36o SCIENCE PROGRESS 



heavy rain in India associated with the south-west monsoon, 

 from June to September, is brought about. 



In former times both the south-west wind»and the rain were 

 attributed to convectional currents due to tne excessive heat 

 over the inland regions of India, but several c^siderations make 

 this explanation appear unsatisfactory. -For instance, the 

 temperature contrast between land and sea is greater in May, 

 before the monsoon sets in, than during the rainy season ; the 

 average temperature is higher in years of small than in those of 

 large rainfall ; lastly, the hottest part of India during the 

 monsoon gets practically no rainfall at all. 



During July, a typical monsoon month, the winds over India 

 circulate round an area of low pressure centred in the north- 

 west. For convenience the air motion near the ground can 

 be divided into ten separate streams. Now the mountain- 

 ranges of India and the Malay Peninsula form a kind of box, 

 having two openings only, one between Burmah and Ceylon, and 

 the other in the north-west ; it is only through these openings 

 that air can freely reach the interior regions. Of the ten air- 

 streams, three cross the western Ghats almost perpendicularly. 

 The warm, damp air ascends 4,000 ft. and condensation takes 

 place at a level of about 500 ft. ; the total rainfall for the month 

 amounts to well over fifty inches in the central parts of the 

 range. Proceeding to Burmah and Assam, three other trajec- 

 tories also encounter mountain-ranges directly, in each case 

 producing extremely heavy rain ; at Cherrapungi, in Assam, 

 where the winds encounter an almost vertical rise of 4,000 ft., 

 the annual rainfall averages 424 inches, the greatest amount 

 recorded in the whole world hitherto. The seventh air-stream 

 moves westwards across Northern India from the Bay of Bengal, 

 entering an enclosed area, but without encountering any sudden 

 barrier ; the air rises only gradually and the rainfall is therefore 

 moderate. The remaining three trajectories pass into India 

 through the gap in the north-west. These are particularly 

 interesting, as it is in the north of this area that the dry region 

 occurs. This dry region is the tract of desert between Multan 

 and Jacobabad, and is also the region of lowest pressure. The 

 presence of the Himalayas and the mountains of Afghanistan 

 and Baluchistan do not permit air to flow in except from the 

 south and east, and owing to the moderate pressure gradient 

 the amount which arrives is not large. The supply would, 

 however, suffice to cause rain, were it not for two factors. First 

 of these is the extremely high temperature prevailing over the 

 desert, which causes the damp air flowing in from the sea to 

 warm up and attain a comparatively low relative humidity. 

 Secondly, as soon as the air begins to ascend it encounters a 

 very dry upper current from the west which blows during the 



