
 
 83-4 
 
 88-7 
 
 83-3 
 
 ... 88-2 
 
 83-0 
 
FORECASTING BY SYNOPTIC CHARTS. 459 
 
 Office, differ little from year to year, but still show a 
 slow progressive improvement. A few details of the 
 methods employed in the United States Signal Office will 
 be very useful to show the practical conditions of weather- 
 forecasting. They are compiled from the publications of 
 that office. From reading in the morning papers the 
 <( Synopsis and Indications " for the day, no one not 
 initiated in the method of preparing them would suspect 
 the magnitude of the work involved in their elaboration. 
 The study requisite for the tri-daily press reports includes 
 the drafting of seven graphic charts, exhibiting the data 
 furnished by the simultaneous reports telegraphed from 
 all the stations, about seventy-five in number. These 
 charts are 
 
 1. A synoptic chart of pressure, temperature, wind's 
 direction, and velocity, the state of the weather, and the 
 kind and amount of precipitation. 
 
 2. A chart of dew-points at all stations. 
 
 3. A chart of the various cloud-conditions prevailing at 
 the time over the United States. The cloud-areas each 
 form of cloud represented by a different symbol are out- 
 lined, and each one is distinguished. The appearance of 
 the western sky at each station as observed at sunset, 
 which affords a strong indication of the weather to be 
 anticipated for the next twenty-four hours, is also marked 
 in tin's chart. 
 
 4. A chart of normal barometric pressure and of 
 variation of the actual from the normal pressures. 
 
 5. A chart of actual changes of pressure occurring, 
 showing separately the fluctuations of the atmosphere 
 during the previous eight and twenty-four hours. 
 
460 WEATHER. 
 
 6. A chart of normal temperature and the variations 
 of the actual from the normal temperature. 
 
 7. A chart of actual changes of temperature in 
 previous eight and twenty-four hours. 
 
 All these charts have to be made out, and the mass of 
 data which they embody to be sifted and analyzed, 
 preliminary to the preparation of every bulletin. Armed 
 with this charted material, the officer preparing the 
 indications proceeds to compile the " Synopsis and Indi- 
 cations," and issue the necessary storm-warnings. The 
 average time which elapses between the simultaneous 
 reading of the instruments at the separate stations, and 
 the issue of the forecasts, is one hour and forty minutes. 
 
 CANADIAN SUCCESS. 
 
 The following particulars of the success obtained by 
 the Canadian Meteorological Office are also interesting, 
 for they give a percentage almost identical with that of 
 the neighbouring States, though obtained by a different 
 organization. 
 
 o 
 
 Storm- Warnings. 
 The percentage of warnings verified was 
 
 1877 ... ... ... ... ... 69-0 
 
 3878 78-3 
 
 1879 ' 83-0 
 
 1880 82-8 
 
 1881 85-0 
 
 This table, like the others, shows progressive improve- 
 ment. The only year for which we have the results of 
 
FORECASTING BY SYNOPTIC CHARTS. 461 
 
 general weather-forecasts is 1881. Then the general 
 percentage of complete success was 82*3 for the whole 
 Dominion, while the proportion of partial and complete 
 success rose to 90*2 per cent. 
 
 AUSTRALIAN FORECASTS. 
 
 We will conclude this chapter with an example of 
 forecasting in Australia, for which we are indebted to 
 Mr. E. Ellery, of the Melbourne Observatory. This will 
 be a valuable illustration of the universality of the general 
 principles we have already laid down. But, first, let us 
 say a few words on the general character of Australian 
 weather. The weather of that great island continent has, 
 like every other country, peculiarities of its own, sub- 
 servient to the great principles common to all the world. 
 
