QC 
 
 99U.li 
 
 HUlr 
 
 ENGINEERING 
 
 MATHEMATICAL 
 
 SCIENCES LIBRARY 
 
 THE RELATION ON PEESSUl 
 TEMP' -,ND Aln CIRCULATION OVKR 
 E SOUTH ATLANTIC 
 OCEAN 
 
 BY 
 M.W.C. HEPrORTH
 
 UNIVERSITY OF CALIFORNIA 
 AT LOS ANGELES
 
 M.O. 177 (1917). 
 
 THE RELATION 
 
 BETWEEN 
 
 PKKSSIfKK. TEMPERATURE, AND AIR CIRCULATION 
 OVER THE SOUTH ATLANTIC OCEAN, 
 
 NOTES WITH REFERENCE TO A SET OF MONTHLY WIND CHARTS OF THE 
 
 SOUTH ATLANTIC OCEAN WHICH WERE PREPARED IN THE METEORO- 
 
 LOUUJALOFFIUfi AND VVKUE ORIGINALLY PUBLISHED BY THE HYDROQRAPHIC 
 
 DEPARTMENT OF THE ADMIRALTY, IN JANUARY, 1904. 
 
 SECOND EDITION. 
 
 M. W. CAMPBELL /HEPWORTH, C.B., R.D. 
 
 Captain It.N.R., Marine Superintendent. 
 
 bt> tfjr Hutljoriti) of tf)r flfceteorologiral iffamnuttee. 
 
 LONDON : 
 !M 'HLIHIIKD BY HIS MAJESTY'S STATIONERY OFFICE. 
 
 To be purchased through any Bookseller or directly from 
 
 H.M. STATIONERY OFFICE at the following addresses: 
 
 IMPERIAL HOUSE, KINGSWAY, LONDON, W.C.2, and 
 
 28, ABINGDON STREET, LONDON, S.W.I; 
 37, PKTER STREET, MANCHESTER; 1, ST. ANDREW'S CRESCENT, CARDIFF; 
 
 23, FORTH STRKKT, EDINBURGH; 
 
 or from E. PONSONBY, LTD., 116, GRAFTON STREET, DUBLIN; 
 or from the Agencies in the British Colonies and Dependencies, 
 Hit- I'nited States of America and other Foreign Count Hi 
 T. FISHER T. \\VIX, LTD., LONDON, W.C. 
 
 1917. 
 Price In. AW. 
 
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 Engineering & 
 
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 Sciences 
 
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 ~ The discu8sion of the meteorological observations for the South 
 * Atlantic and the South American Coastal Regions of the 
 Pacific occupied the attention of the Marine Department of the 
 Meteorological Office from 1898, when the corresponding work 
 upon the Southern Ocean was completed, until 1904. The total 
 number of observations extracted from the Log Books of the Royal 
 Navy, and the Meteorological Logs contributed by the Captains 
 and Officers of the Mercantile Marine whose ships crossed the 
 region referred to, amounted to 946,000. They have been 
 grouped according to five degree squares of latitude and longi- 
 tude, or in special cases according to one degree squares, and the 
 mean results for Wind, Air Pressure, and Air Temperature were 
 embodied in a series of monthly charts prepared under Captain 
 Hepworth's superintendence in accordance with the direction of 
 the Meteorological Council and, in particular, of the late Hydro- 
 grapher of the Navy, Rear-Admiral Sir W. J. L. Wharton, 
 F.R.S., who was ex officio a member of the Council. 
 
 Upon the completion of the Charts the Hydrographer under- 
 took the issue of them as a publication of the Hydrographic Office. 
 The information contained in the Charts will, it is hoped, be found 
 to be of considerable interest and importance to mariners. At 
 the request of the Council, Captain Hepworth undertook the pre- 
 paration of some notes giving the general results which follow 
 from an examination of the Charts. They formed the subject 
 matter of a paper read before Section A. of the British Association 
 at Cambridge in 1904, and are now issued by the authority of the 
 Meteorological Committee. 
 
 Charts for the Coastal Regions of South America were issued 
 first in 1902, and the issue of the completed work for the South 
 Atlantic followed m 1904. 
 
