[anqary 26, 1922] 



NATURE 



109 



The Theory of the South-West Monsoon. 



By L. C. W. BoNACiNA. 



N the widest sense the term "monsoon" in 



cUmatology is applicable to those seasonal 



lodifications, or subversions, of the planetary 



rculation which are established by the differences 

 temperature due to the irregular distribution of 



id and water, especially as seen in such regions 

 Eastern and Southern Asia, where a definite 



mtinental outflow of air in 

 jrinter, and inflow in summer, 



»aracterise the surface circula- 



>n. 



In relation to India the expres- 



)n " south-west monsoon " is 



)wadays quite a commonplace, 



It it cannot be said that the 



ieory of the phenomenon has 

 itherto been properly elaborated. 



le text-books commonly de- 



:ribe the monsoon as a kind of 



lagnified sea-breeze action, an 



tplanation of a complex pheno- 



lenon which can stand only as 

 first approximation. It must 

 emphasised, indeed, that the 



)uth-u^est monsoon of India is 



>t in the main a special local 



Feet of the heated condition of 

 ^India at all, but is part of a 

 general circulation of air with 

 respect to a system of low baro- 

 metric pressure originating prim- 

 arily in the heated condition of 

 the vast Asiatic continent as a 

 whole. It is when one abandons 

 a merely qualitative conception 

 of the monsoonal circulation and 

 considers the latter in the form 

 which it actually takes as a wind- 

 system of particular direction, 

 speed, and structure that the 

 mechanism is realised to be much 

 more complex than is suggested 

 by the above simple statement. 

 For, just' as one cannot under- 

 stand the many puzzling peculiar- 

 ities of the small-scale diurnal 

 sea-breeze effect familiar round 

 the English coast in summer- 

 time without referring to the 

 general barometric-gradient wind 

 of which it is often nothing more important 

 than a coastal modification in direction and 

 speed, so one cannot properly deal with the 

 large-scale seasonal sea-breeze effect in monsoon 

 countries otherwise than as an item in a wider 

 system of circulation. This outlook is the key to 

 the problem, and has enabled Dr. G. C. Simpson ^ 



Lrcture delivered to the Royal 

 921, and published in the Society'* 



to present meteorologists with the most effective 

 analysis that has yet appeared. He shows the 

 futility of trying to explain the monsoon in terms 

 of a single cause, and the necessity of seeing in 

 the phenomenon the final result of a number of 

 interacting factors. 



Before stating Dr. Simpson's theory, it may be 



1 "The South-West Monsoon. 

 Meteorological Society, March i6, 

 Journal for July, 1921. 



Fig. I. — Average wind and pressure distribution around India in May. Pressure in inches. 



well to review the main seasonal divisions of the 

 Indian year. These are : (i) a cool, dry season, 

 November to February, definitely dominated by 

 the north-east monsoon, which really belongs to 

 the north-east trade system as regulated at this 

 season by high barometric pressure in Central 

 Asia; (2) a hot, dry season, March to May, 

 characterised by light air-currents gradually 

 changing round from north-east to south-west, 



NO. 2726, VOL. 109] 