 The same general distribution of pressure holds good 
 there as elsewhere : a low-pressure zone near the equator ; 
 a sub-tropical belt of anticyclones ; an area of low pressure 
 in the temperate zone, incessantly traversed by an endless 
 series of cyclones. Within this latter area the same seven 
 fundamental forms of isobars are perpetually reproduced ; 
 and the same kind of sky is developed in the equivalent 
 part of each shape of isobars, and the same prognostics 
 hold for good or bad weather, as in the northern hemisphere. 
 Only the sequence of the wind as it veers during the 
 passage of a cyclone is the opposite to that in the opposite 
 hemisphere, because the rotation of the wind round the 
 central vortex is in a contrary direction. For instance, 
 we find the characteristic dirty sky and muggy heat of 
 a cyclone on the right or equatorial front in the northern 
 
462 WEATHER. 
 
 hemisphere, with wind veering from south-east to north- 
 west, while in Australia we find the equivalent weather 
 in the left (there also the equatorial) front of the depres- 
 sion, with wind beginning at north-east and going round 
 to south-west. 
 
 After these explanations, we can readily understand 
 the principles on which the following Australian forecasts 
 were issued by the Government Observatory in Melbourne. 
 Let us look back at Figs. 38 and 39, in which we give the 
 isobars and winds over all Australia on November 20 and 
 21, 1884. 
 
 In the first chart (Fig. 38), we see the southern edge 
 of the equatorial zone marked by the isobar of 29*9 ins. 
 over Northern Australia ; the edge of a great tropical anti- 
 cyclone lies over Queensland; and the fragment of a 
 temperate cyclone covers the great Australian Bight. The 
 wind is light and variable at all the northern stations, 
 but rotates round the cyclone in the usual manner. Now, 
 from the peculiarities of Australian weather, the north-east 
 or north winds in front of a cyclone of such moderate 
 intensity are fine, though sultry, but occasionally a small 
 thunderstorm develops, especially near the trough. The 
 cyclone, as a whole, will certainly move towards the east, 
 and the wind at every station will veer according to the 
 universal rule. 
 
 Hence the following forecasts were issued at 3 p.m. 
 "South" and "North" refer to those portions of the 
 colony of Victoria only, and not to the whole of Australia : 
 
 " South. Fine, sultry weather, with northerly tending 
 to westerly and south-westerly winds, with thunder showers. 
 
 " North. Ditto. Ditto." 
 
FORECASTING BY SYNOPTIC CHARTS. 463 
 
 Now, if we look at Fig. 38, we see that the general 
 anticipations have been fulfilled. The depression has 
 moved towards the east, and the wind in Victoria gone 
 round to west and south-west. But a new anticyclone has 
 made its appearance over Western Australia, the cyclone 
 has increased in depth, and thrown out a V-depression 
 into the col between the two anticyclones. Hence the 
 intensity has increased, and the weather is more unsettled 
 on the second than had been expected on the first day. 
 
INDEX. 
 
 Abercromby, cyclone heat, 214 
 , deductions from barograms, 
 
 393 
 , diurnal variation of weather 
 
 in cyclones and anticyclones, 299 
 
 , monsoon rain, 384 
 
 , on prognostics, 18 
 
 , tropical cyclones, 135 
 
 Anticyclones, 26, 47, 137 
 
 , circulation of, 95 
 
 , definition of, 26 
 
 , dryness, cause of, 138 
 
 , pressure over, 138 
 
 , prognostics, 47 
 
 , shape, 47 
 
 Anticyclone weather, 47 
 
 , antithesis to cyclones, 141 
 
 wind, 47, 95. 
 
