UW NKLr METEOROLOGICAL GLOSSARY FFICE. (Fourth Issue) In continuation of The Weather Map, (M.O. 225 i). fcg tf)c authority of tfje iJHetforologiral Committrr. LONDON: N-i ra ' " ins MA.)! - ; i . LIMITED, J. AND .'JIOM | M OEOLOGHCAL 1018, ' -. m*i. M.O. 225 ii. METEOROLOGICAL OFFICE. METEOROLOGICAL GLOSSARY (Fourth Issue) In continuation of The Weather Map, (M.O. 225!,) tg ifje &tttf)orttg of tije jj&eteoro logical Committee. LONDON : PBINTED UNDER THE AUTHORITY OF HIS MAJESTY'S STATIONERY OFFICE Bi DARLING AND SON, LIMITED, ^ACON STREET. E.2. AND TO BE PURCHASED FROM TH-E METEOROLOGICAL OFFICE, EXHIBITION ROAD, LONDON, S.WJ, Price Is. net. For units or systems of units.* F.P.S. is equivalent to foot-pound-second system of units. C.G.S ,, centimetre-gramme-second system of units. B.T.U. ., British Thermal units. .The following are also used : For the expression of Abbrevia- tion. Meaning. Angle O 1 II Degrees, minutes, seconds of arc. Density g/m 3 g/cc Ib/cu. ft. Grammes per cubic metre. Grammes per cubic centimetre. Pounds per cubic foot. Length mm cm m k Millimetre. Centimetre. Metre. Kilometre. Mass ... & Gramme. Kilogramme. Pressure mb cb Millibar. Centibar. Temperature a C F Absolute scale of temperature.* Centigrade scale of temperature. Fahrenheit scale of temperature. Velocity . rn/s mi/hr Metres per second. Miles per hour. Volume cc m 3 Cubic centimetre. Cubic metre. * The abbreviation "a" is al^o us'-vi '-o represent a unit or degree of temperature en the absolute scale. Fov aa iacerval of temperature la is the same as 1 0. METEOROLOGICAL GLOSSARY. TABLE OF CONTENTS. The entries marked (I) are also referred to in The Weather Map. (M.U. 225 i.) Page. Page. Absolute Extremes ... 12 Aneroid barometer .. 30 Absolute Humidity ... 290 Aneroidograph 30 Absolute Temperature 12 Anthelion 30 Accumulated Temper- 293 Anticyclone (I) 30 ature. Aqueous-vapour 31 Actinometer ... 14 Atmosphere (I) 33 Adiabatic 15 Atmospheric Electri- 294 Aerology ... 16 city. Aeroplane Weather ... 16 Audibility 33 Air(i) 19 Aureole 36 Air-Meter 20 Aurora... 298 Airship- Weather 20 Autumn 36 Altimeter 27 Average 37 Altitude 28 Azimuth 37 Alto-cumulus ... 28 Alto-stratus 28 Backing 37 Anabatic 29 Ballon sonde ... 39 Anemobiagraph 29 Balloon Kite ... 42 Anemogram .,. 29 Bar 43 Anemograph ... 29 Barogram 43 Anemometer ... 29 Barograph 43 Anemoscope ... 29 Barometer 43 (13204r 12.) Wt. 26779464. 7000. 3/18. D & S. (S.) Gr. 3. 423582 Glossary. I 'age. i Page. Barometric Tendency 44 Condensation ... ... 70 Beaufort Notation (J ) 44 Conduction ... 71 Beaufort Scale (1) 45 Contingency ... ... 302 Bishop's Ring ... 46 Convection ... 71 Blizzard 46 Corona ... ... ... 71 Blue of the Sky 46 Correction , ... 12 Boiling Points 300 Correlation ... 74 Bora 47 Correlation Ratio ... 302 Breeze ... 48 Cosecant ... 76 Brontometer ... 49 Cosine ... ... 76 Buoyancy 49 Cotangent ... 76 Buys Ballot's Law (I) 57 Counter Sun ... ... 76 Cumulo-stratus ... 77 O.G.S 57 Cumulus ... 77 Calm 57 Cyclone (I) ... 77 Calorie... 301 Cyclostrophic ... ... 77 Celsius ... 57 Centibar 58 Damp Air ... 77 Centigrade 58 Day Breeze ... 77 Centimetre 58 Debacle - ... 77 Cirro-cumulus 58 Dekad ... 78 Cirro-stratus ... 58 Density... ... 78 Cirrus ..- 58 Depression ... 82 Climate 58 Dew ... 82 Climatic Chart 59 Dew-point ... 82 Climatology 60 Diathermancy ... 82 Clouds 60 Diffraction ... 83 Cloud Burst 67 Diffusion ... 84 Clouds, Weight of .. 67 Diurnal ... *5 Col 68 Doldrums ... 88 Compass 68 Drought ... 88 Component 69 Dry Air ... 88 Table of Contents. Page, i Page. Dry Bulb ... 88 Forecast ... 117 Duration of Rainfall ... 303 Freezing ... 118 Dust ... 304 Frequency ... 118 Dust-counter ... ... 305 Friction ... 122 Dynamics ... 89 Frost ... 123 Dynamic* Cooling ... 89 Gale ... ... 124 Earth Thermometer ... 89 Gale Warning... ... 128 Eddy ... 89 Gas ... 129 Electrification of Water- 337 Geostrophic ... 129 drops. Glacier ... ... 307 Electrometer ... ... 91 Glazed Frost ... ... 129 Energy ... 9* Glory ... ... 130 Entropy ... 94 Gradient ... 130 Equation of Time ... 100 Gradient Wind ... 134 Equator ... 100 Gramme ... 138 Equatorial ... 101 Grass Temperature ... 139 Equilibrium ... ... 102 Gravity ... 308 Equinox ... 103 Great Circle ... ... 139 Error ... ... 104 Gulf Stream ... ... 139 Evaporation ... ... 106 Gust ... 140 Expansion ... 109 Gustiness ... 142 Exposure ... 109 Extremes ... 110 Hail ... 142 Halo ... 143 Fahrenheit ... 110 Harmattaii ... 145 Fall ... Ill Harmonic Analysis ... 311 False Cirrus ... ... 306 Haze ... 145 Fluid ... Ill Heat ... 145 Fog ... 112 High (I) ... 152 Fog Bow ... 116 Hoar Frost ... 152 Fohn ... 115 Horizontal ... 152 (jrlossary. Page. | Page. Horse Latitudes ... 153 I Katabatic 182 Humidity (I) ... 154 i Khamsin 182 Hurricane ... 155 Kilometre 183 Hydrometer . . . ... 158 Lake 183 Hydrosphere ... ... 158 Land Breeze 1*3 Hyetograph ... 158 Lapse ... 183 Hygrograph ... ... 158 Lenticular 185 Hygrometer ... ... 159 Level ... 185 Hygroscope ... 159 Lightning 186 Hypsometer ... ... 160 Protection against 324 Line Squall 188 Ice ... . 321 Liquid 189 Iceberg ... 161 Low (I) 189 Incandescence ... 161 Lunar ... 169 Index ... ... 162 Index Error ... ... 162 Mackerel Sky ... 190 Insolation ... 162 Magnetic Needle 190 Inversion ... 163 Magnetism 327 Ion ... 164 Mammato-Cumulus ... 190 lonisation ... 322 Mures 1 Tails ... 191 Iridescence ... 166 Maximum 191 Irisation ... 166 Mean ... 191 Isabnormals ... ... J66 Meniscus 191 Isanomalies ... 166 Mercury 192 Isentropic ... 167 Meteor ... 192 Iso ... 168 Meteorograph ... 193 Isobars ... ... 168 Meteorology ... 193 Isohels 1 Metre ... 193 Isohyets 1 g Tcsn Microbarograph 193 Isopleths [ k JLbU. Millibar 194 Isotherm J Millimetre 195 Isothermal . 181 Minimum 195 Table of Contents. Page. Page. Mirage ... ... 195 Pocky cloud 212 Mist ... 196 Polar 212 Mistral ... .., 197 Pole 212 Mock San ...'197 Potential 213 Mock Sun Ring ... 197 Potential Temperature 213 Monsoon ... 197 Precipitation 329 Moon ... 198 Pressure ... ... 213 Prevailing winds ... 213 Nadir . 198 Probability 214 Nephoscope ... 198 Prognostics ... ... 215 Nimbus ... 198 Psychrometer 216 Normal ... 198 Pumpimr 216 Purple Light ... ... 217 Observer ... 203 Pyrheliometer ... 217 Ombrometer ... ... 203 Orientation ... 203 Radiation 330 Orographic Rain ... 203 Rain ... 217 Ozone ... 204 Hainband . ... 218 Rainbow .. ' ... 218 Rainday 219 Pampero . . . 204 Raindrops, size of, &c. 334 Paranthelion . . ... 204 Rainfall ., ... 219 Paraselenae . . ... 204 Rainfall, duration of . . . 303 Parhelia , . ... 204 Raingauge 219 Pentad ... 205 Rain-spell ... ... 219 Periodical ... 205 Reaumur 219 Persistence . . ... 206 Reduction 220 Persistent Rain ... 206 Reduction to Sea Level 220 Phases of the Moon ... 208 Refraction 222 Phenology ... 210 Registering balloon ... 223 Pilot-balloon ... ... 210 Regression-equation ... 339 Pluviograph ... ... 211 Relative humidity ... 223 Pluviometer ... . 212 Reversal 224 Glossary. Page. Page. Ridge 225 Solar Radiation Ther- 238 Rime ... 225 mometer. River 225 Solstice ... 238 Roaring Forties 226 Sounding ... 238 Spells of Weather ... 238 Spring ... ... 239 St. Elmo's Fire 226 Squall .,. 239 Saturation 226 Stability ... 239 Scotch Mist 340 Standard Time ... 239 Screen 226 State of the Sky ... 239 Scud 226 Statics .. 240 Sea-breeze 227 Station ... 240 Sea-level 227 Statoscope ... 240 Seasons 227 Storm ... ... 241 Secant ... 231 Storm Cone ... 241 Secondary 231 Strato-cumulus ... 241 Seismograph ... 231 Stratosphere ... ... 241 Serein ... 231 Stratus ... 241 Shamal 231 Summer ... 241 Shepherd of Banbury 232 Sun ... 241 Silver Thaw 235 Sun-dial ... 343 Simoon 235 Sun-dogs ... 242 Sine 235 Sun Pillar ... 242 Sine curve 236 Sunset Colours ... 242 Sirocco 237 Sunshine ... 242 Sleet 237 Sunshine-recorder ... 243 Snow ... 237 Sun spot Numbers ... 243 Snow crystals 237 Surge ... 244 Soft hail 343 Synoptic ... 244 Solar Constant 237 Solar Day 238 Tangent ... 244 Solarisation 238 Temperature (I) ... 244 Table of Contents. Page. Page. Temperature Gradient 246 Vapour Tension ... 266 Tension of Vapour .... 247 Vector ... ... 266 Terrestrial 247 Veering ... 267 Terrestrial Magnetism 327 Velocity ... 267 Thaw 247 Vernier ... 268 Thermodynamics ... 247 Vertical ... 268 Therm ogram ... ... 247 Viscosity .,, 269 Thermograph 247 Visibility ... 269 Thermometer 248 Vortex ... 347 Thunder 248 Thunderstorm ... 249 Time ... 252 Water ... 271 Tornado 252 Water-Atmosphere ... 273 Torricelli 253 Waterspout ... 274 Trade Winds 253 Water-Vapour ... 274 Trajectory ... ... 262 Waves ... ... 274 Tramontana 263 Waves of Explosion ... 276 Transparency ... ... 263 Weather (I) ... ... 276 Tropic ... ... ... 263 Weather maxim ... 276 Tropical 263 Wedge ... 279 Tropopause ... ... 263 Weight ... 279 Troposphere 263 Wet Bulb ... 279 Trough 263 Whirlwind ... 280 Twilight 344 Wind ... 280 Twilight Arch ... 264 Wind Rose ... 351 Type 264 Winter ... ... 288 Typhoon 265 Wireless Telegraphy ... 288 Upbank Thaw 265 Zenith ... ...288 V-Shaped depression... 266 Zodiac ... ... 289 Vapour Pressure ... 266 j Zodiacal Light ... 289 10 TABLE OF CONTENTS OF THE WEATHER MAP. AN INTRODUCTION TO THE GLOSSARY. (Now issued as a separate volume.) PAGE. Meteorology and Military Operations 3 Weather Records nd Climate 5 The Necf ssity for Forecasts of Weather ... 7 Modern Meteorology the Work of an Organisation, not of an Individual ... 8 The Meteorologist at Headquarters ... ... ... ... 8 A Map of the Weather ... 9 The Beaufort Notation ... .., 10 A Map of the Winds 12 The Beaufort Scale 13-14 The Atmosphere . , 15 Water Vapour : Evaporation and Condensation .,. ... 17 Temperature and Humidity , 19 A Map of Temperature Pressure and Its Measurement 21 The Barometer 23 A Map of the Distribution of Pressure Isobars ... ... ... ... ... ... ... ... 25 A Map of all the Elements together 26 Lessons from Weather Maps 26 Buys-Ballot's Law and the General Relation of Pressure to Wind .., 26 Weather and Temperature 30 The Sequence of Weather 31 The Travel of the Centres of Cyclonic Depressions ... 35 Barometric Tendency ... ... ... ... ... ... 41 Veering and Backing of Wind 42 Types of Pressure Distribution . 43 The Upper Air. The Dynamics and Physics of the Atmosphere ... ... ... ... ... ... ... 44 Distribution of Rain and Cloud in Cyclonic Depressions... 50 Climatic Supplement, Charts and Diagrams ... 55 11 METEOROLOGICAL GLOSSARY. CONTAINING INFORMATION IN EXPLANATION OF TECHNICAL METEOROLOGICAL TERMS. Details as to the use of meteorological instruments are given in' the Observers Handbook, and as to the numerical computations in the Computer's Handbook, to both of which reference is made in the Glossary when required. In accordance with the practice of the Oxford Diction- ary the initial word of each article is in black type, and words in the body of the text are printed in small capitals when they are the subjects of articles in another part of the glossary. For the articles in this glossary I am principally indebted to the Staff of the Observatory at Benson, W. H. Dines, F.R.S., and E. V. Newnham, B.Sc. ; and of the Branch Office at South Farnborough, Captain C. J. P. Cave, R.E., and R. A. Watson Watt, B.Sc., with Major Taylor, Professor of Meteorology, R.F.C. ; and to the staff of the Forecast Division, especially F. J. Brodie, E. L. Hawke, B.A., and Second Lieutenant T. Harris, R.E., who have passed the MS. through the press, and W. Hayes who prepared many of the illustrations. The revision of the work for the present fourth issue has been carried out by Dr. C. Chree, F.R.S., Superintendent of the Observatory, Richmond. Some new articles have been added at the end of the volume, pp. 290-354. NAPIER SHAW. Meteorological Office, 26 October, 1917. 12 METEOROLOGICAL GLOSSARY. Absolute Extremes. The word extreme is often used with reference to temperature to denote the highest and lowest temperatures recorded at an observing station in the course of time. As the observations are generally summarised for a year, extreme temperatures come to mean the highest and lowest temperatures of a year. When the survey is taken over a longer period, 10 years, 20 years, 35 years or 200 years, according to the duration of the observations, the highest and lowest temperatures observed during the whole period are called the absolute extremes. The journalistic expression would be u the record," high or low as the case may be. The absolute extremes for the British Isles are, highest 310'8a., (100 F.) ; lowest 245'8a., (-17 F.). Those for Belgium, highest 311'2a., (]QO-8 F.) ; lowest 243-2a., (-21-6 F.). The Surface of the Globe, highest 329-7a., (134 F.) ; lowest 203'2a., (-93-6 F.). In the Upper Air, lowest 182'Ja., (- 131'6 F.) at a height of 16^ km. over Java. Absolute Temperature. The temperature of the centigrade thermometer, increased by 273, more properly called the temperature on the absolute, or thermodynamic scale. The absolute scale is formulated by reasoning about the production of mechanical work at the expense of heat (which is the special province of the science of thermodynamics, see ENTROPY), but for practical purposes the scale may be taken as identical with that based on the change of volume and pressure of one of the per- manent gases with l^eat. For thermometric purposes aiming at the highest degree of refinement the hydrogen scale is used, but for the purposes of meteorological Absolute Temperature. Ui reckoning the differences of behaviour of the permanent gases, Hydrogen, Oxygen, Nitrogen are unimportant. In physical* calculations for meteorological purposes the absolute is ihe natural scale ; the densities of air at any two temperatures on the absolute scale are inversely proportional to the temperatures. .Thus the common formula for a gas, P _ ' Po ^ P (273 + " PO C^ 73 + *o) where p is the pressure, p the density and t the tempera- ture Centigrade of the gas at one time, p Q , p , to the corre- sponding values at another, becomes JL ffo where T and Jo are the temperatures on the absolute centi- grade scale. Its most important feature for practical meteor- ology is that from its definition there can be no negative temperatures. The zero of the absolute scale is the tempera- ture afc which all that we call heat would have been spent. In the centigrade scale all temperatures below the freezing point of water have to be prefixed by the negative sign . This is very inconvenient, especially for recording obser- vations in the upper air, which never gives tsmperatures above the freezing point in our latitudes at much above 4 kilometres (13,000 feet), and often gives temperatures below the freezing point nearer the surface. The absolute temperature comes into meteorology in other wavs ; for example, the rate at which heat goes out into space from the earth depends, according to Stefan's Law, upon the fourth power of the absolute temperature of the radiant substance. See RADIATION. 14 Glossary, Temperatures can also be m expressed in an absolute scale of Fahrenheit degrees of 'which the zero is approxi- mately 45',' below the Fahrenheit zero. Some common temperatures on the absolute scales and their equivalents in Centigrade and Fahrenheit are : Centigrade. Fahrenheit. af. The boiling point of 4 269 -452-2 7-2 helium. The boiling point of 77 -196 -320-8 138-6 nitrogen. The freezing point of 234-2 -38-8 -37-8 421-6 mercury. The freezing point of 273 32 491-4 water. The mean temperature 282-7 9'7 49'5 508-9 of London. " Temperate " as shown 285-8 12-8 55 5H'4 on an ordinary ther- mometer. The best temperature 290 17 62-6 522-0 for a living 1 room. A hot summer day 300 27 80-6 540-0 The temperature of the 3io 37 98-6 558'o human body. The temperature of the 6,000 10.000 Sun. Actinometer. An instrument for measuring the intensity of RADIATION received from the sun. In Michelson's Actinometer, for example, the essential element consists of two strips of different metals fixed together. These are heated by the solar radiation which they absorb, and the amount of bending which results Actinometer. 15 from their unequal expansion is a measure of the rate at which they are receiving radiant energy. AdiabatiC The word which is applied in the science of thermodynamics to the corresponding changes which may take place in the pressure and density of a substance when no heat can be communicated to it or withdrawn from it. In ordinary life we are accustomed to consider that when the temperature of a body rises it is because it takes in heat from a fire, from the sun or from some other source, but in the science of thermodynamics it is found to be best to consider the changes which occur when a sub- stance is compressed or expanded without any possibility of heat getting to it or away from it. In the atmosphere such a state of things is practically realised in the interior of a mass of air which is rising to a position of lower pres- sure, or sinking to one of higher pressure. There is, in consequence, a change of temperature which is called mechanical or dynamical, and which must be regarded as one of the most vital of meteorological phenomena because it accounts largely for the formation and disappearance of cloud, and probably for the whole of our rainfall. Tyndall' illustrated the change of temperature due to sudden compression by pushing in the piston of a closed glass syringe and thus igniting a piece of tinder in the syringe. The heafcing of a bicycle pump is a common experience due to the same cause.* On the other hand the refrigeration of air is, often obtained simply by ex- pansion, particularly in the free atmosphere. * Dangerous heating- may result on firing a gun from the sudden compression of gas within the bursting charge of the shell if there are cavities in the explosive therein. 16 Glossary. To plan out the changes of temperature of a substance under compression and rarefaction alone, we have to suppose the substance enclosed in a case impermeable to heat the word adiabatic has been coined to denote impermeable to heat in that sense. The changes of temperature thereby produced are very great, for example : For adiabatic change of pressure decreasing 1 from 1000 nib. by The fall of temperature from 29oa, 62-6 F.. is nib. in. 254-3 Aerology. The study of the free air; a word that has come into use recently to indicate that part of meteorology which is concerned with the study of the upper air. Some of the results are given under BALLON-SO.NDE and PILOT-BALLOON. Aeroplane weather. The weather most suitable for aeroplanes is calm clear weather with little or no wind. The only conditions which make it impossible for a good pilot to fly a modern aeroplane are a strong To face p. 16. Diagram showing the pressure in the upper air corresponding with the standard pressure (1013-2 mb.) at the surface and adiabatic lines for saturated air referred to height and temperature. (From Neuhoff Smithsonian Miscellaneous Collections, Vol. 51, No. 4, 1910.) The pressure is shown by full lines crossing the diagram, and the adia- batic lines for saturated air by dotted lines. Temperatures are given in the absolute scale. The short full lines between the ground and the level of 1,000 metres show the direction of the adiabatic lines for dry air. Aeroplane Weather. 17 GALE or a FOG. On the other hand many weather con- ditions may prevent useful work from being done by an aeroplane when it is in the air. The weather affects civilian and military flying in quite different ways. When testing aeroplanes, with a view to finding their rate of climb, top speed when flying level, landing speed, or other aerodynamical quantities it is usually necessary to choose a calm day, when eddies, or large ascending or descending currents, or other conditions prejudicial to accurate testing, are unHkely to occur. In flying across country the chief danger is that the engine will stop when the aeroplane is over ground on which it is impossible to land. When the engine has stopped the aeroplane must come down somewhere in- side a circle whose radius is about equal to five times the height of the aeroplane above the ground. An aeroplane flying at a height of one mile will have an area of about 75 square miles in which it may choose its landing ground, while at a height of 2,000 feet on a calm day the machine has less than 12 square miles to choose from. In England it is almost always possible to pick out a possible landing ground in a circle containing 75 square miles ; but it is frequently impossible to do so in an area of 12 square miles. For this reason clouds below 6,000 feet are one of the chief dangers of cross-country flying, and the lower they are the more dangerous they become. In flying under war conditions eddies and vertical currents are almost immaterial provided they are not so violent as to impede observations. On the other hand low clouds make observations over an enemy's lines almost impossible, owing to the accuracy of modern anti-aircraft gnus. Detached clouds iiupede, but do not put a stop to, reconnaissance. On days when the clouds are too low to 20 Glossary : Lithoplate IX. the upper regions, collections of water globules which are called clouds ; in meteorology the dust is regarded as an impurity, and the clouds as an addition to the air, not part of it. The dust, though an impurity, is important, as it makes the formation of cloud and rain possible when- ever the temperature of a mass of air gets below the DEW-POINT. Air-meter. The name given to an apparatus designed to measure the flow of air. It consists of a light wheel with inclined vanes carried by the spokes, and a set -of counting dials to show the number of revolutions of the wheel. Its accuracy can be tested on a whirling table. As generally sold it is the most portable form of ANEMO- METER, its box not being more than four inches each way. But it cannot be used with success by a careless observer. Airship-weather. Favourable weather.- The most favourable conditions for airships are calms or light airs, with good seeing from above, extending over the whole area to be traversed, and persistent for the whole period, say 24 hours. Detached low clouds may be an advantage, but precipitation, whether in the form of rain, snow or hail, would spoil the occasion. The favourable conditions thus defined are characteristic of the central part of an area of high barometric pressure which, in technical language, is called an ANTICYCLONE. Thus, for operating across the North Sea, the primary meteorological conditions will be favourable when the barometer readings at Helder, Yarmouth and Grisnez are higher than those at surround- ing places, because the three places named will then be in the central region of an anticyclone, cf. Fig. 3, p. 25. / During an anticyclone the pressure is, as a rule, above the normal for the place ; thus, pressures above 1,020 AIRSHIP WEATHER. FIG. 1. DISTRIBUTION OF TEMPERATURE, WEATHER, WIND, AND PRESSURE, 6 P.M. 6th SEPTEMBER, 1915. Normal Ttroperature ^^^*^~* ~~ of the Se*tS*pt.*il ISOBARS are drawn for interrals of fire milH- WEATHER. Shown by lh following symbols - bar*. f\ olear sky. f) iky i clouded. bl " rowa the scale 0-12, is iodi- o%Ud by the number of father, l overcatt sky. A rain falling ^ow. A bail* S f OR. Tthwodtr. "K thunderstorm. TEMPERATUR.-OiTen in dfrta Fahrenheit Airship - Weather. 