Ma7%k 15, 1888J 



NATURE 



46t) 



THE MONSOONS} 



■pVERY School Board pupil who reads a shilling 

 •^-^ primer of physical geography knows that the mon- 

 soons are periodical winds which blow over the Indian 

 Ocean at different seasons of the year ; but very few, 

 even among regular meteorologists, are fully aware of the 

 interesting but complex nature of the details of these 

 phases of atmospheric circulation. 



The two publications which are the subject of this 

 notice contain a vast amount of information and research 

 connected with these winds. The charts of the Bay of 

 Bengal consist of a series of maps of mean pressure, 

 resultant wind, and ocean currents for every month of the 

 year ; with a page of descriptive letterpress for each. 

 They were compiled by Mr. W. L. Dallas, and are 

 published in the inconvenient size of 23 by 18 inches. 



The memoir on the winds of the Arabian Sea is a long 

 and exhaustive report by the same author, with a critical 

 and theoretical discussion of the results obtained all over 

 the North Indian Ocean. This is published in a large 

 quarto form, and contains small-scale charts of mean 

 pressure, and of both wind force and direction, for every 

 month of the year ; while similar maps of temperature 

 and vapour-tension are given for the months of April, 

 May, and June only. 



Space will permit us to notice only the extreme con- 

 ditions which characterize the months of January and 

 July, or the most pronounced periods of the north-east 

 and south-west monsoons respectively. It will be well to 

 take the north-east monsoon first, as it is much the simpler 

 of the two. One of the most important results of Indian 

 research has been to modify the crude idea that the north- 

 east monsoon blows directly all the way from the great 

 Siberian winter anticyclone down to the equator. Now 

 it has been shown that there is in the month of January 

 a small anticyclone over the Punjab, and an area of high 

 pressure over the Persian Gulf. 



This fact is of far more than local importance. The 

 typical distribution of pressure over the globe consists of 

 an equatorial belt of low pressure, with a belt of anti- 

 cyclones on either side, about the line of the two tropics. 

 Heretofore we have been constrained to look on the 

 Siberian anticyclone— centered near the Arctic Circle — 

 as the representative of the tropical belt of high pressure, 

 but now there is the strongest presumption that these 

 smaller anticyclones are the true, but diminutive, equi- 

 valents of the tropical belt. 



There is a curious irregularity in the sweep of the 

 isobars and winds over India towards the equator. The 

 charts indicate a local depression all along the west coast 

 of India, and a slight general protrusion of pressure over 

 the Bay of Bengal. This latter is important, as we shall 

 have to refer to the converse condition in the opposite 

 monsoon. 



The conditions of the south-west monsoon are a good 

 deal more complicated, for in July we have to explain the 

 following relations of pressure and wind. A belt of high 

 pressure runs along the twentieth degree of south lati- 

 tude almost from Australia to long. 70"^ E. ; but then the 

 isobars mount up to the equator along the coast of Africa. 

 An irregular area of low pressure lies over Scinde, but 

 the banc slopes all round are by no means symmetrical. 

 The most noticeable irregularity is an area of relatively 

 low pressure over the south-west of the Bay of Bengal, 

 so that the mean isobar of 29-80 which runs towards the 

 north-east from Africa to near Bombay, bends then to 

 the south-east until it arrives near Trincomalee, in 

 Ceylon, when it turns again to the north-east. The 



' "Weather Charts of the Bay of Bengal and adjacent Sea norlh of the 

 Equator. Issued by the Meteorological Department of the Government of 

 India, (Calcutta, 1887.) 



" On the Winds of the Arabian Sea and Northern Indian Ocean" By 

 W. L. Dallas, late of the Meteorological Office, London. Published by 

 the Meteorological Department of the Government of India. (Calcutta 

 1887.) • 



wmd conforms partially to this distribution of pressure. 

 South of the Lme the south-east trade blows with great 

 uniformity, crosses the equator with a regular sweep 

 into the Arabian Sea, turning first to the south-west, 

 and eventually to the west, between Karachi and Bom- 

 bay. But in the Bay of Bengal the situation is rather 

 different. The depression, before noted, is associated 

 with a west-north-west wind over Southern India, but a 

 light westerly current and rainy weather prevails all over 

 the south of the Bay, from the latitude of Ceylon, down 

 to the equator, while a strong south-west monsoon blows 

 all up the Bay itself. Hence a ship going up to Calcutta 

 will find the south-east trade replaced by light irregular 

 winds between south and west, with much rain, from the 

 equator to about 10° N., before she encounters the fresh 

 south-west monsoon in the upper part of the Bay. 



Mr, Dallas gives many interesting details in this 

 memoir, such as a discussion of the so-called "soft 

 place" in the monsoon between Bombay and Aden. 

 This is a region described in the East Indian sailing 

 directories as lying along, or about, the ninth parallel in 

 the Arabian Sea ; but the present series of observations 

 afford very little evidence of the existence of this tract of 

 quiet conditions. 



The author seems to find some difficulty in explaining 

 the cold air found along the African coast during the 

 height of the monsoon, but this is almost certainly due 

 to the cold water which wells up from below, as the hot 

 surface water is driven to the north-east. The weather 

 shore of a tropical coast in a steady atmospheric current 

 is always cold for the same reason. 



Mr. H. F. Blanford has worked out the precipitation of 

 the south-west monsoon in his great memoirs on " Indian 

 Rainfall," and has brought out most clearly a great 

 meteorological principle. He finds that even with the 

 saturated atmosphere of the Indian Ocean— air that con- 

 tains nearly twelve grains of water in a cubic foot, as 

 compared to about six grains in our own climate— no 

 great precipitation takes place without dynamical cooling. 

 That is to say, unless the air is forced upwards by local 

 obstacles, &c., and so cooled by expansion, no great 

 rainfall can occur. 



But the great question, about which there is still some 

 doubt, is the precise relation of the south-west monsoon 

 to the south-east trade. Dove started the theory that the 

 south-east trade turns to south-west after crossing the 

 equator, owing to the influence of the earth's rotation ; 

 and there can be little doubt that in the Arabian Sea the 

 trade wind does sweep directly across the Line and grow 

 into the monsoon. 



But in the Bay of Bengal, Mr. Blanford finds that the 

 south-west monsoon is not linked up habitually with the 

 south-east trade, though it is so occasionally ; and he 

 considers that the monsoon is drawn from a reservoir 

 of air over the equatorial zone fed by the south-east trades, 

 but that it is not the south-east trade simply diverted 

 from its ordinary course. 



This opinion is based on the following facts, brought 

 out by the charts under review, for — 



(i) The south-east trades are strong, but the winds are 

 light and variable from the equator to io° N., above 

 which fresh winds are again developed. There is, then, 

 some hitch in the sweep of the south-east current across 

 the equator. 



(2) The winds just north of the line are nearly from the 

 west, while they flow from south-west at the top of the 

 Bay. Theoretically the wind should get more and more 

 westerly the further north we go. 



(3) The south-east trade is tolerably dry ; the equa- 

 torial belt of westerly winds is very wet and squally ; 

 while the precipitation of the south-west monsoon is not 

 very great out at sea. 



Though these facts undoubtedly indicate some irre- 

 gularity in the flow of the south-east trade across the 