 Aspect of slope, 208 
 Audibility, 61 
 
 Augustin, rain at Prague, 303 
 Australian forecasts, 461 
 
 weather, 198 
 
 Avalanches, effect on air, 239 
 
 Salafres, 98 
 Barber, 223 
 Barograms, 151 
 
 , convex or concave, 394 
 
 , deductions from, 393 
 
 Barometer, 392 
 
 , apparent failure of, 399 
 
 Barometer, failure of, 399 
 
 , fine weather with low or 
 
 falling, 414 
 
 , forecasting by single, 393 
 
 in cyclone, 39 
 
 in secondary, 45 
 
 in squalls and thunderstorms, 
 
 236 
 
 , jumping, 164 
 
 , on board ship, 415 
 
 , rain with rising, 401, 403 
 
 , rain with steady, 410 
 
 Barometric anomalies, 166, 401, 
 
 403 
 
 rate, 163, 395 
 
 waves, 167 
 
 Bebber v. temperature on cyclone 
 
 paths, 427 
 Bezold, 245 
 
 Blanford, Calcutta rain, 302 
 , dependence of monsoon rains, 
 
 376 
 
 Blizzards, 223 
 " Boen," 248 
 
 Break in the rains (India), 261, 386 
 Breakers, 373 
 British forecasts, 441 
 
 , percentage of success, 448 
 
 Buchan, wind at sea, 305 
 
 , hot and cold periods, 313 
 
 Bull's-eye, 135 
 Burst of the monsoon, 261 
 2H 
 
466 
 
 INDEX. 
 
 Calm, 183, 194 
 
 , centre of cyclone, 135 
 
 Canadian forecasts, 460 
 
 Cats' tails, 98 
 
 Changes of weather, 50, 158, 294 
 
 , difference from variations of 
 
 weather, 158, 298 
 
 Cirro-cumulus, 103 
 
 (Jirro-filum, 84 
 
 Cirro-nebula, 116 
 
 Cirro-stratus, 100 
 
 Cirro-velum, 101 
 
 Cirrus, 71, 83 
 
 , before barometer, 400 
 
 , dangerous, 9S 
 
 , filature, 86 
 
 , fine weather, 98 
 
 , formation of, 74 
 
 , haze, 116 
 
 , origin of, 100 
 
 , over cyclones and anticyclones, 
 
 93 
 
 , prognostic value, 98 
 
 , radiation of, 88 
 
 Cirrus-stripes, 84 
 
 , lie of, 86 
 
 , , relation to isobars, 92 
 
 , motion, 84, 86 
 
 , origin, 84 
 
 , relation to cyclones and anti- 
 cyclones, 92 
 
 , etriation of, 87, 97 
 
 , vanishing points of, 87 
 
 Clouds, 70 
 
 -, anticyclone, 48 
 
 --, cirro-cumulus, 103 
 
 > cirro-nebula, 1 17 
 
 , cirro-stratus, 100 
 
 , cirrus, 83 
 
 , cumulo-cirrus, 107 
 
 , cumulo-nimbus, 111 
 
 , cumulo-stratus, 108 
 
 , cumulus, 71 
 
 , cyclone, 36 
 
 , diurnal, 299 
 
 , forecasting by, 120 
 
 , forma Vioii at definite levels, 
 
 120 
 
 Clouds, fleecy, 103 
 
 , height of, 119 
 
 , local, 282 
 
 , nimbus, 1 1 1 
 
 , nomenclature, 71 
 
 , perspective, 87 
 
 , prognostics, 70 
 
 , scud, 117 
 
 , secondary in, 42 
 
 , strato-cirrus, 101 
 
 , strato-cumulus, 108 
 
 , stratus, 82 
 
 , striated, 97, 10 L 
 
 , vaults, 252 
 
 , woolly, 103, 114 
 
 , wrack, 117 
 
 , wreaths, 117 
 
 Col, 147 
 
 , definition of, 26 
 
 Cold, 204 
 
 , great, 221 
 
 , in Great Britain, 222 
 
 , sources of, 220 
 
 Cumulo-cirrus, 107 
 Cumulo-nimbus, 111 
 Cumulo-stratus, 108 
 Cumulus, 71, 73 
 
 , degraded, 80 
 
 , festooned, 77 
 
 , high, 82 
 
 , line, 82 
 
 , minor varieties, 81 
 
 , relation to cirrus, 74 
 
 .roll, 111 
 
 , turreted, 82 
 
 Cyclical periods of weather, 319 
 Cyclone, 27, 125 
 
 , axis, 127 
 
 , calm centre, 135 
 
 , central eye, 135 
 
 , circulation of, 93 
 
 , crossing Atlantic, 421 
 
 , definition of, 26 
 
 , double symmetry, 32 
 
 , filling up of, 165 
 
 , front, 29, 38 
 
 . general circulation, 127 
 
 , height of, 134 
 
INDEX. 
 