 By way of illustration of some of the general features of the 
 average meteorological conditions of the South Atlantic, some 
 additional Charts on small scale have been prepared. They show 
 the variations in the course of the year of the position and 
 intensity of the areas of high pressure in the regions under 
 consideration, and the position and extent of the equatorial region 
 
 (11354 12.-) W't. 7107^874. 350. 7/17. P fr S. G. 3,
 
 of doldrums in relation thereto, and the average distribution of 
 rales and of fog. Captain Hepworth's notes call attention to the 
 points which are illustrated by these additional Charts, and also 
 to the effect of the air circulation upon the distribution of tempera- 
 ture, which is found to diverge considerably from the parallels of 
 latitude, even over an area of such uniform geographical character 
 as the South Atlantic. 
 
 For the facts upon which the notes depend, the Wind Charts 
 themselves should be referred to, but it is thought that sailors and 
 others interested in marine meteorology may wish to have the 
 general conclusions in a separate form. 
 
 W. N. SHAW, 
 
 Director. 
 
 Meteorological Office, 
 
 63, Victoria Street, S.W. 
 August, 1905. 
 
 NOTE. 27 March, 19.17. Not much has been added to our 
 knowledge of the South Atlantic Ocean since the M.O. Charts were 
 published in 1904, and the call for a new edition of Captain 
 Hepworth's notes can therefore be met with some slight alterations 
 of the text and the conversion of the pressure-readings in order to 
 correct them for latitude in accordance with international con- 
 vention, and to bring them into accordance with the practice of 
 the Meteorological Office, adopted also since January, 1917, by the 
 Bureau Central Meteorologique of Paris, of expressing pressure 
 in millibars. 
 
 NAPIER SHAW. 
 Meteorological Office, 
 South Kensington, S.W. 7.
 
 THE RELATION 
 
 BETWEEN 
 
 PRESSURE, TEMPERATURE, AND AIR CIRCULA- 
 TION OVER THE SOUTH ATLANTIC OCEAN. 
 
 WIND AND BAUOMETEU. 
 
 The South Atlantic offers an excellent field for the study of 
 air circulation on a comprehensive scale and under normal con- 
 ditions. An ocean covering 1 an extensive area, connected north 
 and south with great oceans and completely open to the south, it is 
 at the same time free from tne disturbing- influences of island 
 groups, and is bounded east and west by continents having 
 coastlines that are, for the most part, exempt from large 
 irregularities of outline. The atmospheric circulation over the 
 South Atlantic may therefore be regarded as one vast wind system, 
 its air currents undisturbed over the northern half by the 
 occurrence of aerJal eddies for the most part ; and over the southern 
 half, although at times interrupted, yet not effaced by them. 
 
 The relation between Pressure, Temperature, and Air Circu- 
 lation, over this area, also over the eastern margin of the South 
 Pacific Ocean, is well shown by a set of monthly wind charts pre- 
 pared in the Meteorological Office and published by the Admiralty, 
 entitled \Vind (.'harts for the Soutlt Atlantic, Ocean. These charts 
 embody the general results of a very large number of observations 
 of wind and simultaneous readings of the barometer and therrno- 
 meteru 
 
 The anticyclone, or area of high bar. metric pressure, is the great 
 feature of the system. Round this central high pressure a general 
 circulation of air is indicated. 
 
 This circulation of air is assisted on the eastern side of the 
 ocean by the relatively low pressure over South Africa ; on the 
 northern, side by the low pressure over the equatorial regions of 
 the Atlantic ; on the western side by the relatively low pressure; 
 over South America ; and on the southern by the lower pressure 
 of higher 'latitudes. The direction of the wind about the core of 
 high pressure, stated generally, is Southerly to South-easterly 
 between it and the African coast; South-easterly and Easterly, 
 towards the equator ; Masterly to Northerly on its western side; 
 ;ind .Vorth -westerly to South -westerly on its southern. Over the 
 eastern and northern segments of the system the flow of the air 
 current is steady ; over the western and southern, particularly over 
 the former, the circulation is less persistent. Southward of 80 
 8. lat., and even farther to the northward, on the western side of 
 the South Atlantic, the apparent normal circulation of the surface
 
 wind, iii harmony with the course of the average isobars, is largely 
 masked by the effect of low pressure systems travelling eastward 
 or south-eastward. 
 