21 millibars (30*1 inches) are generally to be found in anti- cyclones, and this has given rise to the statement that a high barometer in itself indicates favourable conditions for airships and vice versa. This is often, but not always, the case. If one plots the pressure on a map the favour- able area extends outward from the central region where the highest pressures are found until the pressure begins to fall away rapidly, arid then one finds strong winds and, possibly, also rain or snow. An anticyclone is indicated on a map by drawing lines of equal pressure, ISOBARS, which naturally enclose the area of highest pressure. The lines of an anticyclone, generally speaking, run in roughly parallel curves and are easily recognisable on a map, such as the one reproduced here, Fig. 1. The shape is sometimes that of a regular curve, more or less like a circle or an oval, as in this instance, but often it is quite irregular and straggles over a large region. Anywhere near what may be called the top of it, ^.e., the region of highest pressure^ there are calms or light airs. Further away the winds begin to range themselves in circulation round the central region easterly winds on the uouth side, westerly on the north. Further out, as one gets towards the regions of low pres- sure, the winds become brisker, and on the margins of an anticyclone they may be very strong, but on the eastern side they are generally steady, not changeable or squally. It is characteristic of an anticyclone that when it is once set up and well marked it generally lasts two or three days ; sometimes it is persistent for a week or 10 days, occasionally even more. An anticyclone is, in fact, typical of settled weather, and consequently the setting in of a large anticyclone over the area of 22 Glossary : Lithoplate X. operations may be regarded as providing an ample period of favourable weather. An anticyclone in our neighbourhood generally drifts eastward or north-eastward. It has northerly winds on its eastern or front side, so that the setting in of a northerly wind, veering to N.E., generally means that an anticyclone is coming over, and as it will take two or three days at least to pass, the navigator is practically sure of a few days of fair conditions, and while the central region is going over and the wind is slacking down from north-east to calm, and then changing to south or south-west, there is practically certain to be a perfect day, possibly two or three, for operating an airship. All that an airship navigator has to do, therefore, to hit upon a favourable time fo/ a raid is to choose the occasion when there is an anticyclone with its central area over southern England or the Channel, advancing slowly east- ward, as most of them do. He may then reckon on two or three days' favourable weather, and if he watches the map may extend his forecast day by day as he notes the behaviour of the anticyclone. An anticyclone is such a well-recognised creature in meteorological maps that observations from one half of it are sufficient to go upon in forming a judgment as to its existence. Its end comes with the southerly wind that marks its western side, so that a southerly, wind, even a light one, is a warning which no hostile navigator is likely to disregard. In winter it is often foggy in anticyclonic weather, particularly when the anticyclone is going away, and sometimes it is cloudy and gloomy, but there is never heavy rain in the central region, and seldom any rain at all. Unfavourable weather. The most unfavourable weather for hostile operations with airships is cyclonic Plate X. AIRSHIP WEATHER. FlG. 2. DISTRIBUTION OF TEMPERATURE, WEATHER, WIND, AND PRESSURE, 7 A.M. 1st NOVEMBER, 1915. ISOBARS are drawn for intervals of five milli- WEATHER. Shown by the following gym bols : ^. bftrs - C\ clear ky. O k T i clouded. WIND.- Direction Is shown by arrcrws fljing X ;s: . with the wind. dD 8R y ^ clouded. (Jj) sky j clouded. Force, on the scale 0-12, is indi- /Tix overcast sky. A rain falling Umber f I8atheri - Y ow. A hail s fog =uiist, T thunder. 15 thunderstorm. TEMPERATURE. t/iven in degrees Fahrenheit. x f J A irsh ip- Wea ther. 23 weather represented on the map by a region of relatively low barometer round which strong winds circulate. (See figure 2.) It is the opposite of anticyclonic weather. The cyclonic depression passes rapidly across the map and the weather goes through a well-known cycle of phases in the course of twelve to twenty-four hours. A well-developed cyclonic depression makes successful air raiding impossible, partly because of the strength of the wind, which may reach 50, 60 or 70 miles an hour in the upper regions, and still more because of its variability (the wind is gusty and squally and is also liable to regular changes), which may give the ship as much lee-way as traverse. This, in darkness, means losing the course and probably losing the bearings. Besides, there is often heavy rain or snow with cyclonic weather. In the South of England cyclonic weather generally begins with a southerly or south-westerly wind, and as there is frequently a succession of depressions passing along the same track there are successive fallings and risings of the barometer and successive phases of southerly wind, veering to N.W. with the rising barometer, and backing again to S.W. with a falling barometer. Between two successive depressions there is often a day of perfect weather, light, transparent airs and clear skies. An airship commander who started at the right time might use this brilliant interval to make a raid, but with- out extremely expert forecasting, which would require ample telegraphic information, it is too dangerous. He is more likely to wait until the setting in of a northerly or north-easterly wind marks the beginning of an anticyclone. Risky weather. Between these two extremes of easily identifiable weather, favourable or unfavourable, there are a number of conditions which may be called risky, 24 Glossary. periods of slow transition between anticyclonic and cy- clonic, or periods of vague type without marked features. , These require an expert knowledge of meteorology if they are to be dealt with successfully. -So far as we know, hostile aircraft have used special meteorological observa- tions (with pilot balloons) to identify a case in which a strong north-easterly wind, too strong for easy navigation, fell off and became much lighter in the upper air. That is characteristic of easterly and north-easterly winds, but there are exceptions. To make use of the proper occasion in this particular is certainly clever, but it is risky ; because we can only take advantage of such cases when we happen to find them, and we do not know any law of their distribution. In this connexion it may be re- marked that airships are not likely to take the air at night in a strong wind. Not knowing precisely the dis- tribution of air currents, it is impossible to lay a course for an objective across wind, so the objective must be approached ultimately up wind. That means the slowest speed at the most critical point. The most risky weather for an airship is when a cyclonic depression with southerly wind in its front advances rapidly eastward and replaces the light airs in front of it. Light airs, it has been remarked, are characteristic of the central region of an anticyclone, but they are also characteristic of the region in front of an advancing depression. Depressions sometimes advance at a rapid rate, say 25 miles an hour 600 miles a day always in that case from the W 7 est or south-west. In winter the risk which an airship runs depends a good deal upon the position of the centre of the depression. On the northern side of it, or in its rear, there is often snow, in the latter case with strong northerly winds. Airship- Weather. FIGURE 3. 25 Chart of Barometer Reading's Variations of the Barometric pressure at five stations during the passage of an anticyclone in September, 1915. 26 Glossary. FIGURE 4. Chart of Barometer Readings Oct.- Nov., 1915 1020mb Helde-r Yarmouth IQIQmk Holqhead C.GrisNer woo* Skudesnaes Variations of Barometric pressure during the passage of two consecutive cyclonic depressions in October-November, 1915. Airship- Weather. 27 This description is quite typical, and is exactly applic- able to the case of February 17th, 1915, when two airships were lost. Prognostication. It will be understood from what has been written above that forecasting favourable weather for air raids for a few days ahead is an easy matter when an anticyclone establishes itself on the map. It requires only the most elementary knowledge of weather-study. Similarly it is quite easy to recognise a day or two in advance the periods of unfavourable weather. The best way of doing this is to have a daily map upon which the positions of the anticyclone or cyclonic depressions are easily recognised. But if a chart of consecutive barometer readings is preferred, the charting of the readings at Helder, Yarmouth, Holyhead, Grisnez, Skudesnaes may be recommended. Figure 3 shows the chart for the passage of the anticyclone of Figure 1 , and Figure 4 the chart for the passage of the cyclone of Figure 2. Forecasting for the more risky weather is a matter for experts, and entails a careful study of weather maps. What was curious about the late summer and autumn of 1915 was the frequency of anticyclonic periods, and their coincidence with times of new moon. A succession of depressions is generally characteristic of the weather of North-Western Europe, and, consequently, a competent meteorological establishment would naturally lay itself out to catch the occasional opportunity. But this was not at all necessary in the season of 1915. No particular skill in forecasting was required. Altimeter. An aneroid barometer graduated to show height instead of pressure. The most that an aneroid barometer can do is to give a satisfactory measure of the 28 Glossary. pressure of the air. The pressure is very largely affected by the height, and, therefore, whatever indicates the pressure gives a rough indication also of the height. The accurate determination of the height of a position requires a knowledge of the temperature of the air at successive steps, so that a mean temperature may be obtained for the column between the position and the earth. There are various short ways of making estimates of the temperature of the column, but in any case the temperature at the top and bottom should be noted.* Altitude. The angle in a vertical plane subtended at the eye of the observer by the line drawn from the top of an object to the horizon. The word is also used commonly as synonymous with height. AltO-CUmulus. A form of cloud of middle height (10,000 feet to ^5,000 feet). It consists of fleecy groups of cloudlets called by the French " gros-inoutons." The separate cloudlets are thick enough to show a darkening of the white ; the similar groups of smaller and higher clouds, cirro-cumulus, show no shadow. See CLOUDS. AltO-StratUS. A sheet of continuous cloud of middle height, of considerable size and moderate thickness, some- times covering the whole sky. It must be distinguished from cirro-stratus which is higher and thinner, and stratus without any prefix, which is the lowest form of cloud- sheet. * The pressure at the foot of the column must also be known, and the lack of this knowledge is a source of error with a machine travelling over great distances. A special note on the subject prepared for the Handbook of Meteorology can be obtained from the Meteorological Office. Anabatic. 29 Anabatic. Referring to the upward motion of air due to convection. A local wind is called anabatic if it is caused by the convection of heated air ; as, for example, the breeze that blows up valleys when the sun warms the ground. See BREEZE. Anemobiagraph. See ANEMOGRAPH. . Anemogram. The record of an anemograph. Anemograph.. An instrument for recording the velocity or force, and sometimes also the direction of the wind. The best known forms of anemograph are the Robinson Cup anemograph similar to that designed by Beckley for Kew Observatory, the Tube anemograph with direction recorder similar to that designed by Dines for Benson Observatory (which might be called the Harpagraph or gust-recorder), the anemobiagraph designed by Halliweli for Negretti and Zambra, the Dines Tube recorder, with direction recorder designed by Rooker for R. W. Munro. The Royal Observatory at Greenwich and the Observa- tory of the Mersey Docks and Harbour Board, near Liverpool, have pressure-plate anemographs by Osier. Anemometer. An instrument for measuring the velocity or force of the wind. Anemometers register in various ways ; by counting the number of revolutions of cups in a measured time, by the difference of water level in a tube, and in other ways. Information as to the construction and use of various anemometers is given in the Observer's Handbook, Anemoscope. An instrument for indicating the existence of wind and snowing its direction. The one 30 Glossary. best known to the Meteorological Office is that designed by Mr. J. Baxendell which is provided with recording mechanism. It is an observatory -instrument, not a portable one. Aneroid Barometer. An instrument for deter- mining the pressure of the atmosphere. It consists of a shallow air-tight metal box, usually nearly exhausted of air. The distance between opposite faces of the box alters with change in the surrounding atmospheric pressure, the alteration being shown on a dial by a hand actuated by a suitable train of levers. An aneroid is light, portable and convenient, but should be compared occasion- ally with a mercury barometer, as an appreciable change of zero sometimes occurs. It is also subject to "creep," e.g., after a recent large fall of pressure such as may occur when it is used as an ALTIMETER it will, though under a really constant pressure, show a small spurious further fall, which in the course of an hour may amount to 1 or 2 per cent, of the previous fall. u Creep " in the same direction may be perceptible for several hours, but its rate continually diminishes. Aneroidograph.. A self-recording aneroid. An aneroid-barometer provided with mechanism for record- ing the variations of pressure of the atmosphere. See BAROGRAPH. Antlielion. A colourless MOCK SUN (see HALO) appearing at the point of the sky opposite to and at the same ALTITUDE as the sun. Anticyclone. An anticyclone is a region in which the barometric pressure is high, relatively to its surround- ings, and is generally shown on the weather charts by a series of closed isobars, the region of highest pressure Anticyclone. 31 being the central region of the anticyclone. In a well- marked anticyclone the isobars are roughly circular or oval curves, the wind blows spirally outwards in accord- ance with Buys Ballot's Law, and the pressure in the central parts is very seldom under 1,015 millibars or 30-00 inches. See Plate XIII. Certain parts of the earth, notably large parts of the latitude belts of about 30 N. and 30 S., also continental areas in the winter in temperate latitudes, are anti- cyclonic regions. In the Azores-anticyclone in summer the pressure is usually about 1,025 millibars, or 30*25 inches, and in winter about 1,020 millibars, or 30*10 inches, and in the Siberian anticyclone of winter the pressure is often as high as 1,050 millibars, or 31 '00 inches. Anticyclones are characterised by calms and light winds and an absence of rain; the desert regions of the earth are anticyclonic regions. But in the temperate zones, short of gales and strong winds, almost any weather may occasionally occur in an anticyclone. In England they are generally accompanied in winter by dull, cheerless weather and fogs, and in summer by bright, hot weather. The causes of anticyclones are still unknown. We have learnt in recent years that the temperature of the air in them between the heights of 2 and 10 kilometres (1-6 miles) is higher, but at still greater heights lower than its environment. For the anticyclone in relation to weather refer to the Weather Map, and see also AIRSHIP-WEATHER and ISOBARS. Aqueous Vapour.* Aqueous vapour isalways present in the atmosphere, and, although it never represents more . * See also HUMIDITY, p. 154, and ABSOLUTE HUMIDITY, p. 290. 32 Glossary. than a small fraction of the whole, it has physical properties that give it great importance in meteorology. In a closed space wherever there is a free surface of ice or water, evaporation takes place until the water- vapour exerts a definite pressure of saturation, depending only upon the temperature, and not upon the pressure of the surrounding air. This pressure of saturation is very much greater at high than at low temperatures as is shovm in the following table : Temperature. Pressure of Saturation in millibars. Temperature. Pressure of Saturation in millibars. F a. 10 260-8 20 266'3 30 271-9 40 277-5 50 283-0 2-4 3*7 5'8 8-5 I2'2 l? a. 60 288-6 70 294-1 80 299-7 90 305-2 100 310-8 17-6 24-7 34-6 47*8 65-0 A cubic metre of dry air at 1,000 mb. and 289a. weighs l,206g. The mass of water contained in saturated air at different temperatures is given in the following table : Mass in grammes Mass in grammes Temperature. of water vapour per cubic metre Temperature. of water vapour per cubic metre of saturated air. of saturated air. F. a. F. a. 32 273-0 5 70 294-1 18 40 277-5 7 80 2997 25 50 283-0 9 90 305-2 34 60 288-6 13 100 310-8 45 Vapour. 33 It is easily seen from these figures that saturated air must at once yield rain or snow if cooled, and even air that does not contain all the aqueous-vapour possible will ultimately deposit moisture if sufficiently cooled. In the passage from the liquid to the gaseous state great quantities of heat are absorbed, 536 calories for every gramme ^evaporated at the boiling point, and even more if the water is initially cold. Conversely much heat is yielded up when condensation occurs. See p. 70. Tyndall has shown that the heat radiated from a black body at the boiling point of water is readily absorbed by aqueous vapour, which must, therefore, have a correspond- ing power of radiation. Spectrum analysis shows also that some of the visible radiation of the sun is strongly absorbed by the earth's atmosphere. Atmosphere. See Weather Map. M.O. 225 i., p. 15. Atmospheric Electricity. See p. 294. Audibility. The audibility of a sound in the atmos- phere is measured by the distance from its source at which it becomes inaudible. On a perfectly clear, calm day the teound of a man's voice may be heard for several miles, provided there are no obstructions between the source of sound and the listener ; but- quite a small amount of wind will cut down the range of audibility enormously. The sound is not cut down equally in all directions; to leeward, for instance, a sound can usually be heard at a greater distance than it can to windward of the source. This is accounted for by the bending which the sound-rays undergo, owing to the increase in wind-velocity with height above the ground, the rays to leeward of the source being bent downwards while those to windward are bent upwards so that they pass over the head of an 34 Glossary. observer stationed on the ground. The decrease in all directions in the range of audibility of a sound when there is a wind appears to be due chiefly to the dissipation in the energy of sound as it passes through eddying air. A plane wave-front becomes bent in an irregular manner when it passes through air in irregular or eddying movement. It, therefore, ceases to travel uniformly for- ward. Part of its energy is carried forward, while the rest is dissipated laterally. If there were no dissipation of energy in a sound-wave the intensity of the sound would decrease inversely as the square of the distance from the source. Experiments show that, under normal conditions when there is a light wind blowing, the rate of decrease in intensity of sound at a distance of half a mile or more is considerably greater owing to the dissipation of energy than would be expected from the inverse square law. If, for instance, a whistle can be heard at a distance of half a mile, four whistles blown simultaneously should be audible at a distance of a mile ; but the range is actually only increased to about | of a mile. 4 Sounds are usually heard at greater distances during the night than during the day. On calm nights the range of audibility of a sound may be as much as 10 or 20 times as great as it is during the day. This effect is clue partly to the increased sensitiveness of the ear at night owing to the decrease in the amount of accidental disturbing waves, partly to the inversion of temperature which com- monly occurs on calm, clear nights, and has the effect of bending the sound- waves downwards, but chiefly to the diminution of the amount of disturbance in the atmos- phere at night. Between the source of sound and the extreme range of Audibility. 35 audibility areas of silence sometimes appear, in which the sound cannot be heard. This effect has in some cases been attributed to a reversal in the direction of the wind -in the upper layers of the atmosphere. The lower wind would bend the sound rays upwards to windward of the source. On entering the reversed upper wind current these, rays would be bent down to the earth again, and would reach it at a point separated from the source of sound by an area of silence. This explanation is quite adequate in many cases in which the places, where the sound is heard again, are to the windward of the source. There are, however, many cases in which areas of silence appear to leeward of the source, and many others in which an area of silence occurs in the form of a ring enclosing the source and surrounded by an area of distinct audibility. In most of the cases where a ring-shaped area of silence has been observed the outer region of distinct audibility begins at a distance of about 100 miles from the source, and may extend to 150 miles or more. The well-known Silvertown explosion is a good example of a case in which a detached area of audibility was separated from the source of sound by an area of silence. In the accompanying map, which is reproduced by permission from the Quarterly Review, the two areas of audibility are shown. It will be seen that the outer area, which includes Lincoln, Nottingham and Norwich, lies about 100 miles from the source of sound. The inner area sur- rounding the source is not symmetrical, being spread out towards the north-west and south-east. Definite evidence was obtained that no sound was heard at various towns within the area of silence. No very satisfactory explanation of these cases has so far been offered. The wind-distribution necessary to 13204 B 2 36 Glossary. explain them on the wind-refraction theory would be very complicated and would, moreover, in some cases, be of a type which no meteorologist has yet observed. The effect of FOG on the audibility of sound has been the subject of a considerable amount of discussion. The idea that sound is muffled by a fog seems to be commonly accepted ; but on the other hand the experiments of Henry and Tyndall have failed to give any indication of such an effect. They seem rather to show an increase in audibility in a fog. The effect of the waterdrops them- selves has been shown to be too mall to affect the propagation of sound waves to an appreciable extent, while the weather conditions usually associated with the production of fog, the homogeneous state of the atmos- phere and the INVERSION of temperature, are such as to give rise to increased audibility. In calm weather the direction of a hidden source of sound may be estimated to a few degrees by turning the head till the sound appears to come from the point towards which the observer is facing. The .observer, however, is seldom confident that he has attained such accuracy. In windy weather it is more difficult to esti- mate the direction of sound. Aureole. The luminous area surrounding a light seen through a misty atmosphere. Aurora. See p. 298. Autumn. Autumn, in meteorology, comprises the three months of September, October and November, the first three months of the farmer's year. In astronomical text-books it is defined as the period commencing with the autumnal equinox and ending with the winter solstice, i.e., from September 23rd to December 22nd, but Audibility To face p. 36. ^Stfrtv Canter &pry c (Reproduced "by permission from the Quarterly Review. July, 1917.") 38 Glossary. CURVES SHOWING CHANGE OF TEMPERATURE WITH HEIGHT ABOVE SEA- LEVEL OBTAINED FROM BALLON- SONDE ASCENTS 1907-8. V PYRTON HILL 4 PYRTON HILL iPITCHAMWftK pwnw HILL pTDMMMK [MANCHESTER r; toe. MANCHESTER 8 DEC 3, (MANCHESTER' ' SELLACK MANCHESTER PYRTON HiUL WTCHAMfMK MANCHESTER CRiNAN'HARfiS MANCHESTER CRINAN HARB< (&CNNANHUW OITCHAM F*RX JPYRTON HILL pITCHAM PAW (MANCHESTER Jwa. PYRTON HILL . PYRTQN HILL flfae. MANCHESTER IWRTON HILL [MANCHESTER PYRTON HILL MANCHESTER icpTifSELLACK (OITCHAM f 5. OITCHAM R^RH ISELLACK icpre. 01 SOTit PYRTON HIU Z. PYRTON HILL 5. [Oil ISELLACK OITCHAM WRK Z7Ae. PYRTON HILL -40 -EO 2.Q TEMPERATURE IN DEGREES FAHRENHEIT , 3^0 40 250 2^0 -270 fEMPCRATURE ABSOLUTE 300 r The separate curves represent the relation between temperature and height in miles or kilometres in the atmosphere. The numbers marking the separate curves indicate the date of ascent at the various stations as shown in the tabular columns. The difference of height at which the isothermal layer is reached, and the difference of its temperature for different days or for different localities, is also shown on the diagram by the courses of the lines. Brdlon sonde. 39 Ballon SOnde. A small balloon usually made of india- rubber, inflated with hydrogen, and used for carrying self -registering instruments into the free atmosphere and thus obtaining records of the pressure, temperature and humidity aloft. The balloons used in England usually have a diameter of about one metre at starting (40 inches nearly), those on the continent nearly two metres. The balldons generally rise until they burst, on account of the diminished external pressure of the air, which may not happen until they have reached a height of 20 kilometres (12 miles) or more. After the balloon has burst, the material acts as a kind of parachute and breaks the fall of the instruments, so that they reach the ground without injury. A label is attached, offering a reward to the finder for the return of the instrument, and in that way valuable records are secured. Sometimes the balloon fails to burst, but develops pin- holes through which the gas leaks. A long TRAJECTORY is the result. One of our records was returned from a Bavarian forest. In other countries, where much heavier recording instruments are used, two balloons in tandem are em- ployed, one of which bursts, and the other regulates the fall. This mode of arranging the apparatus, with a simple modification, is available for use at sea and many soundings of the air over the sea have been obtained. We have not taken part in that side of the inquiry. Very remarkable facts about the temperature of the free air have been disclosed by soundings with ballons- sondes. Their general characteristics are shown in the diagram on page H8 which exhibits graphs of tempera- ture and height for 45 soundings obtained for the Meteorological Office in 1907-8. The reader should notice that, with one or two exceptions, the balloons reached a 40 Glossary. O O C^ O co t-H CO O co i-i O C M coo <^ co VQ O ^ O v -d- vo CTiOO H-I CT> co n O t- **"> O "3-OO co t^ CO -rf-O O\ coo MOi-it^^ coOO>O-' v-O VOOOOCOO COCOO OO O I *j"> t^ O ir > O i^)O ^ O "st" t^ ) ~ | O O i ' t^- -j- N O O^ O i PL, i_i (_| n i_ i co co cocO'^-'^- vo O t> l~>- O^ O CO co -rj- ^ *~t~> O O !