 467 
 
 Cyclone, intensity, 28 
 
 , names of various portions, 
 
 29 
 
 paths, 419 
 
 , as indicated by strongest 
 
 wind, 426 
 
 , tendency to follow cer- 
 tain tracks, 420 
 
 , influence of surrounding 
 
 temperature, 427 
 
 , pressure over, 138 
 
 , prognostics, 27 
 
 , propagation, 130 
 
 , rain area of, 32 
 
 , influence on propagation, 132 
 
 , revolving, 362 
 
 , rear of, 29 
 
 , sequence of weather in, 39 
 
 , stability, 131 
 
 , temperature, 210 
 
 , influence on path, 133 
 
 trough, 30, 178 
 
 , tropical and extra-tropical, 
 
 135 
 
 , upper currents, 93 
 
 , weather, 31 
 
 , winds, 31, 93 
 
 Dappled sky, 106 
 
 Dependence of seasons, 375 
 
 Depressions, 126 
 
 Descriptive records of weather, 180 
 
 Dew, 51 
 
 Diabletons, 117 
 
 Diurnal isotherms, 204 
 
 Diurnal variation, 51, 293 
 
 , definition of, 51 
 
 , differs for every shape of iso- 
 bars, 299 
 
 , general view of all, 310 
 
 , independence of general 
 
 changes, 294 
 
 , of ctoud, 299 
 
 , of rain in cyclone, 176 
 , of temperature, 210, 291 
 
 , of weather, 50, 293 
 
 , in anticylone, 300 
 
 , in cyclone, 174, 299 
 
 Diurnal variation of weather, differs 
 
 in each shape of isobars, 299 
 Diurnal variation of rain, 301 
 
 , of velocity, 170, 304 
 
 , of wind, 170 
 
 in cyclone, 171 
 
 , of direction, 171, 30 " 
 
 , over sea and laud, 305 
 
 Doldrums, 330 
 , weather in 1 , 330 
 
 Electricity and rain, 113 
 Eurydice squall. 241, 361 
 Eye of storm, 135 
 
 Ferrel, 200 
 
 Festooned cirro-cumulus, 107 
 
 cumulus, 77 
 
 stratus, 83 
 
 Filature, triangle of, 86 
 Fmley, straight line gales, 189 
 , local rains, 261 
 
 , tornadoes, 271 
 Fitzroy, 103, 401 
 Fohn, 219 
 
 Force and velocity of wind, 202 
 Forecasting, 390 
 
 , aids to, 417 
 
 , checking, 449 
 
 , detail possible, 431 
 
 , examples of, 441 
 from clouds, 120 
 
 , how far in advance issued 
 
 432 
 
 , independent of theory, 430 
 
 , nature of problem, 390 
 
 , prognostics by, 391 
 
 , recurrent types and periods, 
 
 , sources of failure, 437 
 
 , synoptic charts by, 416 
 
 temperature, 231 
 
 , time of preparation, 433 
 
 , unequal barometric chancres 
 
 by, 41* 
 
 , when most successful, 435 
 
 Fracto- cumulus, 112, 117 
 France, hail in, 289 
 
468 
 
 INDEX. 
 