 These low pressure systems occur more frequently frCin May to 
 October, inclusive, than in other months, and are most frequent 
 in July and August. South' of 40 S. lat., the westerly winds, 
 although temporarily interrupted by depressions on their 
 approach, are intensified by them on their retreat. These 
 depressions generally appear on the ocean between the 20th and 
 40th parallels, and the track of those approaching the margin of 
 the relatively high pressure to the eastward is frequently turned 
 south-eastward. The cool northward-flowing air current, on the 
 eastern side of flie ocean, reinforces the area of high pressure 
 which acts as a barrier to eastward moving- depressions; the 
 warm southward-flowing air current, on the western side, induces 
 conditions favourable to t'he formation or advance of disturbances 
 in that region. Thus the most striking characteristic of the dis- 
 triBulion of winds, shown on the charts referred to, is the steady 
 south-easterly current on the eastern and northern sides of the 
 Tropical high pressure, and the varying winds on the western side. 
 The varying character of tEe winds on the western side must be 
 attributed to the passage of low pressure areas mentioned above 
 across the region, or to the south of it. Observations in these 
 latitudes are too few to enable us to identify the character or trace 
 the track of these depressions over the ocean, but observations over 
 the adjacent land area of South America occasionally show low 
 pressure areas which pass away over the ocean not far from the 
 localities referred to. 
 
 Four charts showing isobars over South America, see Plates I. 
 and II., taken from the Daily Weather Reports issued by the 
 Argentine Meteorological Office, may serve to illustrate the 
 relation of the moving depressions to the general distribution of 
 pressure over the South Atlantic. The dates given on the charts 
 indicate the days on which the low pressure systems were 
 observed ; the dotted lines over the ocean show the distribution of 
 mean pressure for fhe month. Areas of low pressure are shown, 
 from time to time, by the Daily Weather Charts of Argentina , 
 to reach the sea on a parallel as far north as 20 8. lat. ; but 
 observations are sparse between the 20th and 30th parallels, and 
 no daily synoptic charts are published for Brazil. 
 
 Log books in the possession of the Meteorological Office, how- 
 ever, record numerous instances of strong winds associated with 
 low pressure systems experienced in the vicinity of the Brazilian 
 coast, between 20 S. and 30 S., 40 W. and'50 W., many of 
 which attained gale force. In the years 1855 to 1899 no less than 
 184 gales were recorded in these Registers, and in all of these 
 instances the force of the wind reached, or exceeded, 8 of Beaufort 
 Scale, and the strongest gale recorded attained force 11 of that 
 scale. The gale forces cannot be assigned to winds from any 
 particular direction ; they come from every quarter. The passage 
 eastward of a cyclonic depression over the River Plate, or its 
 neighbourhood, appears to occur with some regularity at about 
 the end of August.
 
 Plaie I. 
 
 rhe isokan over 
 S. Amrica,in full lints, 
 stiofi the tiixtr'ibution 
 of pressure at 7 a.m. on 
 Jan. 22nd; those over 
 \ the Ocean, in dotted 
 ! Unas, tha mean pressure 
 \ f or ihe month. 
 
 20 O 
 
 JAN. 22, 1903. 
 
 7 A.M. 
 
 4 \ 
 
 { i \ \ 
 05 A 
 
 
 i 
 
 
 
 
 1 
 
 1 T~T 
 
 : 
 
 
 
 
 
 ! 1 ! 
 
 
 
 
 
 S.Ai 
 
 afprassura : 
 June 18th; 
 
 tr 
 
 srs onf 
 oa,in tul 
 distribi 
 n at 7 1 
 MA; the. 
 n, in do 
 metn ot 
 onth. 
 
 iO!O 
 
 'i 
 
 !fiT' 
 
 i.m. /? 
 'a over 
 tied 
 ensure 
 
 J 
 i 
 
 I 
 
 UNE 18, 
 
 7 A.M. 
 _ j,0<5 ( 29 ' 
 
 1903. 
 
 \ 
 
 \ 
 
 ! 7 
 
 
 
 (30>tj 
 
 ^^ 
 
 
 ^ 
 
 ^. 
 
 - v 
 
 
 N % 
 
 -,\ 
 
 
 
 
 - - IOIS (' 
 
 23^7)-. 
 
 
 **-... 
 
 ri 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Illustrating the passage of low pressure systems over the Ocean. 
 
 The Lsobara east of the 40th moridiaa refer to normal pressure diatribufcions. 
 
 Wyman & Sons Ltd., M.O. Preus, London, S.W.-7. Ps. 887*. Pa. 2774. 7107/874. 860. 6/17.
 
 Ploie II. 
 
 r 
 
 The isobars otror 
 S. A;nsrica.i/i i 
 afiew the distribution 
 of pressure at 7 a.m. on 
 Aug. 7th .- thosa over 
 tha Ocean', in ticttad 
 lines, the msan pressure 
 for the month. 
 
 
 AUG. 7, 1903. 
 
 ninst.rating the passage of low pressure 8ystema over the Ocean. 
 