> OO O M 00 co C PH O II s C7\ rh O O co M CO N OO^C^iOO -QOco i-i^O^O^" ' ) rj- rj- vD VO O t> OO O JBallon sonde. H SI H 02 W P to feq O O fe o PQ EH 3 ri ^ g o 02 ^ 2 OQ 3 O -g w S k a g ^? > M > * M "S S P ^ 03 I I I- .CD 3 S r^ - _ f ' ^ to 43' I Qs LO M I>*GO M LOMGO M M 01 ^ GC O COOO VOM * M M OO rt-MGO lOMOO ^ n ^ ro co rj- 10 uox M MSQ COM n M w co rf -Mt^ COCO M \O oo M w \o co w oo voc^co -Nha^coaN C< 01 W CO -t- rt- LO^O vO !>. r^OC GC ^M i>,ir)i-i i>. coco co O co TJ- TJ- LO\O so r^ r^oo GO M iriMGO LOCI OMOMVO C^OO 01 01 CO CO rf IT) LO^O l>s ^00 GO O Tt- OvO H OvO 01 GO CO ON LO ^1 01 01 CO rf rf- LOvO ^O t^ X^GC O 01 \O M OvO M CNJ^ O ^O 01 01 M 01 01 co co *-j- ir) LO\O t^ r^oo OMT^O --t-o r^^ M M 01 01 co co *t- 01 x r^ o M M 01 01 Ol -f-o r^coo rx ro ix H \o M M rj 01 ro co -f- i^i i n M o 42 Glossary* position (somewhere near ten kilometres), after which the temperature ceased to fall, yet the range of temperature at that level for the whole series is larger than the range at the surface. Additional soundings have enabled us to put forward a table of the average pressure and temperature of the free air at different levels in the several months of the year, which is given on pp. 40 and 41. Particulars as to the variation of the meteorological elements, temperature, wind and humidity derived from observations with ballons-sondes, kites and pilot balloons are given in Geophysical Memoirs, No. 5, by Major E. Gold, D.S.O. (published by the Meteorological Office). Balloon Kite. For dealing with the general features of the relation of temperature to pressure or height in the upper air of all parts of the globe the ballon-sonde is most effective. In this general inquiry the details due to the smaller irregularities of diurnal or seasonal variation may be disregarded. Such irregularities are specially noteworthy in the lowest kilometre of the atmosphere and are of importance in aviation and gunnery, because changes in the distribution of temperature are necessarily accompanied by changes in the distribution of pressure, and consequently of wind. The first kilometre or 3,000 feet, therefore, requires special attention. Observations of temperature, humidity and wind can be got by means of kites, when there is wind enough ; by captive balloons, when there is little or no wind ; and by the observation balloon or balloon kite in all weathers, except a gale. Special instruments are required for these observations. A. special form of meteorograph has been designed by Mr. W. H. Dines for use with kites, but suitable provision has still to be made for kite-balloons and captive balloons. Bar. 43 Bar. The unit of atmospheric pressure, being equal to the pressure of one million dynes (one megadyne) per square centimetre. The BAR is equal to the pressure of 29-5306 inches, or 750-076 mm. of mercury at 273a (32F) and in latitude 45. The name was introduced into prac- tical meteorology by V. Bjerknes, and objection has been raised by McAdie of Harvard College on the ground that the name had been previously appropriated by chemists to the C. G. S. unit of pressure, the dyne per square centimetre. The meteorological bar is thus one million chemical bars, and what chemists call a bar we should call a microbar. One bar is 100 CENTIBARS or 1,000 MILLIBARS. See p. 194. Barogram. The continuous record of atmospheric pressure yielded by a self-recording barometer. See p. 156. Barograph. A self-recording barometer, an instru- ment which records automatically the changes of. atmospheric pressure. In one form of mercury barograph the movements of the mercury in a barometer are com- municated by a float to a pen in contact with a moving sheet of paper carried by a revolving drum which is driven by clockwork. The portable barographs which are in common use are arranged to record the variations of pressure shown by an aneroid barometer, and on that account they are some- times referred to as ANEROIDOGRAPHS. Particulars as to the method of using these instruments are given in the Observer's Handbook. (M.O. Publication 191.) Barometer. An instrument for measuring the pres- sure of the atmosphere. The mercury barometer has been found to be the most satisfactory form for general use, The principle underlying this type of instrument 44 Glossary. is quite simple. If a glass tube 3 feet long, closed at one end, is filled with mercury, and the open end is tempor- arily stopped up and immersed in an open vessel of the same liquid, then if the tube is held in a yertical position and the immersed end is re-opened, the mercury will fall until the level inside the tube stands at a height of about 30 inches above the mercury in the trough. The pressure of the atmosphere on the lower mercury-surface balances the tendency of the enclosed column to fall, and the height supported in this way represents the atmospheric pressure at the time. In order to cojnpute the pressure, the length or height of the column of mercury has to be measured. Different mercury barometers vary as regards the method of reading this height, and in all the tem- perature of the mercury and the latitude of the place mast be taken into account. In the aneroid barometer the pressure of the atmosphere causes deformations in a spring inside a closed metallic box, which has been exhausted of air, and these are communi- cated to a pointer moving over a suitably engraved scale. For the purposes of meteorology the pressure of the atmosphere has to be determined to the ten-thousandth part, which is a much higher degree of accuracy than is required in other meteorological measurements. Special contrivances and precautions are therefore required, whicfr are duly set out in the Observer's Handbook. Barometric tendency. The change in the baro- metric pressure within the three hours preceding an observation. See Weather-Map, p. 35. Beaufort notation. A table of letters for weather. See Weather-Map, p. 10. Beaufort Scale. 45 Beaufort Scale. The scale of wind force devised by Admiral Beaufort in 1805. An explanation has been given in the Weather-Ma/), p. 13. Table of Equivalents in Force and Velocity. Pressure of Wind Equiva- | Limits of Velocities. on a Plate lent a velocity & ^ in Ibs. in Milli- in miles f-\ EH Q Statute Miles Nautical ^ , Miles i Metres Feet per square ft. bars ( i o 3 dynes per hour. s 1 per Hour. per H P , S d ' per Second. per cm. 2 ). CQ O Less Less Less Less 1 than i than i than 0*3 than 2 01 'or 2 1 i-3 i-3 o'3-i'S 2-5 08 '04 5 2 4-7 4-6 i -6-3-3 6-1 1 28 13 10 3 -12 7-10 S^-SH 12-18 67 32 15 4 13-18 11-16 5'5-S-o 19-27 I-JI 62 21 * 5 19-24 17-21 8-1-107 28-36 2'3 i-i 27 6 25-31 22-27 10-8-13-8 37-46 3-6 17 35 7 3^-38 28-33 I 3'9- T 7' 1 47-56 5'4 2 ' 6 42 8 39-46 ' 34-4 17-2-207 57-68 77 37 So 9 47-54 41-47 20*8-24-4 69-80 10*5 5-0 59 10 55-63 48-55 24-5-28-4 8i-93 14/0 67 68 11 64-75 56-65 28-5-33-5 94-110 Above : Above Above 12 Above Above 3 3 -6 or Above 17-0 8-1 75 75 65 above. no 46 Glossary. Bishop's ring, so named after its first observer, is a dull reddish-brown ring of about 20 outer radius seen round the sun in a clear sky even at mid-day. That it is a CORONA, and not a HALO, is proved by the fact that at times it has been seen to have a red outer margin (see CORONA). It appeared after the great eruption of Krakatoa in 1883, and remained visible till the spring of 1886, and was no doubt due to. minute particles shot out by the eruption ; these remained suspended in the atmos- phere for a considerable time. The great radius of this corona is explained by the smallness of the particles, and its intensity by their great number. The non-appearance of the other colours of the corona is explained by the presence of particles of many different sizes. Bishop's Ring was seen again after the eruptions of the Souffriere in St. Vincent and Mont Pelee in Martinique in 1902. Blizzard. A gale of wind with the temperature below freezing, the air being filled with fine dry snow. The snow may not be actually descending from the clouds but be merely raised from the snow-covered ground. The fine powdery snow peculiar to these storms is formed only at very low temperatures, so that the phenomenon is practically confined to the polar regions and the large land areas of the temperate zone. During a blizzard the temperature often rises, possibly because the violent wind causes a mixing of the lowest layers of the atmosphere and brings down air that is often warmer than the excessively cold air lyinof near the ground. Blue Of the Sky. Light rays striking particles which are smaller than the wave length of the light are scattered, that is turned aside in all directions. But the short wave's Blue of the Sky. 4? composing the blue and violet end of the spectrum are more completely scattered than the long red and yellow waves. Hence light passing through a medium contain- ing a great number of such particles is left with an excess of red, while light emerging laterally has an excess of blue. It is for this reason that soapy water looks yellow- ish when one looks through it at a source of white light, and bluish when one looks across the direction of illumination. The greater part of the sky appears blue because the light from it consists mainly of light scattered laterally from minute particles in the atmosphere. The smaller the particles the less intense is the light but the greater the proportion of it that is blue. When the particles are larger the proportion of blue is less, as in the whiter sky of a haze. Near the horizon the sky is whiter than at the zenith because the rays of light from that region have passed through a greater thickness of the lower air where large particles are relatively more numerous, Sunset colours are reddish because the rays reaching ue directly have lost much of their blue light by lateral scattering. The sky as seen from high moun- tains and from aeroplanes at a great height is of a deeper but purer blue because there are fewer large particles than at lower altitudes. Boiling* Points. See p. 300. Bora. A cold wind occurring in the northern Adriatic, very violent, which blows from the high plateaus which lie to the northward. These plateaus may become extremely cold in clear winter weather, and passing cyclonic systems allow the air to flow down to lower levels. The actual violence of the wind in a bora is largely due to the weight of the cold air of the plateau causing it to run down the slope like a torrent or cataract 48 Glossary. of water. The wind experienced may therefore be in- dependent of Buys Ballot's Law. The adiabatic warming due to the increased pressure below is riot sufficient to prevent the resulting wind from being cold. In the Meteorological Office it is proposed to call local winds of this character " katabatic " in order to distinguish them from the winds which show the normal relation to the distribution of atmospheric pressure and are called " geostrophic " winds. Breeze. A wind of moderate strength. Glacial-breeze. A cold breeze blowing down the course of a GLACIER, and owing its origin to the cooling of the air in contact with the ice. The movement of the air is due to the gravitation of the air made denser by the cold surfaces on a slope, and a glacial breeze may be classed as a typical example of a katabatic wind. Lake-breeze. A breeze blowing on to the coast of a lake in sunny weather during the middle of the day, part of the convectional circulation induced by the greater heating of the land than of the water. Land-breeze. An off-shore wind occurring at the margin of a sea or lake during a clear night, due to the more rapid cooling of the air over the land than over the water. During the day the conditions are reversed and the wind blows from the sea to the land, constituting a SEA-BREEZE. These phenomena are most marked in the tropics, where the wind arising from other causes is usually not strong enough to mask the convectional effect. See also p. 183. Mountain-breeze. A night breeze blowing down the llrew. 49 valleys, due to the flowing downward of the air chilled by the cold ground. These also, as being due to convection in which the colder air takes the leading part, would be classed as katabatic winds, whereas the one next following in which warmed air plays the leading part would be classed as an u anabatic " wind. Valley -breeze. A day-breeze that blows up valleys when the sun warms the ground. Brontometer, from bronte, a thunderstorm, a com- bination of apparatus for following and noting all the details of the phenomena of weather during a thunder- storm. Buoyancy. Used generally with regard to ships or balloons to indicate the load which a ship could carry without being completely submerged, or the weight which a balloon or airship can carry without sinking. In the case of the balloon or airship the buoyancy is due to the displacement of air by hydrogen which is lighter than air. A cubic metre of perfectly dry air at 1000 mb. and 273 a. weighs 1*28 kilogramme, whereas a cubic metre of hydrogen under the same conditions weighs only *09 kg. It follows that a cubic metre of hydrogen in air at 1000 mb. and 273a. will have a buoyancy represented by the difference, i.e., 1*19 kg. Part of the buoyancy will have to be devoted to sup- porting the envelope which contains the hydrogen and which adds so little to the volume of air displaced by the hydrogen that, for purposes of calculation, we may regard the displacement of the air by hydrogen as the source of the buoyancy, and count the envelope and oth -f- ro Ol I-H O ONOO r-0 n ^04 hH^ 03 01 1 ir< -}- moo O4 OO vno t-^oo O *- rj-04 O O C4 \n 04 M -sf- O coo ON O4 O a l i CD Tf- CO "^-O l-l O vno r^oo O -H c^vnRo,^ O co rj- t~>. vn cOO ON O4 O - g ii i 1 1 hH M hH I-H l-l 04 O4 04 Ol CO co ~ST in <& vno r^oo O "i coxnr^oo HH cOO ON 04 O ^ hH I-H ,_ _ W tH 04 O4 O4 O4 CO CO 2 ^pts 1 O vno ON cooo -* KH O 00 O ri- O 00 O *>. cOO OO CN vn 'I jg 1 '1 j vno !> ON O i-i rh 04 O 00 O co vn t^oo I-H co ,ONOO O vn co vnoo 04 vn 43 rt H BU I-H I-H I-H M O4 O4 04 O4 CO CO c3 "S bo bi) p kj co vno oo O 04 OO t^ t^ 04 co vnoo M vn <1 fhH hH _ HH hH I-H 04 O4 O4 O4 CO CO S, |-S * t-*.O r- O ^h O co vno oo O O4 OO t^.OO CO co vnoo HH vn 4-3 rH vr> 5 - M 1-4 M I-H I-H O4 O4 O4 04 CO CO 43 p | 3 vr*vn p vno t^oo O >-< Svn^as co O t> ONO coo oo hH vn -2& -a M l-l |_| |_| |_4 hH 04 C4 N O4 CO co ~^ vn vn vnoo co ON vn 04 ON i-i M co vno ON i-i rh 04 hH COOO coO ON 04 vn | IS fl HH M M hH M I-H 04 04 O4 O4 CO CO O! d, vno J>-OO O -i a as as O l-l O4 vn 04 coO ON cX O S S3ZSS5 rj- I-H CT> HH co coo ON c>4 O 02 bo S HH ^H |-| hH I-H I-H 04 04 04 O4 CO CO "- so ^ '-i s -1- co coO O t~>. vno t^oo O '-i co vn j> ON M 00 ^f- vo O 00 cOO ON coO b l-l I-H |_, HH M hH 04 O4 O4 O4 CO CO fl Id *-i * p SKg8S 8>S8 co O ON co O ^11 M I-H M M hH M 04 O4 O4 O4 CO CO O 1-^00 O^ O c-i O woo W to eo co < co fO vnoo co -H oo co -H ON C^O t-i M co t-i OO to W O\ vr> - O 1/- > O O O^> O O ^ *$ co TJ- j> cxi o c^ ^ vn oo vo O^ oo co vn co oo co oo co t^ Q d M co *$ -rf- vnvo 1^.00 O *- co xnTj-vnoo w OO -^-Mintxivncooo Ooo -rh^rl-^'^- ^r^vnvO f>.co O M -rhW>O O^^Ot-^fOi-i criCTiO>-< t- vnC\rf-OvO xn T}- vnoo c^oocoO O>C^>- ONOO vn O*> vn O 'rh )_, _< I I h- 1 - C-l Cl CO CO -rf -rf- 1/~>VQ VO t^ s ! O ^1 jjh-ii-H>-it-(t-H(-ii II-HI iciMc.oo be I Buoyancy. 55 With these tables the limiting height of flotation can be approximately determined when the dimensions and par- ticulars of the dead weight, portable load and ballast are known. For example, in the case of an airship which displaces 25,000m 3 of air with a dead weight 11,500kg. 9,000kg. ballast, including 2,000kg. of fuel which can be spared, 7,000kg. portable load, including crew, clothing and food, fuel for the return journey> oil armament and working tackle, the right hand side of the equation is reduced to its minimum when the ballast is disposed of, that is when B = o, in that case p\T works out to be 2*46. This will be found in the table between 3k and 4k, rather nearer 3k in summer than in winter. THE EFFECT OF WEATHER UPON BUOYANCY. The changes of weather affect the buoyancy because they alter the value of p\T at the starting point and at every stage of the course. The value at the starting point determines the amount of ballast that can be carried to begin with, and the value at other stages of the journey determines the height to which the airship can rise with all its ballast expended. Assuming that the limit of ascent is determined by the value 2'4 for the ratio pj T 7 , we find that the variation in the course of the year 1913 was from 3'5k to 4'lk. The effect of low pressure or high temperature can easily be seen but it is not very pronounced. The differ- ence between the worst occasion and the best from January to May 1913 was about 1,600 ft. It is now recognised that above one kilometre, when pressure is low, tempera- ture is also low, and, consequently, at any level the value 54 Glossary. of p\ J^is steadier than the variations at the surface would lead one to expect. In these calculations no allowance has been made for leakage, nor, on the other hand, for compressed hydrogen carried to replace losses. DEPRESSION PRODUCED BY THE WEIGHT OF A DEPOSIT OP RAIN OR SNOW. The catchment area may be about 1000 m 2 , and the figures quoted from Josselin by Modebeck for the weight on that area are Maximum depression. k. Dew (light) Dew (heavy) Rain ... Heavy rain Storm rain Weight. Snow kg. 15 80 200 250 4OO 800 to to to to to to kg. 50 240 290 360 480 1000 03 15 18 21 3 60 According to this table the worst effect, even of snow, would be a depression of 2,000 feet. DEPRESSION PRODUCED BY DESCENDING WIND. The calculation is very hazardous ; supposing the vertical component of the wind in ordinary circum- stances to be from 0'5 to 1*5 m/s, and assuming the ordinary law of resistance, which is true for small areas, the downward force would bo '01 nib. = 10 dynes per cm 2 , say 500 x 111 1 x 10 dynes on the envelope (iakmt- ihe area to be f)00 m 2 ), or 5 x 10 7 dynes. 'This is equivalent to a weight of about 50 kg. and is, therefore, unim- portant, but it would become important if the downward Buoyancy. 57 velocity reached 5 m/s, which it probably may do in a disturbed atmosphere, and in the downrush of a line squall this value might be largely exceeded. Buys Ballot's Law (see The Weather Map). The law is that if you stand with your back to the wind the atmospheric pressure in the northern hemisphere de- creases towards your left and increases towards your right. In the southern hemisphere the reverse is true. The law is a necessary consequence of the earth's rotation. C.G.S. Abbreviation for Centimetre Gramme Second, used to denote the organised system of units for the measurement of physical quantities by units which are based upon the centimetre, the gramme, and the second as fundamental units. Calm. Absence of appreciable wind ; on the Beau- fort Scale 0, i.e., less than 1 mile an hour, or three-tenths of a metre per second. Calorie. See p. 301. Celsius, Anders : an astronomer and physicist, the inventor of the Centigrade thermometer, born at Upsala in 1701. The name Centigrade arises from the division of the interval between the freezing point and boiling point of water into one hundred parts. In continental countries the scale is generally named after the inventor Celsius. It is also not infrequently referred to as the centesimal scale. Note. Celsius divided the thermometric interval between freezing and boiling points of water into 100 parts, but he made the former 100 and the latter 0. The Centigrade scale now in use was introduced by Christin of Lyons in 1743 (possibly earlier). It has also been claimed for Liiine, but the priority is extremely doubtful. 58 Glossary. Centibar. A hundredth part of a "bar" or C.G.S. " atmosphere." See BAR, also see Table of equivalents at the end of this volume. Centigrade. A thermometric scale introduced by ANDERS CELSIUS (q.v .), which has zero at the melting point of ice, while 100 represents the boiling point of water at a pressure of 760 mm. of mercury. A centigrade degree is 9/5 Fahrenheit degrees. On the centigrade " absolute " scale the freezing point is at 273a, the unit being the same as a centigrade degree. Centimetre the hundredth part of a metre. The unit of length on the Centimetre-Gramme-Second system which is universally employed for electrical and magnetic measurements. A metre was originally defined as one ten-millionth part of the earth's quadrant, that is, the distance from the equator to the pole. 1 centimetre is equal to *394 inch, and 2*54 centimetres to 1 inch, to a high degree of accuracy. Cirro-cumulus. A group of fleecy cloudlets or small " flocks " of cloud formed at great heights and showing no shadows. They are called " moutons " in French. See CLOUD. CirrO-StratUS. A layer of thin, transparent cloud at a high level. See CLOUD. Cirrus. Clouds at great height, generally formed into " wisps " of thread-like structure. See CLOUD. Climate. A general summary of the weather for any particular locality. When the weather has been observed for a sufficiently long time in any locality we are able to make a useful statement as to the weather which may be To face p. 58. Cirrus clouds, Thread or Feather clouds at a height of from five to six miles, and generally of a white colour. They are composed of ice-crystals. The picture gives an idea of rather more massive struc- ture than is usual with cirrus clouds, but the sweeps and ^vvisps are very characteristic. Climate. 59 experienced at any particular time of the year in that locality. Technically, the climate of a place is represented by the average values of the different meteorological elements, which should include means for each month, as well as means for the whole year. Average extreme values for different periods and ABSOLUTE EXTREMES are of in- terest, and also the number of rain-days, days of snowfall, frost, hail, thunderstorm, gale, &c., and the frequency of occurrence of winds from different directions. Included also under this heading would be the earliest and latest dates of frost and snow, the average depth of snow lying at different times, and particulars of the temperature of the soil at various depths. In places where the type of weather at a given time of year varies greatly, a long series of observations is needed in order to obtain a fair idea of the different climatic elements. Various kinds of climate are characterised, chiefly with regard to moisture and temperature, as continental which is dry, with great extremes of temperature ; insular or oceanic which is moist and very equable in temperature ; tropical in which the seasons depend chiefly upon the time of occurrence of rainfall ; temperate in which the seasons are chiefly dependent upon the variation of the daily course of the sun in the sky ; arctic in which the year is mainly two long periods of sun and no sun. Local climates, such as those of the Mediterranean in general and of the Riviera in particular, are dependent upon the geographical conditions of land and water. Every climate is to some extent affected by the geographical nature of the sur- roundings of the locality. Climatic Chart (see Weather Map, p. 88). A map showing the geographical distribution of some element of 60 Glossary. climate ; temperature, rainfall and sunshine are the most frequently charted. Charts of normal values of these elements for the British Isles for long periods of years ended 1910 are issued by the Meteorological Office. The word also covers any diagram representing the periodic or secular variation of a climatic factor. Climatology. The study of CLIMATE (q.v.). Clouds. Clouds have been classified into certain typical forms, but there are so many intermediate types that it is sometimes difficult to decide to what class any cloud belongs. Certain types of cloud and the direction from which they are moving indicate certain states of weather ; cer- tain types of cloud being usually within certain heights, a knowledge of cloud forms enables the observer to make a rough estimate of the height of clouds. For practical purposes clouds may be divided into cloud sheets and cloud heaps. CLOUD SHEETS. Many forms of cloud are obviously in more or less extended sheets, sometimes covering the whole sky, sometimes only covering a small patch. They vary much in thickness, sometimes no blue sky is visible through the sheet, sometimes small patches of blue are visible, sometimes half the sky is blue, at other times the sheet of cloud is represented by a few detached clouds in the blue. The formation of a sheet of clouds is not well under- stood. But the following may be on some occasions the method of formation ; it is known that the atmosphere is to a certain extent stratified, and that there may be damp Cloud*. 