 France, thunderstorms in. 248 
 Friction, effect on wind, 201 
 Frost, 57 
 Fuyards, 117 
 
 Gales, 29 
 
 , equinoctial, 314 
 
 , southerly, 345 
 
 , straight line, 1 89 
 
 Germany, forecasts in, 449 
 Globo-cumulus, 78 
 Goat's hair, 98 
 Gradients, temperature, 427 
 
 , wind, 183 
 
 , vertical pressure, 139 
 
 Great cold, 221 
 
 heat, 219 
 
 Grouse, 373 
 
 Hailstorms, localization of, 288 
 Halo, prognostic, 36, 44, 55 
 
 , narrowness of ring, 177 
 
 Hamberg, 305 
 
 Hann, 451 
 
 Hazen, tides and thunderstorms, 
 
 292 
 Heat, 204 
 
 , great, 219 
 
 , primary and secondary effects 
 
 of, 231 
 
 ' . sources of, 217 
 
 Height of clouds, 119 
 Hildebrandson, cloud names, 83, 
 
 103, 111 
 
 , lie of stripes, 92 
 
 , motion of cirrus, 93 
 
 , wind and isobars, 193 
 
 Hinrichs, 244 
 Howard, 71, 82, 111 
 
 , nomenclature of clouds, 71 
 
 Hurricane, 197 
 
 India, monsoons, 259 
 
 , rains, 261 
 
 , temperature, 219, 222 
 
 Indian summer, 316 
 Intensity of weather, 29 
 of a cyclone, 29 
 
 Intensity of secondary, 46 
 
 of type, 371 
 
 Interpretation of meteograms, 170 
 Iowa, squalls in, 244 
 Isobars, 7, 125 
 
 , configuration of, 8 
 
 , origin of, 148 
 
 , relations to wind and weather, 
 
 23, 192 
 
 , seven fundamental shapes, 25 
 
 , what they are, 10 
 
 Isobrontons, 250 
 Isotherms, diurnal, 204 
 , general, 204 
 
 Jump of wind, 41, 145 
 
 Kew, gradients for wind, 186 
 
 Khamsin, 313 
 
 Koppeu, on cloud vaults, 252 
 
 Lammas floods, 315 
 Level of clouds, 120 
 
 of variation, 150 
 
 Ley, 0., 101, 122 
 
 , cirrus stripes, 92 
 
 , cloud names, 78, 84 
 
 , cumulus tops and rain, 112 
 
 , diurnal variation of wind, 305 
 
 , wind and gradients, 187 
 
 Lightning-flash and rain, 114 
 Line squalls, 240 
 
 , cloud vaults in, 252 
 
 , with thunderstorms, 245 
 
 Local variations, 280 
 
 , definition of, 51 
 
 , of cloud, 282 
 
 , of hailstorms, 288 
 
 , of rain, 261, 281 
 
 Loomis, 93, 189, 192 
 Lurid sky, 61 
 
 Mackerel scales, 107 
 
 sky, 107 
 
 Mammato-cumulus, 78 
 Mare's tails, 28, 38, 98 
 Meteograms, 151, 153 
 , interpretation of, 170 
 
INDEX. 
 