 The Isobars east of the 40th meridian roferto normal pressure distributions. 
 
 Vywan & Son, Lsd.. M.O. Press, London, S.W. ~. Pa. 2847. Pa. 2774. 7107/87*. 860. 6/17.
 
 On the eastern margins of the South Atlantic, and South Pacific 
 north of 35 S. dat., and in a measure also on the western margin 
 of the former, nortK of the same parallel, when the conditions of 
 the atmosphere are stable, there appears a marked inclination for 
 the wind to follow the course of the littoral. This probably is 
 connected with the tendency of barometric pressure over the land 
 to conform to the contour of the coast-dine, resulting from a slight 
 temperature and consequent pressure gradient, between land and 
 sea. 
 
 Throughout the 12 months the shape of the isobar bounding the 
 area of highest pressure, 1020 nib. (3(H2 ins.) or 1023 mb. 
 (30'21 ins.), is approximately that of an ellipse. The major axis, 
 however, does not always make the same angle with the meridian ; 
 and the axes vary, not only in position, but also in length, from 
 month to month. 
 
 Over the South Pacific throughout the 12 months there is an 
 area of high barometer readings, the South Pacific anticyclone, 
 which is elliptical in shape, and is indicated by an isobar of 
 1020 mb. (30'12 ins.) except in August, when the highest value 
 shown is 1023 mb. (30-21 ins.). 
 
 Small charts showing the intensity and position of the region 
 of liigH pressure over the <South Atlantic and eastern margin of 
 the Pacific in each month, are given on Plates III. to VI., and, 
 as having a close connexion with the changes shown on these 
 charts, the mean latitude of the belt of doldrums in the Atlantic 
 is also indicated. 
 
 The general features of tlie distribution of pressure, wind, and 
 temperature, at the various seasons of the year, are as follow : 
 
 December to February inclusive. Over the South Atlantic east- 
 ward from the area of highest pressure to the African coast ; 
 northward and north-westward to the equator, and coast of South 
 America, as well as over its eastern and northern segments, a 
 steady trade wind is found mainly from some point between S.E. 
 and E., but the wind becomes a Southerly or even South-south- 
 westerly on the African side, and North-easterly or even 
 Northerly near South America where the trend of the coast-line 
 is the dominant facfor. To the westward of the area of highest 
 pressure, stated broadly, the direction of the prevailing wind 
 following the course of the isobars, is North-easterly, although 
 depressions passing* eastward and south-eastward partly mask the 
 circulation conforming to the average distribution of pressure, and 
 near the South American coast the influence of the land upon the 
 direction of the wind can be clearly traced. South of the 35th 
 parallel, which is about the latitude of Monte Video, the prevail- 
 ing winds have some westing in them, and a good deal of northing, 
 except where they appear to be influenced by the land west 
 of the 40th meridian between tn~e 35th and 40th parallels. 
 
 South of the 40th parallel Westerly winds largely predomi- 
 nate in all months of the year ; and the relation between the slope 
 of the gradient and the strength of the wind is well shown by 
 the percentage of gales for the regions where the isobars close up.
 
 8 
 
 Between the 40th and 55th parallels the pressure gradient is 
 almost always Inwards the south, and .Easterly winds are corre- 
 spondingly rare. 
 
 Over the eastern margin of the South Pacific from 30 S. 
 northward, the trade winds blow steadily; they are chiefly from 
 a South-easterly direction, but near the South American Continent 
 the prevailing- winds flow parallel to the outline of the coast. 
 Southward of 35 S., in accordance with the trend of the isobars, 
 the air current is South-westerly, where not deflected by proximity 
 to land; southward of 40 S. a Westerly air current predominates. 
 
 March to June inclusive. Over the South Atlantic to the east- 
 ward of the high pressure area, as well as over its eastern and 
 northern margins; and from 20 S. northward to the equator, 
 the air current is mainly from some point between S. and E., 
 except where deflected by land influence. Westward of the 
 20th meridian between 20 S. and 35 S. it has a good deal of 
 northing and easting, but is not constant; and this area probably 
 is visited by many small disturbances and is subject to varying 
 influences. South of 35 S. westerly winds predominate. 
 
 Over the south-eastern margin of the South Pacific, the area 
 over which the south-east trade winds prevail, is gradually 
 reduced, so that in -June they are not constant south of the 20th 
 parallel. After March easterly winds become less and less 
 frequent south of the 25th parallel; south of the 40th the westerly 
 winds are well established. 
 