61 and dry layers ; if the pressure over any area is diminished the air in the region will be cooled by expansion ; if sufficiently cooled the dew point will be reached, and any damp layer will become a cloudy layer. Cloud sheets are often seen to be broken up into waves ; the waves may not be formed in the cloud itself ; if waves are set up in the atmosphere they will be propa- gated upwards and downwards ; in a thin cloud layer the air that rises on the wave crest is cooled by expansion, more condensation occurs and the cloud is thicker ; the air that descends in the hollow is warmed by compression and some of the cloud is evaporated, leaving a clear or nearly clear space. Cloud sheets may sometimes be seen forming at several levels at the same time ; over a cyclonic depression there are probably sheets of cloud at several levels. Cloud sheets may be divided up into, three classes which differ in appearance and height. The upper layer, the cirrus clouds, are at heights of from 25,000 to 30,000 feet ; the middle layer, the alto clouds, are from 10,000 to 25,000 feet ; the lower layer clouds are below 10,000 feet. THE UPPER LAYER. The clouds of the upper layer, the CIRRUS CLOUDS, are composed of ice particles, not of water drops as all other clouds. They are of a pure white colour with no shadows except when seen when the sun is low down. They are usually streaky and have a brushed out appearance (mares' tails); they are frequently spoken of afe windy looking clouds, but in spite of this appearance they do not necessarily indicate wind. They are often seen in long streaks stretching across the sky, sometimes in parallel bands, serming by 62 Glossary. the effect of perspective to converge, on the horizon to a V-point. These clouds often move away from the centres of depressions ; when coming from North- West, West or South-West they indicate a depression in a westerly direction, which will probably spread over the observer ; when seen coming from the North they indi- cate a depression to the Northward which will probably move away to the North-East and so not influence the weather. To observe the motion of clouds, especially of the high clouds, get a patch of cloud " on " with the point of a branch or some point of a building; if one moves so as to keep the cloud on the point, one's direction of motion will be towards the direction from which the cloud is coming. It is often difficult to determine the motion of the high clouds without a careful observation of this kind. Bands of cloud do not necessarily move in the direction of their length, nor do parallel bands of clouds necessarily move from their V-point. HALOS and MOCK SUNS (sun dogs) are seen at times in cirrus clouds but in no other forms ; a halo is a ring, sometimes coloured, seen at some distance (22 or more rarely 46) from the sun or moon. Besides the wisps and streaks of cloud of common cirrus there are other forms : CIRRO-CUMULUS : lines or groups of cloud, some- times detached globules, sometimes waves ; mackerel sky (French : moutons) ; these clouds are pure white, with 110 shadows. ClRRO-STRATUS : a bank of tangled web sometimes overspreading the whole sky ; no shadows ; sun pale and " watery." Cloud*. G;> ClRRO-NEBULA : similar to last but no visible structure ; a veil of pale white cloud. Precedes depressions. THE MIDDLE LAYER, ALTO CLOUDS. The alto clouds are composed of water droplets, not ice particles, therefore halos are never seen ; CORON^E may be seen ; these are coloured bands quite close round the sun or moon. The alto clouds are not such a pure white as the cirrus clouds, and shadows are visible on them. They may move away from centres of low pressure, like the cirrus clouds. They are, on the average, only half the height of the cirrus clouds, being from 10,000 to 25,000 feet high. The following are the most important varieties : ALTO-CUMULUS. Very like cirro-cumulus, but the cloud masses are larger, and shadows are visible ; some- times arranged in globular masses (French : Gros moutons), sometimes in waves. ALTO-CUMULUS CASTELLATUS, TURRET CLOUD. The globular masses of Alto-cumulus are developed upwards into hard-edged clouds like miniature cumulus ; some- times a large number of clouds almost of exactly the 'same shape are seen. When coming from a southerly or westerly point, after fine weather, Turret cloud is a sign of approaching thundery conditions. ALTO-STRATUS. Very like cirro-stratus, but a thicker and greyer cloud ; halos never seen ; precedes depres- sions, and does not usually extend so far from the centre as cirro-stratus. , THE LOWER LAYER. The clouds of the lower layer are below 10,000 feet ; thicker clouds with not such a fine structure as the higher clouds. (54 Glossary. STRATO-CUMULUS. Masses of cloud with some vertical structure appearing in rolls or waves, sometimes covering the whole sky ; sometimes the whole sky is covered with cloud, but the hollows of the waves are lighter and obviously thinner ; blue sky is occasionally seen more or less plainly through the thinner parts. This cloud is common in quiet weather in winter, and sometimes per- sists for many days together. In summer it is more broken up, and tends to turn into cumulus clouds. STRATUS. A uniform layer of cloud which resembles a fog, but does not rest on the ground. Small masses of stratus, more or less in the shape of a lens (lenticular cloud), are often seen near thunder clouds ; they appear dark when seen against the white sides of the thunder clouds. NIMBUS. A dark shapeless cloud without structure, from which continuous rain or snow falls ; through open- ings in this cloud, should such occur, layers of cirro-stratus or alto-stratus may almost always be seen. SCUD. Small shapeless clouds with ragged edges ; sometimes seen without other cloud, but usually asso- ciated with nimbus and cumulus. . HEAP CLOUDS. Clouds with considerable vertical structure, not forming horizontal sheets of cloud. Their formation is due to rising currents of air ; as the air rises it is cooled by expansion till it reaches the dew point when cloud begins to form ; as soon as condensation takes place heat is liberated, and though the air cools as it rises still further, it does not cool so rapidly as it would were no heat liberated by the condensing water vapour. The rising current is due to air heated above its surroundings, and the condensation enables the current to rise considerably Between pp. 64 and 65. CLOUDS SEEN FROM BELOW : FIGURE 1. :Strato-Cumulus from below. Cumulo-Nimbus (Thunder-cloud) with "Anvil" extension of. false cirrns. 30th October. 1915. CLOUDS SEEN FROM ABOVE : FIGURE 2. Strato-cuinulus from an aeroplane (4,000 feet). filled with fog, 300 feet deep, in early morning after still night. Clouds. 65 higher than it would have done had no extra heat been available. SIMPLE CUMULUS. This may consist of small clouds with flat base and rounded top or large cauliflower shaped clouds (Woolpack cloud). These clouds commonly form on a summer day, beginning as small clouds, and growing larger by the early afternoon, when the rising currents due to the sun's heat are at a maximum ; they usually disappear before sunset. CUMULO-NIMBUS ; SHOWER CLOUD ; THUNDER CLOUD. Sometimes when cumulus grows to large propor- tions the upper edge is seen to become soft and brushed out into forms somewhat like cirrus (false cirrus) ; at the same time from the under edge of the cloud, which has been growing very dark, rain begins to fall. Sometimes only a slight shower may result ; but if the cloud is large there may be heavy rains and violent thunderstorms. Sometimes the false cirrus is brushed out round the top of the cloud giving it the shape of an anvil ; the anvil cloud is usually associated with very violent thunder- storms and falls of hail. Thunder clouds due to the rising currents of a hot day usually disappear about the time of sunset, but often leave the false cirrus. Cumulus often forms in long lines of cloud presenting the appearance of a succession of clouds or of a wall of cloud extending along the horizon. Any cause that brings masses of air of different temperatures near together brings about the formation of cumulus cloud. It should be noted that rain, hail, and snow only fall from nimbus or cumulo-nimbus clouds, except the slightest and most transient showers, which may some- times fall from alto-cumulus or strato-cumulus. Nimbus causes persistent rain or snow, cumulo-nimbus more or less severe showers of rain, hail or snow. 13204 C 66 Glossary. TABULAR STATEMENT OF THE SEVERAL TYPES OF CLOUDS. I. CLOUD SHEETS. CIRRUS. Mares' tails ; wisps or lines of pure white clouds with no shadows. Upper cloud layer about 30,000 feet. Clouds composed of ice crystals. With these are sometimes seen halos, or rings, at some distance from the sun and moon. Middle cloud layer ; 10,000 feet to 25,000 feet. Clouds com- posed of minute drops of water. Coloured rings sometimes seen quite close to sun and moon, but never halos. Lower cloud layer. Below 7,000 feet.* CIRRO-CUMULUS. Small speckles and flocks of white clouds ; fine ripple clouds : mackerel sky. CIRRO-STRATUS. A thin sheet of tangled web structure, sometimes covering the whole sky ; watery sun or moon. CIRRO-NEBULA. Similar to last, but a veil of cloud with no visible structure. ALTO-CUMULUS. Somewhat similar to cirro- cumulus, but the cloud masses are larger, and show some shadow. ALTO-CUMULUS CASTELLATUS. Turret cloud; alto-cumulus with upper margins of the cloud masses developed upwards into miniature cumulus, with hard upper edges. (Sign of thunder.) ALTO-STRATUS. Very like cirro-stratus and cirro-nebula, but a thicker and darker cloud. STRATO-CUMULUS. Cloud masses with some vertical structure ; rolls or waves some- times covering the whole sky. STRATUS. A uniform layer of cloud resembling a fog but not resting on the ground. NIMBUS. Shapeless cloud without structure, from which falls continuous rain or snow. SCUD. Small shapeless clouds with ragged edges ; sometimes seen without other cloud, especially in hilly country ; but more commonly seen below other clouds, such as cumulus and nimbus. *xThe heights given are only approximate. Thus lower clouds of the strato-cumulus type may rise above 7,000 feet and attain at times the height of the middle cloud layer. Clouds. 67 II. HEAP CLOUDS. CUMULUS. (Woolpack clouds) ; clouds with flat base and con- siderable vertical height. Cauliflower-shaped top. FKACTO-CUMULUS. Small cumulus with ragged tops. CUMULO-NIMBUS. (Anvil-, thunder- or shower-cloud). Towering cumulus with the top brushed out in soft wisps or larger masses (false cirrus) and rain cloud at base. The height of the heap clouds is very variable, Mean height of base, about 4.500 feet ; the height of the top varies from about 6,000 to 25,000 feet. Reproductions of photographs of Strato-cumulus seen from below and from above, also of Cumulo-Nimbus and of a Valley filled with fog are inserted between pages 64 and 65. A photograph representing Cirrus faces page 58, and one representing Mammato-cunmlus, page 190. Cloud-burst. A term commonly used for very heavy thunder-rain. Extremely heavy downpours are sometimes recorded, which in the course of a very short time tear up the ground and fill up gulleys and watercourses ; this happens in hilly and mountainous districts, and is probably due to the sudden cessation of conyectional movement, caused possibly by the supply of warm air from the lower part of the atmosphere being cut off as the storrn moves over a mountain range. With the cessation of the upward current, the raindrops and hail- stones which it had been supporting must fall in a much shorter time than they would have done had the ascensional movement continued. Clouds, Weight of. Measurements on the Austrian Alps of the quantity of water suspended in clouds have given 0*35 g/m 3 to 4- 8 g/m 3 . The water suspended as mist, fog, or cloud may be taken as ranging from 0*1 to 5 g/m 3 . (See Wegener's Thermodynamics of the Atmo- sphere, p. 262.) 13204 C 2 68 Glossary. Col. The neck of relatively low pressure separating two anticyclones (see ISOBARS). One of the most treacherous types of barometric distribution, as it some- times marks a locality of brilliantly fine weather and sometimes is broken up by thunderstorms. See Plate XIV. Compass. A circumference, or dial, graduated into thirty-two equal parts by the points N., N. by E., N.N.E., N.E. by E., N.E. and so on. The cardinal points of the compass are North, South, East and West. The points of the compass are often called ORIENTATION points. A MAGNETIC COMPASS, or MARINER'S COMPASS, is a compass or orientation-card having attached to it one or a series of parallel magnets and supported so that it may turn freely in a horizontal plane. Any magnet sets itself parallel to what is termed the magnetic meridian, but it is only some few countries that have a magnetic meridian the same as the geographical meridian ; they include narrow strips in Arabia, European Russia, Finland, and North Lapland. Even at places within temperate latitudes the angle between the two may amount to 50 or more, and in the Arctic or Antarctic regions the direction of the compass needle may be the opposite of the geographical north and south line. In the neighbourhood of London the needle points about 15 to the west of north, and the amount is slowly decreasing. In the west of Ireland the declination, or variation, as it is called, is more than 20 W. Public wind-vanes are often incorrectly set according to magnetic north instead of true north ; caution must be exercised accordingly. All maps are set out according to true north ; often an orientation mark is given to show the variation of the compass or magnetic declination. Compass. 69 A compass-needle is influenced by magnetic material in its neighbourhood, and therefore in practice should not be used near to articles made of iron or steel. The variation of the compass and correction of its error? are matters of primary importance for aircraft pilots and are provided for by a special manual. Component. A word used to indicate the steps, in their various directions, which must be compounded or combined geometrically in order to produce a given displacement. For example a man going ten steps upstairs arrives in the same position as if he took ten treads forward on the level and then ten rises straight upwards, or equally if he first went up ten rises and then took ten treads forward. The actual distance travelled is the combination of the horizontal distance and the vertical rise. We call the horizontal distance and the vertical rise the components arid the actual distance the geometrical sum or the resultant of the components. It is evident that it is not necessary that the components should be at right angles to each other as the horizontal and vertical are. Any displacement AC is the geometrical sum of the two components AB and BC,, wherever B may be. This analysis of displacement into components, or combination of component displacements to form a resultant displacement, finds an effective illustration in the effect of leeway on the performance of aircraft. The aircraft must necessarily be carried along by the air in which it travels. If we suppose AB to be the travel of the aircraft through the air in an hour, and BC to represent the travel of the wind in the hour, AC, the resultant, will represent the travel of the aircraft with reference to the fixed earth, or its performance. BC is the leeway, AB is the headway made through the air. 70 Glossary. Hence the performance is the geometrical sum of the headway and the leeway. See VECTOR. The law of composition of displacements which is thus set out is equally applicable to the composition of velocities, accelerations, and forces. The resultant is always the geometrical sum of the components, and to obtain the geometrical sura, set out a step or line representing the first component, then another step representing the second component : the resultant is the step from the beginning of the first component to the end of the second. The simplest way of dealing with the magnitudes and angles which come into questions involving geometrical composition is to make a scale-drawing and measure them with a rule and protractor. That is generally accurate enough for most purposes, but the methods of the solution of triangles can be employed if high numerical accuracy is wanted. Condensation. The process of formation of a liquid from its vapour. See AQUEOUS VAPOUR. Conduction. 71 Conduction. The process by which heat is trans- ferred by and through matter, from places of high to places of low temperature, without transfer of the matter itself, the process being one of '* handing on " of the heat- energy between adjacent portions of matter. It is the process by which heat passes through solids ; in fluids, although, it occurs, its effects are usually negligible in comparison with those of CONVECTION. See also p. 146. Convection. In convection heat is carried from one place to another by the bodily transfer of the matter containing it. In general, if a part of a fluid, whether liquid or gaseous, is warmed, its volume is increased, and the weight per unit of volume is less than before. The warmed part therefore rises and its place is taken by fresh fluid which is warmed in turn. Conversely, if it is cooled it sinks. Consequently, if heat is supplied to the lower part of a mass of fluid, the heat is disseminated throughout the whole mass by convection, or if the upper part is cooled the temperature of the whole mass is lowered by a similar process. There are two apparent and important exceptions in meteorology. Fresh water, when below the temperature of 39*1 F., 277a., expands instead of contracting on being further cooled. Hence a pond or lake is cooled bodily down to 39*1 but no further, as winter chills the surface before it freezes. Secondly, heat applied to the bottom of the atmosphere may stay there without being dis- seminated upwards when the atmosphere is exceptionally stable in the circumstances explained under ENTROPY. Corona. A coloured ring, or a series of coloured rings, usually of about 5 radius, surrounding the sun or moon. The space immediately adjacent to the luminary is 72 Glossary. bluish-white, while this region is bounded on the outside by a brownish red ring, these two together forming the " aureole." In some cases the aureole alone appears, but a complete corona has a set of coloured rings surrounding the aureole, violet inside followed by blue, green, yellow to red on the outside. This series may be repeated several times outwards. The corona is produced by diffraction, that is by the bending of rays of light round tl^e edge of small particles, in this case minute water drops, but sometimes dust (see BISHOP'S RING). If the diffracting particles are of uniform size the colours are pure ; a mix- ture of many sizes may give the aureole only. The more numerous the particles involved, the greater is the intensity of the colours, while the radius of the corona is inversely proportional to the size of the particles. Thus a corona whose size is increasing indicates that the water particles are diminishing in size, and vice versa. The corona is distinguished from the halo, which is due to REFRACTION, by the fact that the colour sequence is opposite in the two, the red of the halo being inside, that of the corona outside. Correction. The alteration of the reading of an instrument in order to allow for unavoidable errors in measurement. The measurement of nearly all quantities is an indirect process, and generally takes the form of reading the position of a pointer or index on a scale. When we wish to know the pressure of the atmosphere we read an index on the scale of a barometer ; when we wish to know the temperature of the air we read the position of the end of a thread of mercury in a ther- mometer ; to determine a height we use the pressure, though the scale may be graduated in feet or metres. Correction. 73 Almost all measurements are, in fact, ultimately re- duced to reading a position or length on a graduated scale and, generally speaking, the reading depends mainly, it is true, on the quantity which the instrument is in- tended to measure, but also partly upon other quantities. Thus the readings of barometers are generally affected by temperature as well as pressure, those of thermometers by alterations in the glass containing the mercury or spirit. It is the object of the designer and maker of instruments to get rid of these disturbances of the reading as far as possible, either by the selection of special materials or by introducing some device whereby the disturbing effect is automatically corrected. In that case the error, and often the instrument, is said to be compensated. But in most cases the amount of the error has to be determined and allowed for by a suitable correction. An ANEROID BAROMETER is often compensated for tempera- ture, and for a mercury barometer the effect of temperature is made out and tabulated and a correction introduced, which is derived from a table, when the temperature of the " attached thermometer " has been noted. In a similar manner the correction of a barometer reading for the variation of GRAVITY at different parts of the earth's sur- face is worked by means of tables, the variation of gravity with latitude having been previously reduced to a formula, by means of observations from which the figui e of the earth has been determined and the change of gravity has been ascertained. In some measurements, such as the determination of height by the use of an aneroid barometer, corrections are numerous and complicated : the uncorrected reading may even be only a rough approximation not sufficiently accurate for practical purposes. 74 Glossary. Correlation. Two varying quantities are said to be correlated when their variations from their respective mean values are in some way or other mutually connected with each other. A kind of measure of this connection is given by the " correlation coefficient." This is a decimal lying between + 1 and 1, which is easily calculated. Values of + 1 and of 1 show that the two quantities -are directly or inversely proportional ; that is to say, for any departure from the normal of the one quantity there is a corre- sponding departure from the normal of the other, always in the same sense or always in the opposite sense. On the other hand values that are nearly nothing show that there is little if any connexion. It may be of interest to state that the coefficient of correlation between the phases of the moon and the barometric pressure at Greenwich is insignificant. Modern statistical methods have been frequently applied to meteorological problems. Mr. W. H. Dines, F.R.S., has used the method of correlation with great success in his work on the upper air. His results are published by the Meteorological Office in Geophysical Memoirs No. 2. As examples of correlation coefficients we may give those found by Mr. Dines between the pressure at a height of 9 kilometres and the mean temperature of the air column from 1 to 9 kilometres. For different sets of observations the values found were '88, '96, 1 90, -90, 94. The inference to be drawn from such correlation coefficients is that the variations of temperature of the air column from 1 to 9 kilometres are directly dependent upon the pressure at the top of the air column. Mr. Dines gives correlation coefficients from work on the upper air in various other papers. Correlation. 75 Sir" Gilbert Walker, of the Indian Meteorological Service, has used the method of correlation to predict the amount of the Indian Monsoon Rainfall. He has also investigated the connection between sunspots and tem- perature, sunspots and rainfall, sunspots and pressure by statistical methods. From, various papers the following examples of correla- tion coefficients have been selected : Correla- tion Coefficient. Number of Observations. Variables Correlated. Reference. *49 42 (1867-1908). Height of Nile flood and South Ameri- can mean pressure in March, April, May. G. T. Walker, Cor- relation in Seasonal Variations of Weather. (Memoirs of the Indian Met. Dept., Vol. XXI., Part II.) -'47 30 (1880-1909). Rainfall at Java, October to March, with rainfall at Trinidad the fol- lowing six months. R. C. Mossman. South- ern Hemisphere Seasonal Correla- tions. (Symons Met. Mag., Vol. 48, 1913.) -'43 34 (1877-1910). Annual mean tem- perature at Cairo and annual mean temperature in Eng- land, S.W., and South Wales. J. I. Craig, a See-Saw of Temperature. (Quar. Journal, Roy. Met. Soc., April 1915.) -62 36 (1875-1910). Mean pressure for March at Azores and in Iceland. J. P. Van der Stok. 76 Glossary. Correla- tion Coefficient. 