January 26, 1922] 



NATURE 



III 



west monsoon current (numbered arrows). It 

 should be studied in relation to Fig. 4, showing 



-Chief alignments of mountains, and air-stream lines of south-west monsoon, in and around 

 (After G. C. Simpson.) 



the mean rainfall over the same area in July, the 

 typical monsoon month. The disposition of the 

 mountain ranges is such as 

 very effectively to entrap in 

 a kind of box the humid 

 air brought into the Indian 

 area by the south-west mon- 

 soon, with the consequence 

 that the air is mechanically 

 forced to ascend with copious 

 condensation of moisture as a 

 result of cooling by adiabatic ex- 

 pansion. Where the ranges 

 obstruct the air-currents at right 

 angles, as in the case of the 

 Western Ghats, KL, or the 

 Khasi Hills, HI, enormous falls 

 of rain occur during the four or 

 five months of the wet season. 

 The Khasi Hills, moreover, 

 contain a spot, Cherrapunji, so 

 peculiarly favourable to oro- 

 graphic precipitation that the 

 average annual rainfall is as 



-h as 424 in., nearly all of 



nich falls during the monsoon 

 period. In the Gangetic Plains 

 the heavy rainfall is largely due 

 "to the convergence of air- 

 streams III., IV., v., and vii., 

 assisted by the Himalayan wall, 

 CD, at the base of which the 

 forced ascent of air 



shadow " of the mountains, but the desert region 

 in the north-west of India is nearly rainless for a 

 complexity of reasons — 

 partly because the trend 

 of the neighbouring moun- 

 tains is not such as to 

 force upward the compara- 

 tively small amount of air 

 which flows into this 

 comer of the country; 

 partly because, with the 

 initial conditions thus un- 

 favourable to cloud pro- 

 duction, what little air 

 does arrive there from the 

 sea is heated up so that 

 its relative humidity is 

 lowered and the tendency 

 to drought consequently 

 increased ; and partly be- 

 cause over this part of 

 India the upper-air cur- 

 rent from the north-west, 

 as revealed by direct kite 

 observations, is warm and 

 dry, a condition most 

 unfavourable to condensa- 

 tion of moisture in any 

 surface air that may be 

 caused to rise. In the 

 burning-hot Thar Desert a number of interacting 

 factors thus conspire to maintain intense drought 



causes 

 another specially wet submontane strip of country. 

 The dry areas in July are mostly in the 

 NO. 2726, VOL. 109] 



ram- 



+.— Average rainfall (in inches) of India in July. 



during what in India generally is the rainy season. 

 The reason why the mountains provoke so enor- 