 Julu t<> September inclusive. Over the South Atlantic there 
 is a general resemblance between the highest pressure areas of 
 the first and this the third period; but in the one case we have a 
 1019 nib. (30'09 ins.) ellipse travelling 5 to the westward, and in 
 the other case a 1023 nib. (30*21 ins.) ellipse moving 5 to the 
 eastward. This contrary travel in longitude may perhaps 
 be accounted for by the fact that the sun is well south of 
 the equator from December to February, making the African 
 land relatively warmer than the ocean to the westward : 
 whereas he is well to the north of the equator from 
 July to September, making the African land in the same parallels 
 as the anticyclone, relatively cooler than the ocean to the west- 
 ward. The lower pressure over Africa in the first case would 
 restrict the ellipse of higher pressure on the eastern side, and the 
 higher pressure over Africa in the other case would extend it 
 eastward. During these three months the 10.19 mb. (3() - 09 ins.) 
 isobar widens out, east and west, and overlaps the continents of 
 Africa and America, and at the same time moves slightly to the 
 northward. The isobar 1023 mb. (30'21 ins.) does not occupy 
 a ciMiiral position in regard to the isobar 1019 mb. (30'09 ins.), 
 but is situated somewhat to the south thus steepening 
 the gradient for westerly winds, between the 30th and 40th 
 parallels. The region of south-east trade winds is more restricted, 
 in the South Atlantic, than during any other period ; but north of 
 a straight line drawn from Port Nolloth on the African coast to 
 Bahia in Brazil, the air current is still fairly constant from a 
 south-easterly quarter, when it is not deflected by proximity to 
 the land. Southward of this line to 35 S. the normal circulation 
 round the central high pressure is to some extent maintained, but
 
 Plate HI.
 
 Plate IV. 
 
 18
 
 Plate V.
 
 Plate VI. 
 
 Ss 
 
 Z -2J 
 Q I 
 
 |1 
 
 u S 
 
 ii 
 
 I 
 I 
 
 x 
 o 
 
 I
 
 the winds are not constant; southwards of 35 S. the circulation 
 becomes re-established and westerly winds predominate. 
 
 Over the eastern margin of the South Pacific a south-easterly 
 air current largely predominates as far south as the 30th parallel, 
 except where proximity to land induces a more southerly, or even 
 a south-westerly, direction according- to the trend of the coast- 
 line. 
 
 October and November. Over the South Atlantic north of the 
 3()th parallel, as far west as the 10th meridian of west longitude, 
 and north of the 15th parallel westward, the south-east trade wind 
 is steady. West of the 10th meridian between the 15th and 301 li 
 parallels the tendency for northing in the prevailing winds due 
 f<> the trend of the isobars becomes more marked the more westerly 
 the position ; south of the 30th parallel the winds have more west- 
 ing the more southerly the position. The occurrence of easterly 
 gales 'from 30 W. to 40 W. between 45 S. and 50 S. i's 
 remarkable. 
 
 Over the eastern margin of the South Pacific northward of 
 30 S. the south-east trade wind is steady, and is fairly steady 
 between 30 S. and 35 S. near the land, but its direction here, as 
 to the northward, in the neighbourhood of the coast, is largely 
 that of the, coast-line. 
 
 AIR TEMPERATURE. 
 
 There is a general relation to be found between the direction of 
 the prevailing wind and the average temperature of the air over 
 the South Atlantic, on the eastern and western sides of the 
 permanent area of high pressure. 
 
 A deviation of the isotherms from the east and west direction 
 of the lines of latitude is found on the eastern and Avestern sides 
 of the high pressure area ; the south-east trade wind produces a 
 cooling effect upon the air on the eastern side, and the north- 
 easterly wind a warming effect upon the air on the western side. 
 
 Throughout the year there is a marked tendency for air iso- 
 therms to extend along a line drawn from Ihe Cape of Good Hope 
 to the Island of Ascension, under the influence of the relatively 
 cool air of the south-east trades which blow on the African side 
 of the high pressure, and in association with the cool sea surface 
 current setting north-westward. While, at the same time, the 
 relatively warm north-east wind on the South American side of 
 the anticyclone, which is in association with the warm sea .surface 
 current setting south-westward, exercises a similar influence \ipon 
 the air isotherms in the vicinity of South America. 
 
 From December to February, inclusive, the south-east trade 
 \vind carries the eastern ends of the air isotherms considerably 1o 
 the north-west of the average latitude for that temperature. The 
 average maximum limit of effect for the three months is in about 
 2 N". 18 W., although the range during the three months is 
 from 5 N. 20 W. to 5 S. 15 W. 
 