78 -81 80 Number of Observations. 20 (1891-1910). 47 (1869-1915). 20 (1896-1915). Variables Correlated. March barometric gradient at Zikawei (China') and July- August air tem- perature at Miyako in N.E. Japan. Mean pressure for March at Kew and rainfall total for same month at Kew. Sunspot numbers and level of Lake Victoria. Reference. T. Okada (Monthly Weather Review, U.S.A., Jan,, 1916). E. H. Chapman, The Relation between Atmospheric Pres- sure and Rainfall. (Quar. Journal, Roy. Met. Soc., 1916.) M.O. MS. Cosecant. In a right angled triangle the ratio of the hypotenuse to one side is the cosecant of the angle opposite to that side ; the ratio is the reciprocal of the sine. See SINE. Cosine. In a right angled triangle the ratio of one side to the hypotenuse is the cosine of the angle between them. See SINE. Cotangent. In a right angled triangle the ratio of the two sides that form the right-angle is the cotangent of the angle opposite the side taken as the divisor. See SINE. Counter sun. See ANTHELION. Cumulo-stratus. 77 CumulO-StratUS. The name given to a certain com- bination of cloud forms which is no longer used in the international classification. See CLOUDS. Cumulus. The technical name of the woolpack cloud. See CLOUDS. . A name given to a region of low barometric pressure ; now usually spoken of as a DEPRESSION or a LOW. See ISOBARS. Cyclostrophic. See GRADIENT WIND. Damp Air. As distinguished from dry air in me- teorology, damp air implies a high degree of RELATIVE HUMIDITY (q.v.). When its relative humidity equals or exceeds 85 per cent, of saturation air may fairly be called damp. It will deposit some of its moisture in dry woollen fabrics, cordage or other fibrous material, though its water will not condense upon an exposed surface until 100 per cent, is reached. Even the driest air of the atmosphere contains some water vapour, and its relative dampness or dryness can be changed by altering the temperature. Thus the same air may be very dry at 2 o'clock in the afternoon, and very damp, even cloudy, at 8 o'clock in the evening, simply because its tempera- ture has been lowered. At any time of the year, but especially in summer, the dampness of the air is subject to great changes. Day Breeze. See SEA BREEZE or BREEZE. Debacle. Breaking up of the ice in the spring in rivers and seas ; it lasts from two to six weeks, and takes place between the end of January and the beginning of 78 Glossary. May, varying according to locality. The waters are usually free from ice by the end of April-May. At " debacle " the water in rivers rises to the extent of inundating the country for miles around, sometimes stopping all carriage traffic ferry-boats taking their place. This condition may last for as long as three weeks. In Russia there are some 110 stations at which obser- vations of the "debacle" are taken. The event takes place earliest in the Caspian, Black and Azov Seas : it commences at the end of February, and the sea along the coast is free of ice by the end of March the open sea being clear by the end of February. In the Pacific the phenomenon takes place in April, and the sea is clear by May ; in the Baltic usually in March or the beginning of April, the sea being cleared by the beginning of May, but at Reval and Libau this may occur even by the end of March. At Uleaborg the "debacle" is later, there being ice in the sea till the end of May ; and in the Arctic Sea still later, about the end of April, and the sea is not clear till the beginning of June. In Canada, in Ontario, the occurrence takes place in March, freeing the waters by April, and the same is true in the case of the Maritime provinces. In the St. Lawrence it ia a little later, the river being free of ice in May. Dekad, in Meteorology, a period of ten days, but decade is often, used for ten years. Density. The density of air is one of the things you have to know when you want to calculate the lifting power of a balloon of given size. As applied to air, density is a difficult word to explain because the numeri- cal value depends partly upon the composition of the air, partly upon its pressure, and again partly upon its tern- Density. 79 perature. Thus it is often said that moist air or damp air is lighter than dry air, warm air is lighter than cold air, and rarefied air is lighter than compressed air, and all these statements are true provided that in each case we remember to introduce the condition "other things being equal." By the density of a sample of air is to be understood the weight, or better, the mass of a measured volume, a cubic foot, or a cubic metre ; and moist air is lighter than dry air in this sense that a cubic metre of perfectly dry air weighs 1,206 grammes when its barometric pressure is 1,000 millibars, 29*53 inches, and its temperature is 289a. (60*8 C F.), whereas if it were saturated air instead of dry air the cubic metre would weigh 1,197 grammes. But if the barometric pressure rose while the change from dry air to saturation was being effected the density would rise in like proportion ; and if the temperature changed the density would change in reversed proportion to the absolute temperature. The formula for the density of air is, therefore, a complicated one A- where A is the density to be computed, A is the density of perfectly dry air at pressure^ and temperature T . If p = 1,000 mb., 29-53 in., and T = 290a. then A = 1,201 g/m 3 . p is the barometric pressure in mb. of the sample. e is the pressure of aqueous vapour in the sample. The following is a complete example of the deter- mination of the density of a sample of air from a reading of the barometer and wet and dry bulb thermometers. 80 Glossary. Barometer corrected for temperature 1 ,010*1 mb. (29*83 in.). Dry bulb 285*8a. (55*1 P.). Wet bulb 281-3a. (47*0 P.). Vapour pressure from humidity tables = 8*2 mb. (0*239 in.). A = 1,201 g/m3, 1,010-1 - 8*2 x 3/8 290 A = 1,201 x - i )() 00 x 285^8 g / m ' = 1,227 g/m, or 0*0766 Ibs. per cubic foot. Note, One grain per cubic foot is equivalent to 2-29 g/m 3 . Since the reading of a barometer gives the pressure at the level of the mercury in the barometer cistern it must be understood that the density refers to the sample of air at that level. For the value at any other level a correction for level must be made. The variation of the density of air with pressure and temperature is of great importance in meteorology. Pressure is of course higher in an anticyclone than in a cyclonic depression, and it has recently been made cer- tain that above one kilometre (3,281 feet) the air in the high pressure is warmer than in the low pressure, so the effect of difference of pressure may nearly counter- balance the effect of temperature, and in consequence the density of a column of air in a cyclone may be very little different from that in an anticyclone. The influence of moisture upon the density of air is not regarded as having the importance in meteorology which used to be attributed to it when it was held to explain the difference between high and low pressure with the accompanying weather. Density. 81 The following are the average pressures, temperatures and densities of air at different levels above high pressure and low pressure respectively in the British Isles accord- ing to the results obtained with registering balloons by Mr. W. H. Dines, F.R.S. TABLE o^ AVERAGE VALUES OF THE PRESSURE, TEM- PERATURE AND DENSITY OF AIR IN A REGION OF HIGH AND OF LOW PRESSURE. High Pressure. Low Pressure. Height. Pres- sure. Temp. Density. Pres- sure. Temp. Density. lOOO-ft. k. mb. a. g/m 3 mb. a. g/m 3 32*809 IO 273 226 421 247 225 382 29-528. 9 317 2 33 474 288 226 444 26 * 247 8 366 240 53i 335 227 SH 22 * 966 7 422 247 595 388 232 583 19-685 6 483 254 662 449 240 652 16-406 5 552 261 736 5i6 248 724 13-124 4 628 267 818 59i 255 807 9^43 3 713 272 911 675 263 893 6-562 2 807 277 IOI2 767 269 992 3-28l I 913 279 H37 870 275 IIOO o o 1031 282 I27O 984 279 1226 The densities quoted above are calculated on the assumption that the relative humidity is 75 per cent. 82 Glossary. The following are the densities of a few other substances. 'Hydrogen (dry) 88'74 g/m 3 *Carbonic acid gas (dry)... 1,953 g/m 3 Water 1-000 g/cc (at 277a.) Sea water I'Ol to T05 g/cc. Mercury 13'596 g/cc (at 273a.) Petrol ... ... 0-68 to O72 g/cc. See also under BUOYANCY. Depression. A region of low barometric pressure surrounded on all sides by higher pressures. See ISOBARS, and Plate XL Dew. The name . given to the deposit of drops of water which forms upon grass, leaves, &c., when they become cooled, by radiating heat to the sky on a clear night, to such an extent that their temperature is below the saturation or DEW POINT of the air which surrounds them. The last part of the process of the formation of dew is in no way different from that which operates when a glass of ice-cooled water covers itself with water drops indoors, or when a deposit of moisture is formed by breathing on a window-pane. Dew-point. The temperature of saturation of air, that is to say, the temperature which marks the limit to which air can be cooled without causing condensation, either in the form of cloud if the cooling is taking place in the free air, or on the sides of the vessel if it is enclosed. See AQUEOUS VAPOUR. Diathermancy. Diathermanous. The power of allowing heat in the form of radiation to pass in the same way that light passes through glass. Rock salt is * At a pressure of 1,000 mb. and temperature of 273a. Diathermancy. 83 peculiarly diathermanous, water, on the contrary, and glass are not. All forms of energy which are " radiated " and in connection with which the word " ray " is used, such as rays of light, heat rays, X-rays, radio-telegraphic rays, travel by wave motion and have some properties in common. One of the most characteristic is that described as transparency and opacity with regard to light. But the same substances are not similarly transparent for all kinds of rays. X-rays and electric rays make no difficulty about going through walls which stop light ; and sound- waves often find a way where light cannot follow. The question of transparency and opacity for different kinds of rays is one of bewildering complexity. The study of DIATHERMANCY deals with that part of the subject which is concerned with the transmission of heat in the form of wave motion. Diffraction. The process by which rays of different colour are separated one from another when a beam of light passes an obstacle of any shape. In reality the shape of the obstacle must be carefully chosen in relation to the shape of the beam to make the phenomenon easily apparent. Perhaps the simplest experiment is to draw a greasy finger across a plate of glass and to look through the glass at the bright line of an incandescent electric lamp, taking care that the plate is turned so that the lines left by the finger on the glass are parallel to the bright line. Those who are unfamiliar with the experiment will be surprised at the brilliancy of the colours which are produced by the simple process. In more scientific form when the lines are ruled regularly on the glass by a suitable dividing engine we get a diffraction-grating, one of the most delicate of all optical instruments. 84 Glossary. For scientific experiments in diffraction either a bright line of light formed by a slit in front of a lamp with a linear obstacle, or a bright point of light with a circular obstacle may be used, and many remarkable results can be shown with these simple means. For example, when the distances are properly adjusted a bright spot will be found at the central point of the shadow of a circular disc, thrown by a bright point of light, and again on looking past a needle at a line of light parallel to the needle a great play of colours will be seen. The phenomena of diffraction are explained by the hypothesis that the actual transmission of light is not a direct projection of the light along straight lines from any luminous point, but the spreading out of waves with a spherical wave-front central at the luminous point. They are exhibited in the atmosphere principally by the formation of CORONAE round the sun and moon, and sometimes also by the IRIDESCENCE of CLOUDS. Diffusion. The slow molecular process by which supernatant fluids mix in spite of the differences in their density. The molecular forces or motions which come into play in diffusion are perhaps most effectively illustrated by the tenacity with which the mixture main- tains its composition when once the mixing has taken place. For example, whisky is lighter than water and a separate layer of the spirit can be floated on the top of water by judicious manipulation. The spirit and water will then slowly mix by diffusion, even if there be 110 stirring, due to thermal convection or mechanical opera- tion. But when the spirit and water have become mixed no amount of allowing to stand will cause the water to settle to the bottom and leave the whisky in a separate Diffusion. 85 layer at the top. Once mixed they are mixed for ever, owing to the power of diffusion. The process of diffusion follows certain definite rules, which are similar in type to those for the diffusion of heat by thermal conduction, and the diffusion of velocity through a viscous mass, but it takes so long a time for any appreciable effect to be produced, that in practice diffu- sion only completes the process of mixing which has been begun by stirring or convection. Major G. I. Taylor has recently shown that in the atmosphere mixing by tur- bulent motion (see EDDY) follows a similar law, but with a different characteristic constant that brings diffusion by turbulent motion among the operative forces in Meteorology. Diurnal. The word, which means " recurring day by day," is used to indicate the changes in the meteorological elements which take place within the twenty-four hours of the day. Thus, by the diurnal change of pressure is meant a slight rise of the barometric pressure between about 4 a.m. and 10 a.m., and between 16 h. (4 p.m.) and 22 h. (10 p.m.) with corresponding falls between. In this change it is the " semidiurnal " variation which is the most striking because it occurs (with different intensity) all round a whole meridian simultaneously, and sweeps round the globe from meridian to meridian about three and a half hours in front of the sun. Other elements also show noteworthy diurnal variations. We give here diagrams showing the diurnal variation of pressure, temperature, humidity and wind velocity at Kew for January and July as representing winter and summer respectively. 86 DIURNAL VARIATION IN SUMMER. HOURS c so-G CD O c 70-ti 60-: 3 6 9 12 15 18 21 HLMIDITY i\ i-60 HOURS so .310145-1 -2996u -/- WIN _ o 6 - --4 .O-tt^v-icovj-ioocMTj- >f O t> "~> co i-i CM co M- f O CM co HH HH M 00 11 C$ d 00 Tj- >-icor^Mt-^coOt^.t^cMOO OO CM O ONMVOOO cOt^O OOO H "*" M xn rj-coM-tOO'-'"-'.cOTh'1 CM O O CM t^ OO 1>- xn O H-icMcMfMfMCMcOcOcOcMcM w CO HloC p 5 M O "^ O w ON r^ rj- co vO ON Th (N ^J- l> ON O ON O oo l>. ON t^ OO 00 hH ) i h- 1 1 I M -2"- fl vn iO rj-oo^vnOO*nvowTJ-vo :f!'!'o g S n -iONTt--d-MOOOMMOvo vO i-i H- i vn O oo 1^.0 vn CM i-i ^ ^1 || p O xnOO cOrJ-cor^OO coj^i-ivO CM -t-t^>-ivnrMOcoOoo^-CM 9 ' CM Ol CO CO t^ CM i-i 05 rT fl 00 vooOTj-xnt^OONOOoo>-ivn g M i i co ^o t~>- t^. vn co *-* co o|||| PI" M H i M CM vO xn coONQcocovOcMi-i i-i CM CM CO CM i-t i-( eS S ,|.l| S ' i-gj ! , x ^.i -i 1 1 i H glil-o-g-^llgl SnS-'iS^Hi^iKOfcfi 108 Glossary. these conditions, temperature and wind have to be con- sidered, so that the measure of evaporation is the end of a very intricate story. Still, in dry countries like Egypt, South Africa and Australia it is a matter of serioun economic importance. , It is generally given on the analogy of rainfall as the depth of water evaporated. Some results are given on page 107. The differences shown in this table illustrate quite forcibly the influence which evaporation may exercise upon climate, but the figures must not be taken as strictly comparable as the actual amount of evaporation depends upon so many conditions. The measurements given for Egypt are taken from a Wild's gauge which holds only a small body of water and gives a high figure for evaporation. See Keeling : Evaporation in Egypt and the Sudan (1909). Craig : Cairo Scientific Journal, May (1912). Expansion. The increase in the size of a sample of material, which may be due to heat or to the release of mechanical strain, or the absorption of moisture or some other physical or chemical change. The size may be taken as the length or volume, some- times as the area. In the science of heat the fractional increase of length or volume for one degree of tem- perature is called the coefficient of thermal expansion. Thus, the co-efficient of " linear " expansion with heat of the brass used for barometer scales is 0*0000102 per degree Fahrenheit, which means that for 1F. the length of the scale increases by 10,2 ten-millionth parts of its length at the standard temperature (62 F.). The co-efficient of ''cubical" expansion of mercury is '0001010, which means that the volume of a quantity of mercury increases by 1*01 ten-thousandth of its bulk at the standard tem- perature (32 F.) for 1 F. The corresponding expansions Expansion. J09 and coefficients for 1 C. or la. are larger in the ratio of 18 to 10. Expansion of volume alters the density of a substance, and changes of density are therefore numerically related to expansion. The expansion of a gas may be caused either by reduc- tion of pressure or by increase of temperature. So in order to see the effect of temperature alone we muse keep the pressure constant. In these circumstances the coefficient of expansion is '00366 for la. referred to 273a. as standard, or *002 for 1 F. referred to 41 F. as standard. Exposure. In meteorology, the method of presen- tation of an instrument to that element which it is destined to measure or record, or the situation of the station with regard to the phenomenon or phenomena there to be observed. If meteorological observations are to be of much value attention must be paid to the manner of the exposure of the instruments. Details are to be found in the Observer's Handbook. Uniformity of exposure is of the greatest importance and for that reason the pattern of the thermometer-screen has been standardized in most countries, while in these Islands, a standard height above ground for the rain- gauge has likewise been fixed. A SUNSHINE EECORDER demands an entirely unobstructed horizon near sunrise and sunset at all times of the year. The question of the exposure of ANEMOMETERS is one of great difficulty. The extent of the GUSTINESS of the wind as exhibited on the trace of the tube-anemometer is a fair index of the excellence of the exposure. At Aberdeen Observatory, two anemometers are ex- posed, one at an elevation of 30 feet above the other. 110 Glossary. The results are noticeably different ; and the same state- ment applies in an even more marked degree to the two anemometers at Falmouth, one exposed upon the Obser- vatory roof and one upon the tower of Pendennis Castle. Extremes. Generally used with reference to temper- ature or wind ; in the first case to mean the highest and lowest temperatures recorded at an observing station in a day, a month or a year. The maximum and minimum temperatures are the extremes for the day ; in l'.U4 the extremes for Greenwich for January were 55 F. and 20 F., and for July 92 F. and 45 F. When the observations for a series of years are available we may find the normal or average extremes and ABSOLUTE EXTREMES. With regard to wind the highest wind recorded in a gust shown on a tube - anemogram is the extreme wind, and the highest wind-force on the Beaufort Scale noted by an observer in the course of a gale is logged as the extreme for that gale. The strongest gust for the British Isles is given on p. 142, the highest hourly wind velocity is 34*9 m/s (78 mi/hr) recorded at Fleetwood in 1894. ' Falirenlieit, Gabriel Daniel. The improver of the thermometer and barometer, born 1686 at Dantzig. He used mercury instead of spirit of wine for thermometers and avoided negative temperatures by marking the freezing point of water 32 ; the boiling point of water Avas subsequently marked 212. The Fahrenheit scale is still in common use in English- speaking countries, and it has advantages because the size of 'the degree is convenient and temperatures below F. are of rare occurrence at the Earth's surface, except in the polar (Arctic and Antarctic) regions and the con- tinental countries bordering thereupon. In fact, the range Fahrenheit. Ill F. to 100 F. is a very serviceable range for the climates of the temperate zone. But the investigation of the upper air has necessitated the frequent use of temperatures on the negative side of the Fahrenheit zero, temperatures as much as 100 below zero occur, and to have the zero in the middle of the working scale is very inconvenient. In the physical laboratory too, temperatures approaching 5()0F.- below the freezing point are realised in experiments on the condensation of hydrogen and helium. Probably the best scale for all purposes would be in Fahrenheit degrees measured from 459 below the Fahrenheit zero which is computed to be within half a degree of the zero of absolute temperature. In that case 500 would correspond with 41 F. or 5 C. But the grow- ing prevalence of the Centigrade or Celsius scale in countries which do not speak English has led to the use of temperatures measured in the centesimal degrees from the zero of absolute temperature computed as 273 below the freezing point of water. For a table of conversion of the various scales in use see page 355. Fall. "The fall of the leaf" in common use with American writers for Autumn. Fluid. A substance which flows, to be distinguished from a solid which will not flow. Some fluids are very viscous, like pitch or treacle, and take a long time to flow, others are mobile, like water or petrol, and take very little time to flow until the surface becomes level. Gases are included in the general term fluid, because they also will flow through a pipe. Their peculiarity is that they can be not only compressed by pressure but also expanded indefi- nitely on the release of pressure. The density, i.e., the amount 112 Glossary . that can be got into a limited space, is, in fact, almost exactly proportional to the pressure. So when we find a, large mass of gaseous fluid like the atmosphere lying upon the earth's surface, it is dense in the surface layer which has to carry the weight of all there is above it ; and as the pressure gets less and less, upward, the density gets less and less until space is reached. We do not know what happens where the atmosphere merges into space, but we are sure that the earth, with the aid of gravity, carries its atmosphere along without losing any appreciable amount into the void. Fog*. Obscurity of the atmosphere which impedes navigation or locomotion. It may be due to a cloud of water particles at the surface, as sea-fogs and valley-fogs generally are, but an effective fog can be produced by clouds of dust ; that is often the case off the West coast of Africa during the season of the HARMATTAN. In towns true water-fogs are generally rendered more opaque by loading with smoke, and in some cases in towns obscurity of the atmosphere that hardly amounts to fog may be due to the condensation produced by the gaseous products of combustion under the action of sunlight. Sea-fog is apparently due most frequently to the passage of air over sea water colder than itself ; there is first the cooling of the air by the contact with the cold water, and then the mixing up of the air near the surface by the eddy motion resulting in the cooling of a considerable thickness below the dewpoint. (See G. I. Taylor, Scientific Results of the Voyage of the " Scotia" 1913.) Sea-fog is most prevalent in spring and summer when the air is warming rapidly. It does not often occur in the winter. See p. 121. Fog. 113 Land-fog, on the other hand, is an autumn or winter fog ; it is generally due to cold air passing over relatively warm, moist ground. The process of cooling may be either by radiation or by a change of wind, but again eddy motion is necessary to mix the warm, moist air close to the ground with the cold flood. Fogs of this kind are not infrequent in the early mornings of summer, but they persist sometimes through the day in autumn and winter. Autumn is their special season. Anticyclonic weather with light airs is very favourable for land-fog, and the ending of a period of anticyclonic weather is nearly always fog. Fog on our coasts is generally included in the forecasts for the British Isles when the wind changes from a Northerly or Easterly to a Southerly point. For the monthly percentage-frequency of fog and mist in the English Channel see p. 121. The conditions for fog in London are set out in a Report 3f the Meteorological Office on fogs. (M.O. publication No. 160.) The frequency of fog at the observing hours, according fco the returns for the past 20 years from British Stations for the Daily Weather Report, is shewn in the following table. It should be noted that up to the end of June, 1908, the morning and mid-day hours of observation were r* c^ ^ E.