 469 
 
 Monsoon, 313 
 
 , north-east, 222, 377 
 
 , , temperature of, 222 
 
 , south-west, 259, 381 
 
 , . burst of, 261 
 
 , , peculiarity of rain, 2GO 
 
 , , temperature of, 219 
 
 Motion of clouds, 89 
 Mountain rain, 287 
 Myths, 3, 181 
 
 Nimbo-pallum, 112 
 
 Nimbo-stratus, 112 
 
 Nimbus, 71 
 
 Noah's ark, 55 
 
 Non-instrumental records, 180, 369 
 
 Non-isobaric rain, 24, 233, 259 
 
 wind, 190 
 
 " Northers " and " Nortes," 189 
 Niibes Memales, 103 
 Nubiculse, 112 
 
 Pamperos, 263 
 
 , clouds in, 264 
 
 , dry, 263 
 
 , relation to line squalls, 266 
 
 , sucios, 263 
 
 , temperature in, 264 
 
 Periodicities, nature of, 317 
 
 Pet day, 57 
 
 Pocky cloud, 78 
 
 Poey, 78, 112 
 
 Poudre snow, 224 
 
 Prague, rainfall of, 302 
 
 Pressure, distribution over the 
 
 globe, 330 
 
 Prognostics, anticyclone, 47 
 , cannot be materially advanced, 
 
 69 
 
 , cyclone, 27 
 
 , early explanations, 17 
 
 , example of failure, 66 
 
 , failure, causes of, 34, 51, 59, 
 
 64 
 
 from damp, 35 
 
 , general theory of, 34, 64 
 
 , modern developments, 64 
 
 , rain, not all from damp, 64 
 
 Prognostics, in straight isobars, 60 
 
 theory of, 64 
 
 in wedges, 54 
 
 use in forecasting, 69, 391 
 
 what they are, 16 
 
 will never be superseded, 69 
 
 with dry air, 56 
 
 Propagation of cyclones, 130 
 
 .Radiation, effect on temperature, 
 224 
 
 weather, 47 
 
 Rain, balls, 78 
 
 , at Calcutta, 302 
 
 , cyclonic, 261 
 
 , diurnal variation, 178, 301 
 
 , local, 261, 284 
 
 , monsoon, 261 
 
 , mountain, 286 
 
 , non-isobaric, 24, 233, 259 
 
 Rain, preceded by different prog- 
 nostics, 68 
 valley, 287 
 
 , with calm, 45 
 
 , with falling barometer, 42 
 
 , with steady barometer, 45, 
 
 400, 410 
 
 with rising barometer, 401, 
 
 403 
 
 , with wind, 42 
 
 Rain prognostics, 33, 43, 56 
 
 , not all from damp, 68 
 
 Recurrent types of weather, 312 
 
 , value in forecasting, 317 
 
 Rainy season, tropics, 386 
 Refraction, 58 
 Ringwood, 196 
 Rothesay, rainfall, 320 
 
 Saint Luke's summer, 316 
 
 Saint Martin's little summer, 316 
 
 Saint Medard, 315 
 
 Saint Swithin, 315 
 
 Scott, equinoctial gales, 314 
 
 Sea-grass, 98 
 
 Seasonal variations, 312 
 
 , definition of, 51 
 
 Seasons, dependence of, 375 
 
470 
 
 INDEX. 
 
 Seasons, rainy, 387 
 
 Secondary cyclone, definition of, 26 
 
 , motion of, 43 
 
 Secondaries, 42 
 
 , thunderstorms in, 44, 254 
 
 , weather in, 43, 254 
 
 , wind in, 43 
 
 Secular variations, 312 
 Showers, tidal, 2yl 
 Simoon, 219 
 Sky, dappled, 106 
 
 , watery, 33 
 
 Snow, 224, 373 
 Soot falling, 61 
 Sources of heat, 217 
 Southerly bursters, 147 
 South-west monsoon, 250 
 Southern hemisphere, winds, 194 
 Spells of weather, 327 
 Sprung, 246 
 Squalls, 37, 233 
 