 Near the American coast the effect of the north-east wind ; - 
 shown in the isotherms even as far to the southward rts 37 S., 
 ranging from 40 S. to 35 S. Consequently, on an -iverage. the 
 air isotherm of 70 F., for sample, reaches the const of Africa in 
 17 S., but dips to 34 S. on the coast of South America.
 
 10 
 
 On the eastern margin of the South Pacific, as a general rule, 
 the relatively cool southerly wind, corresponding- with the 
 prevailing cool sea surface current which sets to tlie north ward, 
 has an appreciable effect in lowering the temperature of the air 
 near the west coast of South America. From. December to 
 February, inclusive, the cool southerly wind aft'ects tlie direction 
 of the air isotherms, forcing their easterly ends to the north-west, 
 from 40 S. to 5 S., thus indicating a lower air temperature 
 than is experienced along the same parallels of latitude farther 
 to the westward. 
 
 Over the South Atlantic during the four months March to June, 
 inclusive, the mean of extreme limits of the cooling effect of the 
 south-east trades as indicated by the air isotherms, is 3 S.16W. ; 
 with a range from 8 S. 15 W. to 5 N. 20 W. The southern 
 limit of influence of the north-east wind, on the western side of 
 the ocean, gradually travels northward, month by month, from 
 30 S. to 23 S., and the average is found in about 25 S. The air 
 isotherm, of 70 F., on an average, terminates in 16 S., on the 
 eastern side, but in 29 S. on the western side of the South 
 Atlantic. 
 
 Over the eastern margin of the South Pacific during these 
 months, the cooling effect is noticeable from 40 S. to the 
 equator. 
 
 Over the South Atlantic throughout the three months July, 
 August, September, the furthest extension of the cooling effect of 
 the south-east trades is in about 10 N. 20 W. ; and the warming- 
 effect of the north-east wind is at its southern limit in 30 S. The 
 air isotherm of 70 F. terminates, on an average, in 9 S. on the 
 coast of South Africa, and in 25 S. on the coast of South 
 America. 
 
 Over the eastern margin of the South Pacific during this period 
 the .southern limit of effect due to the southerly wind is shown 
 to have retreated 5 to the northward and is now in 35 S., while 
 the cooling of the air can be traced to the equator. 
 
 Over the South Atlantic during October and November, the 
 extreme limit of the effect of the south-east trades is in 5 IS". 
 20 W., and the southern limit of that diie to the north-east wind 
 in 30 S. The isotherm of 70 F. terminates in 15 S. on the 
 African coast, and in 28 S. on the American coast. At the same 
 time, over the eastern margin of tlie South Pacific the cooling 
 effect due to the southerly wind is evident from 40 S. to 5 S. 
 
 GALES. 
 
 Depressions of the South Atlantic which are attended with 
 gale force appear to reach that ocean in two ways. They cross 
 the continent of South America somewhere between 25 S. and 
 Cape Horn, more frequently travelling over Patagonia; or they 
 avoid the land altogether and round Cape Horn to the eastward, 
 following the general drift of air and sea surface. 
 
 The percentage curves, see Plates VII. and VIII., seem to show 
 that not infrequently a cyclonic system, instead of rounding Cape 
 Horn or crossing the land, strikes northward or north-weetward
 
 Plate VII. 
 
 GALE FREQUENCY. 
 
 JANUARY FEBRUARY 
 
 SOW. 8O 4O 
 
 E.ZO SOW. O 4O 2O O E.2O 
 
 MARCH 
 
 APRIL 
 
 SOW. BO 4O 2& O E.2O 8OW 8O 4O 2O O 
 
 8OW. 6O 4O 
 
 MAY 
 
 JUNE 
 
 sow. eo 40 20 o EZO sow. eo 
 
 The percentage of gales to the whoie number of wind observations 
 is Sadjoated by tiie fiyare linked to each gale frequency line. 
 
 Wymaii & Sons, Ltd., M.O. l'rw, I-oirfon, S.W. 7. P- S874. Ps, 2774. 7107/874. 360. 6/J7.
 
 Plate VIII. 
 
 OALE FREQUENCY. 
 
 JULY AUGUST 
 
 so*. eo 40 20 o eao BOW eo o ao 
 
 
 JJ 
 
 m^ 
 
 SOW. 6O 
 
 20 o E.ZO 8ow. eo *o 20 o E.2O 
 
 SEPTEMBER 
 
 OCTOBER 
 
 O i.2O 
 
 NOVEMBER 
 
 DECEMBER 
 
 sow. eo 40 
 
 SOW 60 4O 2O O E.2O 
 
 The 
 
 he percentage of gales to the whole number of wind observations 
 indicated by the figure linked to each gale frequency line. 
 