-S !^SE ^s J " : r^J 'rtf. >< ^ ^ r O W M 00 COO M O CO IO o s -is. ro M O IT) O CO M rf M K H 6 co ro Th co O l>x CO LO M O ^ |>> M CO M H s O M C1 CO g & R 01 pj ^2 -P o O O C 120 Glossary. In this case the number of years over which the obser- vations extend is given, quite an arbitrary number, and thus the numbers for frequency of occurrence have to be considered with reference to the number of years selected. It is, however, usual to reduce frequency figures to a yearly average. Here, for example, is the average frequency of GEOSTROPHIC winds from different quarters over the South- East of England and Northern France obtained from observations for the nine years, 1904-1912. Frequency of winds (geostrophic) from different quarters. Average Number of Days in the several months of the year in which the Wind is from a specified quarter. South-East of England and Northern France. N.E Fi SE. s s.w. w N.W. N Calms. Total. January ... 3 2 2 2 9 6 2 I 4 31 February .. 3 I I 4 7 6 3 2 I 28 March 4 2 I 3 7 5 2 2 5 31 April 5 2 I 2 6 5 2 4 3 30 May 4 2 2 2 5 4 2 2 6 29 June 5 I I 3 6 4 3 4 3 30 July 4 I I i 6 5 3 4 6 3 1 August ... 2 I I 2 10 6 3 2 4 3 1 September 4 3 2 2 5 4 3 4 o J 3 October ... 3 3 2 5 7 3 3 2 4 ^2 November 3 2 I 3 7 7 2 1 4 30 December.. i 2 2 7 10 4 2 I 2 31 The treatment of fractional parts of a day accounts for the two discordant totals. 121 In view of the awkwardness of having to bear in mind the possible number of occurrences, while considering the actual or average number, it is convenient to use the percentage frequency instead of the actual frequency. This plan is often adopted for giving the results of observations at sea, which are made six times a day, or every four.hours. For example," the percentage frequency of fog in the English Channel is given by the figures in the following, table : Percentage Frequency of Fog and Mist in the English Channel. [Based on four-hourly observations from ships during the 15 years 1891-1905.] Percentage of Percentage of Month* Number of observations. whole number of observations. whole number of observations. Fog. Mist. January 1,187 2'5 15-8 February . . . 1,185 2'8 22*9 March 1,241 3'8 2 4*5 April M 2 4 4-8 24-4 May 1,501 4*4 26*9 June 1,363 5'6 30-2 July 1,260 3*8 26*3 August 1,206 3' 2 I7-3 September ... 1,264 r,3 I7'0 October i,454 i-6 16-5 November ... ' 1,284 I *I 15-0 December ... 1,294 i*5 18-2 122 Qlossanj. Friction. A word used somewhat vaguely in meteor- ological writings in dealing with the effect of the surface of the sea or of the land, with its obstacles in the form of irregularity of surface, hills, buildings, or trees upon the flow of air in the lower layers of the atmosphere. The effect of the irregularities of surface is to produce turbu- lent motion in the lowest layer which gradually spreads upwards, if the wind goes on blowing, and consists of irregular eddies approaching to regularity in the case of a cliff eddy which can be noticed when a strong wind blows directly on to a cliff and produces an eddy with a horizontal axis. An account of the eddy caused by the Eastern face of the rock of Gibraltar is gi^en in the Journal of the Aeronautical Society, Vol. 18, 1914, p. 184. The general effect of this so-called friction is to reduce the flow of air past an anemometer so that the recorded wind velocity is below that which would be experienced if the anemometer were high enough to be out of the reach of the surface effect. Numerical values for this effect are of great practical importance, because they are concerned with the change of velocity in the immediate neighbourhood of the ground. But it is not easy to obtain them, because every exposure near land or sea is more or less affected, and, therefore, no proper standard of reference can be obtained by direct observation. Recourse is, therefore, had to the computation of the wind from the distribution of pressure, the so-called " geostrophic " or GRADIENT WIND. From the comparison of a long series of geostrophic and observed winds we conclude that over the open sea, or on an exposed spit of flat sand like Spurn Head, the wind loses one-third of its velocity from "friction," and at other well-exposed stations the loss is, on the average, as Friction. 123 much as 60 per cent., but for any particular anemometer it is different for winds from different quarters because the exposure seaward or landward is different. Infor- mation on this point for a number of Meteorological Office stations is given in a memoir by Mr. J. Fairgrieve (Geophysical Memoirs, Vol. 1, p. 189), and information for other stations is in process of compilation at the Meteorological Office. The consequence of this 4 effect can sometimes be seen in weather maps. On one occasion when the whole of the British Isles was covered with parallel isobars running nearly West and East, all the stations on the Western side gave the wind as force 8 (42 mi/hr) while those on the Eastern side gave force 5 (21 mi/hr), so that the velocity was reduced by one-half in consequence of the "friction" of the land. If the velocity at the exposed Western stations be taken at two-thirds the velocity of the wind free from friction, we get the following interest- ing result which is probably correct enough for practical use : One-third of the velocity is lost by the sea friction on the Western side, and one-third more by. the land friction of the country between West and East. Frost. According to British meteorological practice frost occurs when the temperature of the air is below the freezing point of water (see FREEZING) ; it may be either local, as a ground-frost, a spring-frost or a night- frost often is, or general, such as a frost which gives bearing ice in the course of three or four days. But the word would hardly be used unless there were water or plants or something else to be frozen, so that its use is generally restricted to the lowest levels of the atmosphere. We should hardly speak of a frost in relation to the cold of the upper air, or even of a mountain top. 124 Glossary. Meteorologically, the difference between the conditions for a general frost and those for a local frost is so great that different words are needed. The American meteor- ologists have some reason, therefore, in speaking of a general meteorological frost as " a freeze." The British Isles are accessible for a freeze or general frost in two ways, first by Northerly winds bringing cold air from the Arctic regions over the North Atlantic, round a high pressure lying over the Greenland-Iceland region, secondly by Easterly or South-Easterly winds coming round a high pressure over Scandinavia and Northern Europe, which in the winter is persistently cold. The Northerly wind has to cross a considerable stretch of the north-eastern extension of the Gulf -stream water, so that it has to travel quickly to avoid being warmed. Consequently, Northerly " freezes " are generally short and sharp. The more prolonged frosts are generally caused by the Easterly winds which have only a short stretch of sea to cross. A long freeze may begin with a Northerly wind, and snow, followed by a persistent Easterly wind. The short frosts, or night frosts, may occur with very light winds from any quarter except between South and West ; they are characteristic of clear nights, with great loss of heat from the ground by radiation to the clear sky. The conditions are set out in detail in a pamphlet prepared in the Meteorological Office and reprinted in " Forecasting Weather" Chapter XII. Low temperatures are often quoted as degrees of frost, meaning thereby the nu&iber of degrees below the freezing point of water. Gale. Wind with an hourly velocity of more than 17m/s, or 39mi/hr. The figure is selected as the lower Gale. 125 limit of force 8 on the Beaufort Scale. A wind estimated as force 8 or more is counted a gale. The relation of the estimation to the measured hourly velocity is subject to some uncertainty on account of the incessant fluctuations of velocity in a strong wind which are known as GUSTINESS. They are not shown in the records of the cup anemometer which were used for com- puting the equivalents. The number of gales recorded for any locality depends largely on the exposure of the anemometer, as the table on the next page shows. Judging by this table, anyone who is unacquainted with the practical difficulties of anemometry would be tempted to draw the conclusion that the localities represented by Kew, Falmouth, Aberdeen, Valencia and Yarmouth are immune from gales, or nearly so. For any purpose of aerial navigation such a conclusion would be egregiously untrue. It is true of the anemo- meters, but not of the free air above them. The records, which in these particular cases go back to 1868, are good enough when we are concerned only with comparing the wind of to-day with that of yesterday, or any other day in the last forty-seven years, and of determining normals for reference, the diurnal and seasonal variation, and so on ; but when we want to compare one locality with another we must face the problem of making allowance for the exposure of the anemometer. To meet this requirement we propose to give the basic characteristics of the localities under our observation as regards wind in terms of GRADIENT WIND, or more strictly geostrophic wind. It is a very voluminous inquiry, but is now nearly completed, and some of the results will be included in this volume. 126 Glossary. ,3 '3-^ Q i ^o M i' O O CO H O i-OOO M M M 0} s M M o To o ^ 0> *9UUJ[ o M H O O O O O CO Is J^" 1 4H te i "jCraj^T o M -t- M CO O 01 <*J .3 -** ^ r-H 0> CJ C8 t> }<5{ jiidY . o cn -f M LO O 00 O 4s ZQ a 0-3 ^* -+- o CD .43 P -d fl * rt ^ - !>, r^ ^t-i>. o O^ H CO - *> ^> ^ -^ |1s A'j^n.iqy^ in -i- -h O O^ O O^ W M o 1 j f'S cr 1 -^ ^"8 -^uu,, 01 -t- n oc LO o w 01 01 O 00 N Is * s si |>-: J9qraao9([ IJ^ ON M co o vO M M M rf o o 01 M So * eq 'V Pa *$l P Wqra8A o N ^ 90 H !>. 00 O w "S ^ |ig a o ^ _ci a $ ^qov.0 ,0 r^ co o o M 01 O 'Tf 3 1 'S S Ig^s "I9qui9^d9g M X M -f- O 01 O o 1 g c S) *s s '^snSny 01 M CO O 000 5 $ Ch B-SI .s ^!^nj[' o r^ o M o o o o ^ ^ % Number of recorded b d o 1 VI Valencia G P c -M .s" Fleetwood Holyhead Scilly Falmouth (Observator Deerness Aberdeen Yarmouth CD M Gale. 127 The seasonal variation may be expressed as follows : O fl OD 0nMoooc*0Mp>oovoo>- 1 42 real C^ rH -o^o^^^o^cr^ h rH .5* 'fe *? ! j9qni9AO^[ xi "^~ QO - 'jaqo^OQ oo *-t o OVOOOOOOOOOQ a\ o r* O " 43 a? f 3 o ^ g QD c j8qniaid8Qj t^O rl-^Oxort-C^cow - C^w W QQ QQ S N N N W $<* .9 2 ^ o ^nSny SSSSffSffaSffsaaS 1^ C8 03 I*V> ^ r^J 43 pq J A I n f HI HI HI f rS s '8unp ^^^^^OSN ^ N ^cso^*- SH *"* HI 1- 43 ^f ^ S O '^Bj^r s-sass's R'g.sa- :? *3 .2 .3 " ludy re? a 2 a-:? gulag's- pd HH <> CC qow M ^8*3* ""a:rs'' rf 2 -s 2 > QQ ^Cisnjqa^ ^g^^^-.OK.-^^JJ-OO i 2 ^ S 1 Xnuimr ^^^^^^^^^s:- 2 |f rg 2 2 -2 r* o d 1 frS CO r^ fl 2 r ^ ?H 0) 1 *0 1 COASTS. ^ > ^ ^' ^ ^ ' 1 s = s | ^j*li & JS 2sl (1 The " favourit 21 to 19 against. 128 For special localities tables of local statistics must be consulted. Some guidance may be obtained from the diagrams given under the heading wind. Gale- Warning*. Notice of threatening atmospherical disturbances on or near the coasts of the British Islands are issued by telegraph from the Meteorological Office to a number of ports and fishery -stations. The issue of a warning indicates that an atmospheric disturbance is in existence which will probably cause a GALE (Force 8 by BEAUFORT SCALE) within a distance of (say) 50 miles of the place to which the warning is sent. The place itself may be comparatively sheltered, and the wind may not attain the force of a gale there. The meaning of the warning is simply " Look out. Bad weather of such and 'mch a character is probably approaching you." The fact that such a notice has been received is made known* by hoisting in a conspicuous position a black canvas cone (gale-cone) 3 feet high and 3 feet wide at the base, which has the appearance of a triangle when hoisted. The " South cone " (point downwards) is hoisted in anticipation of gales and strong winds From S.E. veering to S.W., W., or N.W. S.W. W. or N.W. W N W vv . ,, J.-N . vv . The " North cone " (point upwards) is hoisted in antici- pation of gales and strong winds From S.E., E. or N.E., backing to N. N.W. veering to N., N.E., or E. N. N.E. or E. . N.E. E. * The display of cones and issue of notices to the general public has been suspended during the war. Gas. 129 The warning is intended to continue from the time the telegram leaves the Meteorological Office until 8 p.m. the following day. The gale-warning service of the British Isles was established under the direction of the late Admiral FitzRoy in 1861, and has been maintained in operation ever since, with a slight interruption in 1867. Gas. The name used for any kind of fluid which has unlimited capacity for expansion under diminishing pressure. It is to be distinguished from a liquid which has only a limited capacity for expansion under reduced pressure. A liquid may occupy only the lower part of a vessel like a bottle ; it will flow to the bottom of the vessel and leave a "free" surface. But a gas cannot be located in that way ; its volume is determined not by the amount of material but by the size of the vessel which contains it and by the pressure upon its boundaries. There are many different kinds of gas, such as nitrogen, hydrogen, carbonic acid, coal-gas, marsh-gas, and so on ; but the word is often used when coal-gas is meant, and recently it has been used for heavy poisonous gas of unspecified composition. In scientific practice gas means any substance which obeys approximately the gaseous laws ; these laws are two, viz. : 1. When the temperature is kept constant the pressure of a given mass of gas is inversely proportional to the volume which it occupies, or the density is directly proportional to the pressure. 2. When the volume is kept constant the pressure is proportional to the absolute temperature, or when the pressure is kept constant the. volume is proportional to the absolute temperature. GeostropMc.- See GRADIENT WIND. 13204 E 130 Glossary. Glazed Frost. When rain falls with the air-tempera- ture below the freezing point a layer of smooth ice, which may attain considerable thickness, is formed upon all objects exposed to it. This is known as glazed frost. The accumulation of ice is frequently sufficient to bring down telegraph wires. In these islands the phenomenon is one of comparative rarity. It must be distinguished from SILVER THAW, which occurs when a warm, damp wind supervenes upon severe cold, the moisture condensing on still-freezing surfaces and thus producing a coat of ice, similar in appearance to glazed frost. Super-cooled water-drops are said to be the cause of glazed frost. Glory. The system of coloured rings surrounding the shadow of the observer's head on a bank of cloud or fog or even of dew. It is a diffraction-effect due to the bending of rays of light round small obstacles, water- drops in this case. As in all diffraction effects the violet ring is nearest the centre, followed outwards by blue, green, orange, and red on the outside; the blue and violet are seldom seen. A Glory may be seen surrounding the shadow of an aeroplane on a cloud. Gradient. A convenient word rather overworked in modern meteorology. We use it in pressure gradient, temperature gradient, potential gradient, to denote different ideas. In pressure gradient for any locality we imagine the distribution of sea-level pressure to be mapped out by isobars ; take a line through the locality at right angles to the isobars nearest to it on either side and measure the step of barometric pressure which corresponds with a measured distance along the line from high pressure to low. This use of gradient was Gradient. 131 introduced by Thomas Stevenson, C.E., of the Board of Northern Lights. It corresponds with an engineer's use of the word gradient in specifying a slope from a map of contours, but to get the pressure-gradient we have first to determine the line along which the slope is steepest, so that pressure-gradient has a definite direction. There is a convention that the distance to be taken is 15 nautical miles and the step of pressure is to be given in hundredths of an inch. The gradient will work out at practically the same figure if the distance is a geographical degree and the step of pressure is given in millimetres. To get the same figure for the gradient with the step of pressure in millibars the distance would have to be taken as 45 nautical miles. But numerical values of the gradient are very seldom quoted. Temperature gradient may be based on the same idea and give the rate of change of temperature, along the horizontal through a locality, at right angles to the isotherms, as obtained from a chart of isotherms properly corrected for height. But it is much more frequently used to indicate the step of temperature for a kilometre step of vertical height. Used in this sense temperature gradient may be positive or negative, and by international agreement the temperature gradient is positive when the step is towards lower temperature for increasing height, because temperature generally decreases aloft ; but it does not always do so. The change from positive to negative temperature gradient is called an INVERSION of temperature gradient or simply an u inversion ", and so an " inversion " comes to mean a region where tempera- ture increases with height. Potential gradient is used for the change of atmospheric 13204 E 2 132 Glossary. electrical potential in the vertical, and for that alone. It is generally given in volts per metre. That also may be positive or negative and is taken to be positive when the potential increases with height. The word LAPSE (q.v.) has been adopted as a better name than gradient for these rates of change in the vertical. The pressure gradient is the one which comes most frequently into practical consideration, as it is closely related to the direction and force of the wind, so that the idea of pressure gradient should always be present in the mind of a student of weather maps, though the gradient may seldom be evaluated in figures. On looking over a map the localities where the gradient is steep will always be noticeable by the closeness of the isobars. The deter- mination of the pressure gradient is comparatively easy when the isobars in the locality are free from local irregularity and nearly parallel. There is then no difficulty in identifying the direction of the gradient, because the line drawn at right angles to successive isobars is approximately straight for a sufficient distance on the map. Experience is required to make a workable estimate of the gradient when the isobars are irregular. In practice tho gradient is not taken by setting out a length of 15 or 60 or 45 nautical miles, but by scaling the distance apart of consecutive isobars. It is most con- venient to express this distance in nautical miles, because 60 nautical miles make up a degree of latitude, and every map made for meteorological purposes is scaled according to latitude. If, for example, isobars are drawn for steps of JSQ' millibars, and the shortest line drawn to bridge two Gradient. 133 isobars across a station scales out at 75 nautical miles ; the gradient is 1 millibar (0*03 m.) for 15 nautical miles, or 3 on the conventional scale of pressure gradients. For calculation in C.G.S. units it is convenient to have the gradient expressed in terms of millibars for 100 kilo- metres. It is best to use a large scale map for obtaining pressure gradients so that intermediate isobars can be inserted by estimation when those drawn for the ordinary steps are not regular, but with the best maps the estimation of the gradient is sometimes uncertain on account of local irregularities of pressure which may be indicated on la barogram but cannot be allowed for in a map based on telegraphic reports from stations 100 miles (160 kilometres) apart. Some of the steepest authentic gradients that have been noted on British weather maps are : Date and Place. Gradient. Inter- Millibars national meaeure. per 100 kilometre. 1912, August 26, East Anglia (Norwich floods). 1907, February 20, between Ice- land and Faroe. II'O I0'0 13*2 12 'O 1912, November 26 land. , West of Scot- 9*7 II'7 134 Glossary. Gradient Wind. The flow of air which is necessary to balance the pressure-gradient. The direction of the gradient wind is along the isobars, and the velocity is so adjusted that there is equilibrium between the force pressing the air inwards, towards the low pressure, and the centrifugal action to which the moving air is subject in consequence of its motion. In the case of the atmosphere the centrifugal action may be due to two separate causes ; the first is the tendency of moving air to deviate from a GREAT CIRCLE in consequence of the rotation of the earth ; the deviation is towards the right of the air as it moves in the Northern hemisphere, and towards the left in the Southern. The second is the centrifugal force of rotation in a circle round a central point according to the well known formula for any spinning body. In this case we regard the air as spinning round an axis through the centre of its path. This part of the centrifugal action is due to the curvature of the path on the earth's surface. Both components of the centrifugal action are in the line of the pressure gradient : the part due to the rotation of the earth is always tending to the right in the Northern hemisphere, the part due to the curvature of the path goes against the gradient from low to high when the curvature is cyclonic, and with the gradient when it is ancicyclonic, so that in the one case we have the gradient balancing the sum of the components due to the earth's rotation and the spin, and in the other case the gradient and the spin-component balance the action due to the earth's rotation. The formal reasoning which leads up to this result is gi /en at the end of this article. The method used therein Gradient Wind. 135 for calculating the effect of the rotation of the earth was suggested to the writer in 1904 by Sir John Eliot, F.R.S., Director of the Indian Meteorological Service. For the sake of brevity in reference to these two components it is very convenient to have separate names for them. . Let us call the one due to the rotation of the earth the geostrophic component,* and the one due to the curvature of the path the cyclostrophic component. Consider the relative magnitude of these components under different conditions. It will be noticed that the geostrophic component depends upon latitude, the cyclo- strophic component does not, so, other things being equal, their relative importance will depend upon the latitude ; so we will take three cases, one near the equator at latitude 10 within the equatorial belt of low pressure, one near the pole latitude 80 of undetermined mete- orological character, and one, half - way, between, in latitude 45, a region of highs and lows travelling Eastward. Using V to denote the wind-velocity, when the radius of the path is 120 nautical miles the cyclostrophic com- ponent is equal to the geostrophic in latitude 10 when V is 5*6 metres per second ; in latitude 45 when Y is 22*9 metres per second ; in latitude 80 when V is 31-9 metres per second. It will be seen that in the equatorial region the cyclo- strophic component is dominant as soon as the wind reaches a very moderate velocity. * A table to find the geostrophic component is given on pp. 172, 173. 