 , barometer in, 236 
 
 , in Iowa, 244 
 
 , line, 240 
 
 , simple, 234 
 
 , thunder, 235 
 
 Stability of cyclones, 131 
 Storms, what, 29 
 
 crossing Atlantic, 421 
 
 Straight isobars, 59 
 
 , definition of, 27 
 
 , prognostics in, 60 
 
 Strato-cirrus, 101 
 Strato-cumulus, 108 
 Stratus, 71, 82 
 
 , festooned, 83 
 
 Striated clouds, 87 
 
 , origin of, 101 
 
 Stripes of cirrus, 84 
 
 Sun spots and weather, 319 
 
 , value in forecasting, 325 
 
 Superimposition of variations on 
 
 curves, 158 
 Surge, 166 
 Synoptic charts, 7 
 
 , construction of, 19 
 
 , how developed prognostics, 
 
 65 
 
 Synoptic charts, forecasting by 
 means of, 416 
 
 Temperate zones, weather in, 333 
 
 , types of pressure in, 334 
 
 Temperature, changes, examples of, 
 226 
 
 , disturbance of cyclone, 213 
 
 , diurnal variation of, 210, 2U4 
 
 , forecasting, 230 
 
 , mean diurnal range, 296 
 
 Theoretical meteorology, 11 
 
 Thermal slope, 205, 20y 
 
 , aspect of, 208 
 
 Therm ograms, 151 
 
 Thunder coming against wind, 256 
 
 Thunder heads, 81 
 
 Thunderstorms, 233 
 
 , barometer in, 236 
 
 , conditions of, 255 
 
 , dependence on damp, 258 
 
 , independence of isobars, 251 
 
 , in France, 249 
 
 , in secondaries, 44, 254 
 
 , frequency in different coun- 
 tries, 257 
 
 , shape of, 245 
 
 , tidal influence, 292 
 
 , tracks of, 250 
 
 with line squalls, 245 
 
 with V-depressions, 245 
 
 Tidal showers, 291 
 
 thunderstorms, 292 
 
 wind, 291 
 
 Tides, irregular, 373 
 
 Tornadoes, 263, 267 
 
 , cloud, 269 
 
 , descriptions of, 274 
 
 , Fiuley on, 271 
 
 , relation to cyclones and V's, 
 
 271, 277 
 
 , smokiness of, 269 
 
 , wind in, 269 
 
 Trade winds, nature of, 332, 349 
 
 , weather in, 331 
 
 Trough of cyclone, 30, 178 
 
 , relation to velocity, 136 
 
 of V-depression, 144 
 
INDEX. 
 