 Wyman & Son*, Ltd., M.O. PT<JM, London, 9.W. 7. PB. 2847. P^ 2774. 7107/874. 860. 6/17.
 
 11 
 
 up the South Pacific at a distance of 5 to 10 from the west 
 coast of South America. 
 
 It seems probable that systems rounding Cape Horn drill nortli- 
 east and east, their centres eventually pass south of the Cape of 
 Good Hope, and continue their eastward movement in tho 
 Southern Ocean. 
 
 FOG. 
 
 Attention may here be called to the effects of a warm moist 
 wind over cold water, and of a cold wind over warm water. 
 
 A warm humid wind passing over water of a lower temperature 
 is cooled, and the excess of vapour, after the point of saturation 
 has been reached, may be condensed, and form mist or fog. In 
 like manner a cold wind, blowing over water of a considerably 
 higher temperature than its own, chills below the point of satura- 
 tion the vapour as it rises, when it becomes visible. For the lower 
 the temperature of the atmosphere the less aqueous vapour can it 
 hold in an invisible state. 
 
 In the first case the fog, as a rule, does not extend more than a 
 few hundred feet above the sea surface, and may occasionally be 
 seen over from the masthead of a vessel ; in the second case it rises 
 to a comparatively high altitude, but does not always extend to 
 the surface, so that objects may occasionally be seen at a distance 
 by an observer descending to a position near sea-level. It must 
 not be concluded that these are the only causes productive of fog 
 at sea, but the regions of greatest fog frequency are regions in 
 which these causes occur most conspicuously. 
 
 Over the South Atlantic, north of the 30th parallel, fog is 
 rarely met with, except near the land on either side of the ocean. 
 For over this area the wind circulates, for the most part, steadily 
 round the central area of highest pressure, and the relation 
 between air and sea .surface temperature is fairly stable. Any- 
 where south of the 30th parallel, where a relatively warm 
 moisture-laden wind passes over a relatively cold current, fog 
 may be expected, and is found to become increasingly frequent 
 the higher the latitude reached, to the southern limit of observ- 
 ation, see Plate IX. This is probably due to increase in gale 
 frequency with latitude, the passage of cyclonic systems bringing 
 in their train rapid fluctuations in air temperature, varying 
 inequalities between the temperature of air and sen surface, 
 together with the failing winds and calms which usually intervene 
 between successive disturbances. 
 
 THE DOLDRUMS. 
 
 Attention has already been called on p. 7 to the migration in 
 the course of the year of the belt of calms and variable airs known 
 as the doldrums of the equator. These are the regions between 
 the two "trades," and they are -localities of frequent rainfall. 
 The weather of the doldrums is a subject of special interest on 
 account of the part which it was supposed to take in the formation 
 and maintenance of the trade-winds. A table giving a summary 
 of observations on ships in that region is therefore given here.
 
 II 
 
 
 
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 Plate IX, 
 
 DISTRIBUTION AND FREQUENCY OF FOQ 
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 The upper figures show the number of wind observations, 
 
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 DISTRIBUTION AND FREQUENCY OF POO- 
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 Wyroan <fc Sons, Ltd., M.O. Press, London, S.W. 7. P. 1847. P. 2774. 7107/874. 860. fl/17.
 
 13 
 
 INDEX. 
 
 . 
 
 Air circulation, South Atlantic ... ... ... ... ... 5 
 
 Air temperature ... ... ... ... ... ... ... y 
 
 Anticyclone, South Atlantic ... 
 
 Winds of I." I." r/G 
 
 Argentina, Daily Weather Report of 
 
 Barometer, Wind and .-, 
 
 Barometric pressure, effect of littoral on 7 
 
 and passage of depressions <J 
 
 Brazil coast, Low pressure over ... ... ... ... i; 
 