136 Glossary. EQUATION FOR GTEOSTEOPHIC WIND. 2 he Relation between the Earttts Rotation and the Pressure Distribution for Great- Circle-Motion of Air. The rotation co of the earth about the polar axis can be resolved into co sin about the vertical at the place where latitude is and co cos about a line through the earth's centre parallel to the tan- gent line. The latter produces no effect in deviating an air current any more than the polar rotation does on a current at the equator. The former corresponds with the rotation of the earth's surface counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere under the moving air with an angular velocity co sin 0. We therefore regard the surface over which the wind is moving as a flat disc rotating with an angular velocity co sin 0. By the end of an interval t the air will have travelled Vt y where V is the " wind-velocity," and the earth underneath its new position will be at a distance Vt X co t sin 0, measured along a small circle, from its position at the beginning of the time t. Taking it to be at right angles to the path, in the limit when t is small, the distance the air will appear to have become displaced to the right over the earth is Fco -sin 0. This displacement on the "igt 2 " law (since initially there was no transverse velocity) is what would be produced by a transverse acceleration 2 co Fsin 0. . . the effect of the earth's rotation is equivalent to an acceleration 2 co Fsin 9, at right angles to the path directed to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. In order to keep the air on the great circle, a force corresponding with an equal but oppositely directed acceleration is necessary. This force is supplied by the pressure distribution. Gradient Wind. 137 EQUATION FOB CYCLOSTROPHIC WIND. Force necessary to balance the acceleration of air moving uniformly in a small circle, assuming the earth is not rotating. Let A be the pole of circle PRQ. Join PQ, cutting the radius OA in N. Acceleration of particle moving uniformly along the small circle with V' 2 V 2 velocity Fis -^ along PN = =-. - where R = radius of earth ; Jri\ jj, sin p and o is the angular radius of the small circle representing the path. The horizontal component of this acceleration, that is, the component F 2 cosp F 2 along the tangent at P, is p . = -- cot p. GENERAL EQUATION CONNECTING PRESSURE GRADIENT, EARTH'S ROTATION, CURVATURE OF PATH OF AIR AND WIND VELOCITY. I. Cyclonic motion. The force required to keep the air moving on a great circle in spite of the rotation of the earth must be such as to give an acceleration 2w V sin directed over the path to the left in the Northern Hemisphere. It must also compensate an acceleration due to the curvature of the path, F- cot p JR, by a force directed towards the low pressure side of the isobar. For steady motion these two combined are equivalent to the acceleration due to the gradient of pressure, i.e., *- where I) is the 138 Glossary. density of the air, and y the pressure gradient, directed towards the low pressure side. y F 2 .*. = 2 (j Fsin + cot p. 1 II. Anticyclomc motion. In this case 2 w F sin and ^ are directed outwards from the region of high pressure, and the equation becomes y F 2 -/ = 2 u> Fsin cot p. D R Gramme. The unit of mass on the C.G.S. system. It is one- thousandth part of the standard kilogramme, Gramme. 139 which was originally constructed to represent the weight of a litre (cubic decimetre) of water. A gramme is equivalent to 15*4 grains, or rather more than one-thirtieth of an ounce. A pound is equivalent to 454 grammes. For GRAMME- CALORIE see p. 301. Grass-temperature. For estimating the effect of RADIATION from the Earth's surface at night a minimum thermometer is exposed just above the surface of short grass, so that the bulb does not actually touch the grass. Abroad the thermometer is sometimes laid on the grass itself. Gravity. See p. 308. Great Circle. A line on the earth's surface which lies in a plane through the centre of* the earth's figure. All meridian lines are great circles so is also the equator, but all lines of latitude, with the exception of the equator, are small circles since their planes do not pass through the earth's centre. The visible horizon is a small circle. The great circle which passes through two points on the earth's surface is made up of the shortest and the longest track between the two points. The shortest track is less than a semicircle, the longest greater than a semicircle. Gulf Stream. A warm ocean current that flows out of the Gulf of Mexico along the coast of Florida. It is ascribed to ttie action of the TRADE WINDS which cause a mass of water to flow into the Gulf from the East. The current near Florida is strengthened by water which branches from the main trade wind current and flows outside the Antilles. The Gulf Stream flows Northward into the region of prevailing Westerly winds, which 140 Glossary. cause a current to flow slowly Eastward across the Atlantic ; this current, also called the Gulf Stream, carries water from the Gulf Stream proper to the coasts of Europe. The air no doubt has its temperature slightly raised by the warm current, but our temperate climate is due to prevailing Westerly and South Westerly winds, which are also the cause of the Eastward extension of the Gulf Stream. Gust. A " coup de vent." The word was used originally for any transient blast of wind, but is now limited to the comparatively rapid fluctuations in the strength of the wind which are specially characteristic of winds near the surface of the earth, and are probably due to the turbulent or eddy motion arising from the FRICTION offered^ by the ground to the flow of the current of air. The subject of gusts, as indicated by a tube-anemo- graph, has been investigated for the Advisory Com- mittee for Aeronautics by the Meteorological Office, and the results are contained in four reports on Wind Structure published in the annual reports of the Com- mittee. The number and extent of the fluctuations are very irregular ; they have been counted as seventeen in the minute, but another count would probably give a different figure. If the wind be regarded as fluctuating between a gust and a lull, the range between gusts and lulls is dependent on the one hand on the mean velocity of the wind, and on the other hand upon the nature of the exposure of the anemometer. Expressing the fluc- tuations as a percentage of the mean velocity we get the following results for various anemometers. (Report of the Advisory Committee, 1910.) Gust. 141 Anemometer. Range of Fluctuation as a Percentage of the Mean Velocity. Southport (Marshside) Scilly (St. Mary's) Shoeburyness, E.N.E. wind W. wind Holy head (Salt Island) Falmouth (Pendennis), S. wind ... W. wind... Aberdeen 30 per cent. 50 30 80 50 25 50 IOO Alnwick 80 Kew IOO In this table a fluctuation of 100 por cent, means that a wind with a mean velocity of 30 miles per hour fluctuates over a range of 30 miles per hour, between 15 miles an hour and 45 miles an hour, in consequence of the gustiness. The most gusty exposure within the experience of the Meteorological Office is at Dyce, in Aberdeenshire, where, for the purpose of inquiry, an anemometer was installed by Dr. J. E. Crombie, with its head projecting 15 feet above the tree-tops of a small wood. Gusts are to be distinguished from squalls. A squall is a blast of wind occurring suddenly, lasting for some minutes at least, and dying away as suddenly. A squall is attributable to meteorological causes, whereas gusts are the result of mechanical interference with the steady flow of air. m/s. 1905. Pendennis, 46-0 1906. Scilly, 38-4 1907. Southport, 36-2 1908. Scilly, 37'6 1909. Scilly, 40*2 1910. Pendennis, 38-9 1911. Eskdalemuir, 40*2 1912. Pendennis, 43'8 IQI3- Southport, 38-4 1914- Quilty, 4I*O ^S- Pendennis, 4O'O 1916. r\ Pendennis, 40-8 142 Glossary. The strongest gusts recorded on anemometers of the Meteorological Office in recent years are : mi/hr. 103 86 81 84 90 87 90 98 86 92 89 91 Gustiness. The name given to the factor which is used to define the range of the gusts shown on the record of an anemometer. The gustdness of an interval is the factor, (maximum velocity minimum velocity) -f- mean velocity. The figures given for the fluctuations of wind in the records of various anemometers given a^ove may be called the " percentage gustiness " of the winds. To obtain an estimate of the relative gustiness of the winds in the upper air, Mr J. S. Dines used the pull of a kite wire defining the gustiness as (maximum pull minimum pull) -f- mean pull. Using this method it appears that gustiness falls off rapidly in the first 500 feet of ascent, and thereafter it is irregular. (Second Report on Wind Structure, p. 10.) Hail. Usually described as frozen raindrops, though hailstones are often very much larger than any raindrop an possibly be. Hail is formed in the columns of rapidly Hail. 143 ascending air that are part of the mechanical process of a rain-storm or thunderstorm. They are associated \vi h the cumulo-nimbus type of cl>ud. The convection currents which begin with insta' ility in the atmosphere result, first in heavy cloud, and then in raindrops still carried upward in air which is automatically becoming colder in consequence of the diminished pressure. So the drops may freeze, and then any further upward journey may result in condensation in the form of ice on the already formed hailstone. To maintain a mass of water or ice in the air a very vigorous ascending current is required. If a raindrop reaches a certain size it is broken up into smaller drops by the current which is necessary to keep it from falling, but when the hailstone is once formed there is no limita- tion of that kind upon its growth. From their structure, which is often very composite, it is clear that hailstones have a long history, and from their size, which may be large enough to give measurements, it is said, of three or four inches in diameter, a pound or more in weight, they must have required ascending currents of great velocity to support them. There is, however, evidence to show that some of the strongest winds of the earth are katabatic winds, that is, they are due to falling air, so it requires only a special adjustment of the temperature of the environment to give rise to currents of rising air, anabatic winds, of the most violent character. (SOFT HAIL, see p. 343.) Halo. The term halo is an inclusive one applied to all the optical phenomena produced by regular REFRAC- TION, with or without accompanying reflection, of the 144 Glossary. rays of the sun or moon in clouds consisting of ice- crystals (see CLOTD, Cirro-Nebula). The most common halo is a luminous ring of 22 radius surrounding the sun or moon, the space within it appearing less bright than the rest of the sky. The ring, if faint, is white if more strongly developed its inner edge is a pure red, while yellow and green follow, more faintly. Next in order of frequency of occurrence is a similar but larger ring of 46 radius. MOCK SUNS are simply more brilliant patches occurring at certain definite points in a halo system. There is a great variety of minor and rarer halo phenomena. (For some of these see Observer's Handbook, p. 57.) In polar regions, where ice crystals extend much lowe^ in the atmosphere, halo systems attain great brilliance and complexity. Halos are very varied in form, they are produced by the REFRACTION of the sun's rays, or the moon's rays, through a cloud of ice crystals forming what is called cirro-nebula or cirrus-haze, one of the highest forms of cloud. They are of great interest from the point of view of the physics of the atmosphere, but they have no^ meteorological significance. In weather lore they are' often spoken of as presaging storms and it is possible that the ice cloud is one of the earliest results of the fall of pressure with which the storm is associated ; but the formation of a halo is not by any means a necessary step in the preparation of a storm ; many storms arrive without announcing their coming in that way. Moreover, the appearance of a halo at the end of a spell of dirty weather is said to be a sign of clearing. We may perhaps conclude that cirro-nebula, with no other clouds in the sky to interfere (the condition for seeing a halo), may be found at the beginning or the end of a depression. Harmattan. 145 Harmattan. A very dry wind which is prevalent in Western Africa during the dry season (November to March). During these months, (the winter of the Northern Hemisphere) the air over the desert of Sahara cools rapidly, owing to its clearness and lack of moisture, so that it tends to flow outwards to the coast, especially south-westwards to the Gulf of Guinea, and replace the lighter air there. Being here both dry and relatively cool, it forms a welcome relief from the steady damp heat of the tropics, and from its health-giving powers it is known locally as " The Doctor," in spite of the fact that it carries with it from the desert great quantities of impalpable dust, which penetrates into houses by every crack. This dust is often carried in sufficient quantity to form a thick haze, which impedes navigation on the rivers. Harmonic Analysis. See p. 311. Haze. Obscurity of the atmosphere which may occur in dry weather and may be due to dust or smoke, or merely to irregularities of density and consequent irregular refraction of the light by which distant objects are seen. During HARMATTAN winds off the West Coast of Africa, dust haze is thick enough to be classed as fog. At sea the weather is often classed as hazy when there is no distant horizon, and yet no visible mist or fog. The obscurity may, however, be due to water particles. It would therefore be desirable to limit the use of the word haze to occasions when the air is not very damp, that is when there is a noticeable difference between readings of the wet and the dry buib thermometers. Heat. The name used for the immediate cause of the sensation of warmth, a primary sensation which is easily recognised and needs no explanation. As used in relation 146 Glossary. to the weather, heat and cold are familiar words for opposite extremes of temperature of the air. What the American writers call a heat-wave is a spell of hot wenther in which the maximum temperatures reach 90 or 100 F. (above 305a.), and a cold-wave is a spell of the opposite character during which temperatures in the neighbour- hood of the Fahrenheit zero, or 32 degrees of frost, may be experienced. In continental climates, during the passage of severe cyclonic depressions, the transitions from heat to cold are sometimes extremely abrupt and far-reaching ; a difference of temperature of 50 F. in a few hours is not unknown. We have visitations of similar character in this country, but they are less intense. A few days in succession with a temperature over 80 F. would suffice for a heat-wave, and a few days with 10 of frost would certainly be called a cold-wave. One of the most noticeable features of our climate is the succession of cold spells which interrupt the genial weather of late spring and early summer. They are not very intense, but a drop in the mean temperature of the day from 55 F. to 45 F., which roughly defines them, produces a very distinct impression, As used in connexion with the study of the atmosphere heat has another sense which must not be overlooked. It denotes the physical quantity, the reception of which makes things warmer, and its departure makes them colder. If you wish to make water hot, you supply heat to it from a fire or a gas-burner or, in modern days, by an electric heater, a very convenient contrivance for getting heat exactly where you want it. On the other hand, if you want water to become cooler, you leave it where its heat can escape, by CONDUCTION, aided by CONVECTION or by RADIATION. You can also warm water by adding Heat. 147 some hot water to it, or cool it by adding cold water to it. Either process suggests the idea of having the same quantity of heat to deal with altogether, but distributing it, or diluting it, by mixing. The idea of having a definite quantity of heat to deal with, and passing it from one body to another is so easily appreciated and so generally applicable, that the older philosophers used to talk confidently of heat as a substance which they called Caloric, and which might be transferred from one body to another without losing its identity. They measured heat, as we do still, by noting by how much it would raise the temperature of a measured quantity of water. For students of physics the unit of heat is still a gramme-calorie, the heat which will raise a gramme of water through one degree centigrade. To raise m grammes from ^C to t 2 C, m( 2 -i) gramme calories are required. The amount can be recovered, if none has been lost meanwhile, by cooling the water. If we wish to be very precise, a small correction is required on account of the variation in what is called the capacity for heat of water at different temperatures, but that need not detain us. For students of engineering the unit, called the British Thermal Unit, is a pound-Fahrenheit unit instead of the gramme-centigrade unit, and the heat required to raise m pounds of water from ^F to 2 F is m (t 2 - tj B.T.U. It is in many ways a misfortune that students of Physics and Engineering do not use the same unit. It is no doubt a good mental exercise to learn to use either indiscriminately without confusion, but it takes time. From measurements of heat we get the idea that with different substances the same change of temperature requires different quantities of heat ; the substances have 148 Glossary. different capacities for heat. We define capacity for heat as the heat required to raise a unit of the substance (1 gramme or 1 Ib.) through 1 degree. It is a remarkable fact that of all common substances water has the greatest capacity for heat. It take* one unit to raise the temperature of a unit mass of water one degree, it takes less than a unit, sometimes only a small fraction of a unit, to raise the temperature of the same amount of another substance through one degree. We give the name specific heat to the ratio of the capacity for heat of any substance to the capacity for heat of water. Numerically, specific heat is the same as the capacity for heat in thermal units. The specific heat of water is 1, the specific heat of any other common substance is less than 1. The specific heat of copper is only 1/11. So the heat which will raise the temperature of a pound of copper 1 will only raise the temperature of a pound of water 1/11, or the heat which will raise the temperature of a mass of water 1 will raise the temperature of the same mass of copper 11. This peculiar property of water makes it very useful for storing heat and carrying it about. From that point of view it is the best of all substances for cooling the condenser of an engine, for distributing heat at a moderate temperature in a circulating system, and for many other economic purposes. In meteorology its influence is very wide. Large masses of water, of which the ocean is a magnificent example, are huge store houses which take up immense quantities of heat from the air when it is warm and give it out again when the air is cold, with very little change in its own temperature, so that a large lake, and still more the ocean, has a great influence in reducing the extremes HeaL 149 of temperature of summer and winter, and of day and night, in the countries which border it. There is another remarkable storage of heat in which water takes a predominant share that is dealt with in physical science under the name of latent heat. Water at 288a. (59 F.) cannot be evaporated into water-vapour unless every gramme of it is supplied with 589 calories of heat, which produce no effect at all upon the temperature. -.The water is at 288a. to start with, and the water- vapour is at exactly the same temperature and yet 589 calories of heat have gone. They are latent in the water- vapour but produce no effect on the thermo- meter. You can get them back again easily enough if you condense the vapour back again into water, but you must manage somehow to take away the heat while the condensation is taking place. The separation of the u waters that are above the firmament from the waters that are below the firmament," or in modern language, the evaporation of water from the sea or a lake or the wet earth and its condensation in the form of clouds and rain, implies the transference of enormous quantities of heat from the surface to the upper air, the dynamical effect of which belongs to another chapter of the romantic story of heat which deserves more than the few words which we can afford for it. Readers can find an interesting account in Tyndall's Heat a Mode of Motion. The idea of heat as an indestructible substance, caloric, which could be transferred from one body to another without loss, became untenable when it was found that when air was allowed to expand iri a cylinder it cooled spontaneously to an extent that corresponded exactly, so far as could be ascertained, with the means then available, with the amount of mechanical work that the 150 Glossary. cylinder was allowed to do. It was the last step in the process of reasoning by which men had come to the con- clusion that, when mechanical work was devoted to churning water or some other frictional process, heat was actually produced, not brought from some other substance but created by the frictional process. It took many years for men to reconcile themselves to so novel an idea, and a good deal of ingenuity was devoted to trying to evade it, but it has now become the foundation stone of physical science. Heat is not an unalterable indestructible substance but a form of energy. It can do mechanical work in a steam-engine or a gas engine or an oil-engine, but for every foot-pound* of work that is done a corresponding amount of heat must disappear, and in place of it a corresponding amount of some other form of energy is produced. A good deal of heat, besides, may be wasted in the process so far as practical purposes are concerned. In a steam engine, of the whole amount of heat used, only one tenth may be transformed, the rest wasted, as we have said ; but it is still there raising the temperature of the water of the condenser or performing some other unproductive but necessary duty. There is, therefore, a numerical equivalent between heat and other forms of energy. We give the relation : 1 B.T.U. is equivalent to 777 foot-pounds of energy. 1 gramme-calorie = 42,640 gramme-centimetres. = 41,830,000 ergs. * Afoot-pound of work is the work done in lifting one pound through. a distance of one foot. A gramme-centimetre is the work done lifting one gramme through one centimetre. An erg is the absolute unit of work on the C.G.S. system ; 1 gramme centimetre = 981 ergs. Heat. 151 We have led up to this statement in order to point out how extraordinarily powerful heat can be in producing mechanical energy. If, in the operations of nature, one single cubic metre of air gets its temperature reduced by l c 0. in such a way that the heat is converted into work by being made to move air, the equivalent of energy would be a cubic metre of air moving with a velocity of nearly 45 m/s. (101 mi/hr.). So familiar have we become with heat as a form of energy that we measure the heat of sunlight in joules* and the intensity of sunshine in watts per square centi- metre, i.e.. the number of joules falling on one square centimetre per second. THE SPECIFIC HEAT OF AIR. The foregoing statement is necessary to lead up to a matter of fundamental importance in the physics of the atmosphere, namely the heat that is required or used to alter the temperature of air in the processes of weather ; in technical language this is the capacity for heat of air or the specific heat of air. We have explained that when air is allowed to do work on its environment, in expanding, heat disappears, or more strictly is transformed. So the amount of heat required to warm air through a certain number of degrees depends upon how much expansion is allowed during the process. The most economical way of warming air from the * A joule is a more convenient unit than the small unit, the erg ; one joule = ten million ergs (10 7 ergs) and one calorie = 4*18 joules. The icatt is a unit of power, that is, rate of doing work ; a power of one watt does one joule of work per second. 152 Glossary, thermal point of view is to prevent its expanding altogether ; it then has " constant volume " and its specific heat is 0*1715 calories per gramme per degree at 273 a. It is remarkable, but true, that if you have a bottle full of air, it will take more heat to raise the temperature of each gramme of it by a degree if you take the stopper out while the warming is going on, than if you keep it tight. The difference between warming a bottle of air with the stopper in and with it out, simple as it may seem, has got in it the whole principle of heat as a form of energy. The effect of leaving the stopper out is that the pressure of the air inside the bottle is the atmospheric pressure for the time being and is therefore practically constant throughout the brief operation. So we get the specific heat of air at constant pressure 0*2417 gramme-calories per gramme per degree, or 1*010 joules. The specific heat of air at constant volume is 0*72 joules. The difference of the two represents the heat equivalent of the work used in expanding unit mass of the gas against atmospheric pressure. High. Sometimes used as a contraction for high barometric pressure. The technical term anticyclone was coined by Sir F. Galton for the purpose, but, whether for the sake of brevity or for some 'other reason, a " high " is often spoken of. Hoar Frost. A feathery deposit of ice formed upon leaves and twigs in the same way as DEW (q.v.) by the .cooling of exposed objects through the radiation of their heat to the clear sky. Horizontal in the plane of the horizon. The surface of still water is horizontal. In dynamics and physics a Horizontal. 