 471 
 
 Types of weather, 327 
 
 , change of, 353, 373 
 
 , dependence of, 375 
 
 , easterly, 363 
 
 , fluctuation of, 353, 372 
 
 , intensity, 371 
 
 , northerly, 357 
 
 , persistence, 372 
 
 , recurrence of, 312> 375 
 
 , southerly, 335 
 
 , westerly. 347 
 
 United States, forecasts, 453, 
 , tornadoes, 267 
 
 Valley rain. 287 
 
 Variations, of weather generally, 
 298 
 
 , cyclical, 319 
 
 , diurnal, 170, 293 
 
 , in velocity and gradient, 187 
 
 , local, 280 
 
 , seasonal, 312 
 
 , secular or cyclical, 312 
 
 Veering of wind, 41 
 
 with sun, 52 
 Velocity of wind, 186 
 Vertical succession of air currents, 
 
 93,95 
 Visibility, 55, 57 
 
 with overcast sky, 61 
 
 V-point of cloud motion, 90 
 V-shaped depressions, 143, 240 
 
 in Australia, 199 
 
 , definition of, 26 
 
 , two kinds of, 144 
 
 Vortices, 130 
 
 Waves, barometric, 167 
 
 Weather, anticyclone, 47 
 
 , bad, with rising barometer, 
 
 56, 401 
 
 , Beaufort's notation, 19 
 - changes, 50, 158, 294 
 
 , cols in, 147 
 
 cyclone, 31 
 
 , dependence of, 375 
 
 , diurnal variation of, 295 
 
 Weather, fine, with low or falling 
 barometer, 414 
 
 in the Doldrums, 330 
 
 in Temperate zones, 333 
 
 in Trade winds, 331 
 
 -, intensity, 29, 371 
 
 , local variation of, 280 
 
 myths, 3 
 
 , ordinary and storms, 29- 
 
 prognostics, 4 
 
 , radiation, 48 
 
 , recurrence of, 375 
 
 , secondaries in, 43 
 
 , spells of, 327 
 
 , straight isobar, 60 
 
 statistics, 5 
 
 types, 327 
 
 , V-depressions, 144 
 
 variations, 51 
 
 , cyclical, 319 
 
 , diurnal, 293 
 
 , local, 280 
 
 . , seasonal, 312 
 
 , secular, 312 
 Wedge-shaped isobars, 53 
 
 , definition of, 26 
 
 , prognostics, 54 
 
 , weather, 54 
 
 , wind, 54 
 
 Weilbach, 82, 83, 103, 112 
 Whipple, 186 
 Whirlwinds, 263, 267 
 Wind, 183 
 
 , anticyclone, 47 
 
 , barber, 223 
 
 , backing, 41 
 
 , Beaufort's scale, 21 
 
 , blizzard, 223 
 
 , cyclone, 31 
 
 , diurnal variation of velocitv, 
 
 170, 304 
 
 , , direction, 173, 306 
 
 . direction, relation to gradient, 
 
 191 
 
 , diurnal variation, 173 
 
 force, 202 
 
 gradients, 183 
 
 , relation to velocity, 186 
 
472 
 
 INDEX. 
 
 Wind gradients, relation to direc- 
 tion, 191 
 
 , hauling, 41 
 
 , inclination to isobars, 192 
 
 , keeping down rain, 63 
 
 .jumping, 41, 145 
 
 , non-isobaric, 190 
 
 , relation to velocity cyclone, 
 
 200 
 
 , rotation in cyclone, 31 
 
 Wind, secondary, in, 43 
 
 , sequence in cyclone, 39, 41 
 
 , theory of, 200 
 
 , veering, 41 
 
 velocity, diurnal variation, 
 
 170 
 
 , relation to gradient, 186 
 
 , relation to force, 202 
 
 | Wrack, 117 
 
 I Wreaths of cloud, 117 
 
 CNIVEKSITY JJ 
 
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 KEGAN PAUL, TRENCH & CO.'S 
 
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 SHAKSPERE'S WORKS. 
 
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 AN INDEX TO THE WORKS OF SHAKSPERE. 
 
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 SPECIMEN OF TYPE. 
 
 4 THE MERCHANT OF VENICE ACT I 
 
 Salar. My wind, cooling my broth, 
 
 Would blow me to an ague, when I thought 
 What harm a wind too great might do at sea. 
 I should not see the sandy hour-glass run 
 But I should think of shallows and of flats, 
 And see my wealthy Andrew, dock'd in sand, 
 Vailing her high-top lower than her ribs 
 To kiss her burial. Should I go to church 
 And see the holy edifice of stone, 
 And not bethink me straight of dangerous rocks, 
 Which touching but my gentle vessel's side, 
 Would scatter all her spices on the stream, 
 Enrobe the roaring waters with my silks, 
 And, in a word, but even now worth this, 
 And now worth nothing ? Shall I have the thought 
 To think on this, and shall I lack the thought 
 That such a thing bechanc'd would make me sad ? 
 But tell not me : I know Antonio 
 Is sad to think upon his merchandise. 
 
 Ant. Believe me, no : I thank my fortune for it, 
 My ventures are not in one bottom trusted, 
 Nor to one place ; nor is my whole estate 
 Upon the fortune of this present year : 
 Therefore my merchandise makes me not sad. 
 
 Salar. Why, then you are in love. 
 
 Ant. Fie, fie ! 
 
 Salar. Not in love neither ? Then let us say you 
 
 are sad, 
 
 Because you are not merry ; and 'twere as easy 
 For you to laugh, and leap, and say you are merry, 
 Because yow are not sad. Now, by two-headed 
 
 Janus, 
 
 Nature hath fram'd strange fellows in her time : 
 Some that will evermore peep through their eyes 
 And laugh like parrots at a bag- piper ; 
 And other of such vinegar aspect 
 
 LONDON : ?KEGAN .PAUL, TRENCH Co., i, PATERNOSTER SQUARE,' 
 
AN INITIAL FINE OF 25 CENTS 
 
 WILL BE ASSESSED FOR FAILURE TO RETURN 
 THIS BOOK ON THE DATE DUE. THE PENALTY 
 WILL INCREASE TO SO CENTS ON THE FOURTH 
 DAY AND TO $1.OO ON THE SEVENTH DAY 
 OVERDUE. 
 
 1933 
 
 CO 
 
 Ct 
 
 
 LD21-100n-7,'33 
 
AS