 British Association ... ... ... ... ... ... ... 3 
 
 Cape Horn, Wind systems rounding ... ... ... ... ... ... IQ 
 
 Cause of fog, A... ... ... ... ... ... .11 
 
 Chart of South Atlantic wind ... ... ... ... 5 
 
 Circulation of air over South Atlantic ... ... ... ... ; ", 
 
 Coasts and trend of isobars ... ... ... ... ... ... ... 7 
 
 Cooling effect of southerly wind ... ... ... ... ... ... 9 \Q 
 
 Council, Meteorological '3 
 
 Cyclones and temperature fluctuations 11 
 
 Daily Weather Charts, Argentina 6 
 
 Depressions, Eastward movement of ... ... ... ... ... ... 6 
 
 Deviation of isotherms ... ... ... ... ... ... ... ... y 
 
 Distribution of wind, pressure, and temperature ... ... ... ... 7~y 
 
 Doldrums, Mean latitude of ... ... ... ... ... ... ... 7 
 
 Rainfall in ... ... ... ... ... ... ... ... 12 
 
 State of sky in ... ... ... ... ... ... ... 12 
 
 Eastward moving depressions ... ... ... ... ... ... ... r, 
 
 Elliptical isobars... ... ... ... ... ... ... ... ... 7 
 
 Fog .'. 11 
 
 ,, , Cause of ... ... ... ... ... ... ... ... ... H 
 
 distribution... ... ... ... ... ... ... ... ... n 
 
 frequency 11 
 
 Frequency of gales 7, 10 
 
 Gale frequency ... ... ... ... ... ... ... ... ... y ? l(j 
 
 Gales near Brazilian coast ... ... ... ... ... ... ... G 
 
 of South Atlantic 10 
 
 Gradient and wind 7 
 
 Hydrographer of the Navy ;', 
 
 Influence of littoral on wind direction --.- .. ... ... ... 7, 1 
 
 Isobars, Elliptical ... , 
 
 Isotherms, Deviation of 
 
 Influence of the winds on... 
 
 Trend of 
 
 Littoral, Wind follows 7. '. 
 
 Low pressure, Coast of Brazil ... ... ... ... ... (5 
 
 South Africa ... ... ... ... ... ... ... 5 
 
 , ,, systems travelling eastward ... ... ... ... ... 6 
 
 Major axis of South Atlantic high pressure ... ... ... ... ... 7 
 
 Maximum gale force of South Atlantic ... ... ... ... ... fi 
 
 Mean latitude of doldrum belt 7 
 
 Meteorological Council ... ... ... ... ... ... ... ... 3 
 
 Pressure distribution, Barometric ... ... ... ... ... ... 7 '.* 
 
 ,, Major axis of high ... ... ... ... ... 7 
 
 Rainfall in doldrums ... ... ... ... ... ... ... ... 12 
 
 Hear Admiral Sir W. J. L. Wharton ."> 
 
 Relation between temperature, pressure and air circulation"'^ 
 
 wind and gradient ... ... ... ... ... ... 7 
 
 River Plate, low pressure ... ... ... ... ... ... ... i
 
 14 
 
 PAGE 
 
 South Africa, low pressure 5 
 
 South Atlantic, air circulation ... ... ... ... ... ... ... 5 
 
 temperature ... ... ... ... ... ... 9 
 
 gales 10 
 
 maximum gale force 6 
 
 wind chart ... ... ... ... ... ... ... 5 
 
 systems ... ... ... ... ... ... ... 5 
 
 South-east trade winds 7-9 
 
 Southerly wind, Cooling effect of 9,10 
 
 South Pacific, air temperature ... ... ... ... ... ... 10 
 
 anticyclone ... ... ... ... ... ... ... 7 
 
 Systems rounding Cape Horn ... ... ... ... ... ... ... 10 
 
 Temperature fluctuations and cyclones 11 
 
 Temperature of the air 9 
 
 Trade winds 8 
 
 Travelling low pressure systems ... ... ... ... ... ... 6 
 
 Travel of high pressure systems ... ... ... ... ... ... H 
 
 Trend of isobars with coast line 7 
 
 Wind and barometer ... ... ... ... ... ... ... ... 5 
 
 Wind and gradient ... ... ... ... ... ... ... ... 7 
 
 temperature ... ... ... ... ... ... ... ... 9 
 
 chart of South Atlantic 5 
 
 distribution 6 
 
 follows littoral 7,9 
 
 maximum force of South Atlantic ... ... ... ... ... 6 
 
 Wharton, Rear- Admiral Sir W. J. L 3 
 
 Winds of anticyclone ... ... ... ... ... ... ... ... 5-6 
 
 Wind systems of South Atlantic ... ; 5 
 
 Printed under the authority of His Majesty's Stationery Office 
 By DARLING AND SON, LIMITED, BACON STREET, E.2. 
 
 154O52
 
 QC 
 
 99k. 
 
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 Hepworth - The relation between pressure, 
 temperature, and air circulation... 
 
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