153 horizontal line is a line at right-angles to the direction of the force of gravity which is vertical and identified by the plumb line. The Visible Horizon, or Distance of Visibilitl| for objects of given height 1,000 900 800 700 600 500 400 500 200 100 10.000 9.000 8000 7.000 6,000 5.000 4000 3.000 2.000 1.000 \ / \ / / / =f I / / o | c ft 7T/ y 5 1 I / <^ ^ / S I/ ^ / / MMes. 10 20 30 40 50 60 70 80 90 100 110 120 130 Miles Diagram showing the relation between the height of an observation point in feet and the distance of the Visible Horizon in miles {neglecting refraction), or the height in feet of a cloud or other distant object and the distance in miles at which it is visible on the horizon. The " sensible or visible horizon " which is visible from a ship at sea, the line where sea and sky apparently join, is a circle surrounding the observer a little below the plane of the horizon in consequence of the level of the earth's surface being curved and not flat. The depth of the " sensible horizon " below the " rational horizon " or horizontal plane is approximately the same as the elevation of the point from which the " sensible 154 Glossary. horizon " is viewed. Apart from any influence of the atmosphere the distance of the visible horizon for an elevation of 100 feet (30 metres) is about 12 miles. The actual distance is about 2 miles greater on account of refraction. It varies as the square root of the height, so that it would require a height of 400 feet to give a horizon 24 miles off. A level canopy of clouds 10,000 feet high is visible from a point on the earth's surface for a distance of about 125 miles, or the visible canopy has a width of 250 miles. Horse Latitudes. The belts of calms, light winds and fine, clear weather between the TRADE WIND belts and the prevailing Westerly winds of higher latitudes. The belts move North and South after the Sun in a similar way to the DOLDRUMS q.v. Humidity, in a general sense means dampness, but in meteorology it is used for RELATIVE HUMIDITY and means the ratio of the actual amount of aqueous vapour in a measured volume of air to the amount which the volume would contain if the air were saturated. (See AQUEOUS VAPOUR.) In practice, at climatological stations, the humidity of air is determined from the readings of the dry and wet bulbs with the aid of tables prepared for the purpose and called humidity tables or psychrometric tables. But humidity is the most variable of the ordinary meteorological elements, as it depends not only on the sample of air under observation but also on its temperature. Hence the record of a self-recording hair- hygrometer which can be obtained in a form not much different from an ordinary barograph gives a most instruc- tive record. In the spring and summer it sometimes Humidity. 155 shows very high humidity in the night and early morning, approaching or actually reaching saturation, and very great dryness, perhaps only from 15 to 20 per cent, humidity, in the sunny part of the day, with very rapid changes soon after sunrise and towards sunset.* These are the changes which correspond with the characteristic changes in the feeling of the air at the beginning and end of the day. Hurricane in French, ouragan, in German, Danish and Swedish, orkan. " A name [of Spanish or Portuguese origin] given primarily to the violent wind-storms of the West Indies which are cyclones of diameter of from 50 to 1,000 miles, wherein the air moves with a velocity of from 80 to 130 miles an hour round a central calm space which, with the whole system, advances in a straight or curved track ; hence any storm or tempest in which the wind blows with terrific violence " (New English Dictionary), The hurricanes of the Western Pacific Ocean are called typhoons in China, and .baguios in the Philippine Islands. Those of the Indian Ocean, which are experienced in India, are called by the Indian meteoro- logists cyclones of the Arabian Sea or of the Bay of Bengal ; while the hurricanes of the South Indian Ocean which visit Mauritius are also called cyclones. Shakespeare uses the word hurricano for a water-spout. Overleaf is a reproduction of a barogram showing the variation of pressure during a cyclone which passed over Cocos Island, Sumatra, in 1909, November 27th. It is interesting to notice that in spite of the rapid fall of pressure with the onset of the cyclone the diurnal varia- tion of the barometer is still apparent and it reappears before the normal level is recovered. 156 Glossary. Coco s Island ttarogram November, 1909 The occurrence of hurricanes shows a marked seasonal variation. The following table is taken from the Barometer Manual for the Use of Seamen. Variation of the Barometer during a Hurricane. 157 lO 10 00 CS CN vO to Tt" M \O tO ' r^oj, CO CO M CS i i . co * VD ^5 aaqraaooQ; H 5 ' ^ 10 ON OO vO 1>> !>. rh a X W . i i i i 11 S -wqowo S' to cs M IT) ft [udy ^ a - ON 10 H H M 00 qo.rej\[ JjJJ ON M to C< to i ^^ 1 CO j <^ tonuqaj t>. ** M 1 M 1 M 1 rs 1 -.fcumn.. to K. " s co i3 r- oc oo ^ >j OO hi "5> CO O r1 I 2 00 > S "^r 10 bo fl ^ " O S fl S 1 P I West Indi P! ^ 1 ^s J 1- ! 02 cq cs 02 o> >; b 1 1 1 ^3 f_* ,4 43 a 9 o a a 49 p o & 9 be 5 CS 0> ft p o> H 1-3 ^ s "-3 -5 <1 r jj fc p 5* N. 90 O'O O'O O'O 1-4 6-7 9'9 7*9 2-4 O'l O'O O'O O'O 28-4 80 O'O O'O 0'2 2-7 7*5 10-3 8'5 3*8 0-5 O'O o-o O'O 33*5 60 O'l I'O 3*9 8-2 I2'oi3'8 I2'6 9-2 4*9 i'5 O'2 O'O 67*4 40 3*3 5*7 9*4 12-9 15*316-2 15-6 13*5 I0'2 6-6 3*8 2-7 115-2 20 9-0 II-2 13*6 15-2 15-815-9 15-8 15*3 H'c 11-7 9*4 8'2 I55*i Equator s. 14-0 14-9 15*3 14-6 13*5 I2'8 13*1 14-2 15-0 14-2 13-6 170-2 20 16-815-9 13*9 II'2 8-8 7*7 8'3 10-5 13*1 is;3 i6'6 17-0 155-1 4 16-6 13-9 9*9 6-0 3-4 2'4J 3*0 5*2 8-8 15-9 17*3 115*2 60 I3'4 9*2 4*4 1-3! o-i o-o, o-i 0-8 3*4 7*8 12-3 14-6 67*4 8o c 90 8'8 3*5 2' I 0-4 o-o O'O O'O O'O O'O O'O O'O O'OJ O'O O'l O'O O'O O'O 2-3 I'O TA II'O 10-5 33-5 28-4 The unit is the amount of energy that would be received on unit area at the equator in one day, at the equinox, with the sun at mean distance if the atmosphere were completely transparent. It is 458'4 times the solar constant, or in gramme calories per minute 885, taking the solar constant to be 1-93. 13204 Glossary. Inversion. An abbreviation for u inversion of temperature-gradient" (see GRADIENT). The tempera- ture of the air generally gets lower with increasing height but occasionally the reverse is the case, and when the temperature increases with height there is said to be an " inversion ". There is an inversion at the top of a fog-layer, and generally at the top of other clouds of the stratus type. Inversions are shown in the diagram of variation of temperature with height in the upper air, p. 38, by the slope of the lines upwards towards the right instead of towards the left, which is the usual slope. In the troposphere inversions do not generally extend over any great range of height ; the fall of temperature recovers its march until the lower boundary of the stratosphere is reached. At that layer there is generally a slight inversion beyond which the region is isothermal, so far as height is concerned. For that reason the lower boundary of the stratosphere is often called the fc< upper inversion ". In some soundings with ballons-sondes from Batavia the inversion has been found to extend upwards for several kilometres from the commencement of the strato- sphere. It is important also to note that frequently in anti- cyclonic weather, and especially cold anticyclonic weather, there is often an inversion at the surface ; the temperature increases upwards instead of decreasing. Ion. The name selected by Faraday for the com- ponent parts into which a chemical molecule is resolved in a solution by the electrolytic action of an electric current. Of the two component ions one is always elec- tro-positive, and the other electro-negative. The electro- Ion. 165 negative ions consist of atoms of oxygen, chlorine or some other corresponding element or radicle, and the electro- positive ions consist of atoms of hydrogen, potassium, or some other metallic element or radicle. Each electro- positive ion is called a cation. It is charged with a definite quantity of positive electricity and travels with the electric current to the cathode, the conductor by which the current leaves the solution, while the electro- negative ion, called the anion, is charged with an equal quantity of negative electricity and travels against the electric current to the anode, the conductor by which the current enters the solution. It is supposed that a solution which will conduct an electric current is ionised by the spontaneous dissociation of the components of its molecules and the consequent formation of free ions carrying their appropriate electric charges. In a solution, recombination and dissociation are constantly going on and the electric current causes the free ions charged with positive electricity to move slowly with the current and those charged with negative electricity to move against the current. Similarly, a gas may conduct electricity to a less extent, but in the same way, as a solution when it contains free ions, which may be produced by the action of radio- active agents, ultra-violet light, very hot bodies, the combustion of flame and in other ways. The conduction of electricity through the atmosphere is now, therefore, attributed to the free ions which exist in it, and its capacity for conducting electricity is attributed to its ionisation. The ions in the air may be atoms of hydrogen or oxygen, or they may be aggregates of those atoms witfe some other material. ] 66 Glossary. A certain number of the ions in atmospheric air doubt- less arise from the radio-active materials in the soil. These materials give rise to an emanation, as it is called, which must gradually reach the surface through the pores of the soil, The supply will naturally depend on- the state of the soil, whether damp or dry, frozen or covered with snow, and presumably also on variations of the baro- metric pressure which promote or check the escape of emanation. Other ions must be produced by light from the sun. These will naturally chiefly arise at considerable heights above the ground, where sunlight is stronger and relatively richer in ultra-violet light than near the ground. In addition there seems to be some other powerful source at high altitudes, possibly some form of electrical radiation from the sun. lonisation. See p. 322. Iridescence or Irisation, words formed from Iris, the rainbow, to indicate the rainbow- like colours which are sometimes seen on the edges of clouds ; tinted patches, generally of a delicate red and green, sometimes blue and yellow, occasionally seen on cirrus and cirro-cumuius clouds up to about 25 from the sun. They may be also seen at times on the edges of fracto-cumuius or strato- cumulus clouds. The boundary between the two tints is not a circle with the sun as centre, as in a CORONA, but rather tends to follow the outline of the cloud. They are probably not due to the refraction of light by water drops, which produces the colours of the rainbow, but to the diffraction of light scattered by the very small water drops, and are to be classed like the corona with the iridescence of the opal and the mother-of-pearl. Diffraction-colours formed in artificial clouds in the Isanomalies. 167 Bame way as in the corona, become more brilliant as the cloud gets older and the drops more uniform in size. Hence it seems probable that an iridescent cloud is an old cloud that has been drifting for some time. Isabnormals. See ISANOMALIES. Isanomalies. This word is a combination of the prefix iSo- and the word ANOMALY, which, like the more common adjective anomalous, signifies departure or deviation from normal. The normals used for reference are obtained on various plans, Normals of temperature have been obtained by taking the general mean of the observed temperatures of successive parallels of latitude and thus assigning a normal temperature to each latitude ; isanomalies of temperature are then the departures of mean temperature for any place from the normal for its latitude ; places that are relatively warm for their latitude have a positive anomaly, and places that are relatively cold for their latitude a negative one. Isanomalies are then lines on a map showing equality of departure of the average temperature of any place from, the normal for its latitude. There are, however, not many meteorological elements which can be said to have a normal value for latitude, and it is usual to employ as normals for any place the average or mean value for a long period of years. In that case departures, or differences from the normal for the corresponding period, of the value for any one period, say a month or a year, are called ABNORMALS or abnormalities, and a chart showing equality of departure from the normal a chart of ISABNORMALS. Isabnorinal is an objectionable compound because it is made up of a Greek prefix and a Latin body ; if the departures are to be called abnormals the lines of equal departure ought to 168 Glossary. be called equi-abnorrnals, so that the tendency is to use ISANOMALIJSS for lines of equal departure of a value from its long period average. Isentropic. Without change of ENTROPY (q.v.) gen- erally equivalent in meaning to ADIABATIC. ISO. The prefix ISO- is the Greek equivalent of the Latin EQUI- and implies the setting out of lines on a chart or diagram to show the distribution of set values of some meteorological element. The words with this prefix can generally be interpreted 'by the reader on this basis : some examples are set out under the separate headings below. Thus : ISOBARS, from baros, are lines on a chart showing equal barometric pressure. ISOHELS, from helios, are lines showing equal dura- tion of sunshine. ISOHYETS, from huetos, are lines showing equal amounts of rainfall. ISOPLETHS, from plethos, lines showing equal amounts of a meteorological element. The word isogram was recommended for this pur- pose by Sir Francis Galton. ISOTHERMS, from therme, are lines showing equal temperatures. Isobars. If some of the air were removed from a room the pressure inside would be reduced and the pressure outside would force air through the windows and doors till the room was again filled with the normal amount of air. If over any area some of the air were by any means removed the pressure over that area would be reduced, and the pressure of the air in the surrounding Isobars. 169 districts would tend to force air into the region of deficient pressure. Such areas of deficient pressure are found to exist, but for the following reason the air does not actually {low into them. Anything moving above the surface of the earth will continue to move in a straight line if no force acts on it ; but the earth in its rotation turns under the. moving body ; the moving body is therefore apparently deflected to the right in the Northern Hemi- sphere. This is true of a cannon-ball, and it is true of a moving current of air. Hence the wind does not actually blow into the area of low pressure ; air from the North is deflected to the West side of the area, air from the West to the South side, and so on ; the wind therefore instead of blowing straight in from all sides blows round an area of low pressure counterclockwise in the Northern Hemisphere. We thus have the apparent paradox of a force tending to push the air into the centre of the low pressure area while the air is actually moving round the centre at right-angles to the force that is acting on it. There are many examples of similar things in nature ; the Earth's motion round the sun for instance ; or the water in a basin when there is a hole in the centre from which the plug has been taken out ; the slightest circular motion sets up a swirl, and the water moves round the basin at right-angles to the force of gravity which is tending to force it towards the hole. In the case of an area of high pressure the air blows out from the high pressure, but it is deflected to the right as in the former case, with the result that the wind blows round the area of high pressure in a clockwise direction. In either case if you stand with your back to the wind the low pressure will be on your left hand, this is BUYS BALLOT'S LAW. In the Southern Hemisphere the reverse is the case. 170 Glossary. If the observed heights of the BAROMETER (reduced to sea level) from a number of places are put on to a map and arrows are put in to represent the direction of the wind, we have a weather map which at first sight looks like a disordered collection of figures ; we may make it clearer, however, by drawing lines through places where the barometer stands at the same height ; thus we may draw one line through all places where the barometer stands at 1,015 mb., another through all places where it stands at 1,010, and so on. Such lines are called ISOBARS. It may happen that we cannot find any station where the barometer stands at say 1,015 mb., but if we find one where it stands at 1,016 and another where it stands at 1,014 we take it that the 1,015 line passes midway between the two stations. When the isobars are drawn in we can readily see the shapes of the areas of . low and high pressure, and we see also that the wind blows in accord- ance with Buys Ballot's law. The areas of low pressure are called CYCLONES, DEPRESSIONS, or simply LOWS ; the areas of high pressure ANTICYCLONE?, or HiGHS. The isobars are analogous to contour lines on an ordinary map, the high pressures" corresponding to the hills, the low pressures to the valleys. The moving air does not go straight from the highs to the lows, but it blows round the highs in a clockwise, and round the lows in a counter- clockwise direction. On the contours of the earth we may descend from a height of say 1000 feet to 500 feet by a gentle slope many miles in length, or in another place we may descend by a precipitous scarp ; in the former case a stream will run down sluggishly, in the second it will be a swift torrent full of rapids and waterfalls. So with the pressure ; we may travel a Jong way between a place where the baro- meter reads, say, 10^0 millibars to another where it reads Isobars. 171 1015 millibars, or we may have to go only a comparatively short distance. The steepness of the gradient on a map is measured by the distance between the contour lines ; the steepness of the barometric gradient is measured by the nearness of the isobars. The strength of the wind depends on the steepness of the barometric gradient, just as the velocity of the stream depends on the steepness of the slope, but the analogy is not quite perfect for the stream runs down the slope across the contour lines, whereas the wind blows nearly along the isobars with a slight inward curvature towards the low pressure. In the case of the wind close to the surface if the five- millibar isobars are 400 miles apart the barometric gradient is sJight, and the wind will be about 10 miles per hour ; if the distance apart is 60 miles the gradient is steep, and the wind will be about 70 miles per hour. The wind calculated from the barometric gradient is called the GRADIENT WIND or, if no allowance is made for the cur- vature of the path of the air, the GEOSTROPH1C WIND ; it is in most cases the wind met with at or about 1500. feet ; nearer the surface, owing to the friction, the actual wind is less than the gradient wind. The gradient wind as stated depends principally on the distance apart of the isobars ; it is modified, however, to a small extent by the variations of density of the moving air and therefore by the height of the barometer and by the temperature ; a table is given on pp. 172-173 showing the geostrophic wind for various pressures and temperatures according to the formula of p. 136. The values are dependent upon the latitude and are given in the table for two latitudes 52 and 40. Further tables are given in the Computer's Handbook MX). 223. Section II. 172 Glossary. fl.ls-.9i gainst ?s*1gS . |H*m g. O 43 Agsli &!i bc-d* g rs j;*lfi' ^ a ."S ^ t> >i ^:iw^ g-a^|5a l^oJ^SJg 1^23^. o o * O ^ p,^^ o o F> M M CO ft 00^ ON M S 5 a 1C ^ M CO ON K r> H-T co *4-3 ^ i CO r^ o T^- cs CO ^t- 10 ON 01 NO 0^ co 01 01 00 r^ ON 2 o O Th rf- vJ OQ g H r^ ON ON CO R CO M 10 co OO OO r^ 10 <* co t^ M 3- S a p -1-3 rf o of JJ 3 1 1 CO 10 rf s CO vO M n M oo M M 10 M ON Ol M co M H M M M 0^ 1| g.2 M VO ro T 0) vO co Tf ON n oo M 01 R M 3 M 10 01 M $ & r^ oo "5 ^ 13 & O i do 10 % N CO 8 01 vO M 10 CO M NO M M M H 8, 00 dS -*^r o o 05 73 d -u oo !>. ro vO LO t^ . CO r^ 10 cs ON oo M lO M NO 01 M OO M lO ON Tj- OO NO r^ d 5 02 +3 3 LO ^q to ON r^ t^ 1O xO co CO Tf n oo M ^ M 01 01 M ? M M ON M 00 CO r^ O M ir> ro 01 g, M lO CO $ c , , , ;: i. with the wiftd. 8k * i doa ded - Iky * clouded - Force, on- the scale 0-12, is indi- |flv overcast sky. A rain falling cated by number of feathers ^ gnow ^ hai ]^ s fog Calm /^S =mist, T thunder. 1C thunderstorm Plate XIV. V-SHAPED DEPRESSION, DISTRIBUTION OF WEATHER, WIND, AND PRESSURE, 7 A.M. 8th OCTOBER, 1916. ISOBARS are drawn for intervals of ten mill!- WEATHER. Shown by thefol!owtogyrnboji bars. WIND. Direction is shown by arrows flying with the wind. Force, on the scale 0-12, is indi- cated by the number of feathers. clear iky. Q ky i clouded. sky \ clouded. ^ sky f clouded. overcwt sky. f rain falling snow. A hail. = tog. Tthnnder. TCthunderiVor Isobars : Lithoplate XIV. Ill At times, especially in summer, very small depressions are apt to form ; several such depressions are sometimes seen on the weather map for the same day ; they follow the same laws as large depressions, but being shallow they do not usually occasion much wind; they bring heavy rains and thunderstorms in the summer. An ANTJ CYCLONE,^ or high-pressure system, is the contrary to a depression ; here the barometer is high in the centre, the isobars are usually more or less circular or oval, they are also usually further apart than is the case in a depression, therefore the winds in an anticyclone are usually lighter. An anticyclone is not so small as the smaller depressions, though the large ones may equal it in size. It often coyers a large area. An anticyclone moves but slowly and irregularly ; it may remain in the same position for many days, or even weeks, at a time. The weather in an anticyclone is usually fine and bright, though extensive cloud sheets may form, and fogs are prevalent in winter ; rain seldom falls, and persistent rain never. The depression and the anticyclone are the main arrangements of isobars, but there are five other shapes each of which has its characteristic weather. The SECONDARY DEPRESSION. Sometimes in the neighbourhood of a depression, usually on its southern side, the isobars take a slight bend outwards, marking the position of a small centre of low pressure ; such a depression usually travels forward in the same direction as the main depression, and may even outstrip it in rate of travel. It usually produces much rain, and sometimes much wind. In the summer secondary depressions are * Itttrstraifttms ~oT~ the various types of isobars are given under the respective heading's ANTICYCLONE, SECONDARY. &c. 17.8 Glossary : Lithoplate XV. often very shallow, and resemble the shallow depressions already noticed ; like them they occasion heavy rain and thunderstorms. When they are secondary to deep depressions they often cause a crowding together of the isobars on their southern side, and thus occasion strong winds. The V-SHAPED DEPRESSION. This is a further extension of the Secondarj T ; the isobars, instead of bulging out slightly, extend out a long way in the form of the letter V. The wind on the east side is from a southerly point, that on the west side from a northerly point, in accordance with Buys Ballot's law ; the east side is a region of cloud and rain, often heavy driving rain ; over the central line there is an abrupt change, very fine weather being experienced on the west side. As the Y-shaped depression moves over any place the observer experiences southerly winds and driving rain, both wind and rain becoming stronger as the central line approaches ; as it passes over there is a sudden change of wind to a northerly point ; the rain stops, and the sky rapidly clears ; the central line is often a region of heavy squalls ; there is a marked fall of temperature with the passage of the central line. See Plate XV. WEDGE OF HIGH PRESSURE. Between two depressions there is oiten a region of high pressure where the isobars are shaped like an inverted V ; the high-pressure wedge usually extends from an anticyclone poleward between two depressions that are skirting the northern edge of the high pressure. The wedge moves forward in an easterly direction between the two depressions ; it is in fact merely the relatively high pressure between the depressions. The front of the wedge is often a region of extremely fine weather with northerly winds, rapidly becoming light as Plate Xl/ SECONDARY DEPRESSION. DISTRIBUTION OF WEATHER, WIND, AND PRESSURE, 7 A.M. 22nd FEBRUARY, 1015. ISOBARS are drawn for internals of ten tan, WIND. Direction ii shown by arrows flyiaf with the wind. Force, on the scale 0-12. is indi- cated by the number of feathers /^N W1AT1IW. -Shown by thetollowlBf symbols - clear iky. Q iky $ clouded. jn sky | clouded.