The Science of the Weather 773 



every year over India. During the winter, the winds blow over 

 this country from the north-east f hese winds are blowing out- 

 wards from the region of high pr 'ire that then covers all the 

 north-east of Asia ; the winds a' 2 .eref ore cool and dry. But 

 with the arrival of summer the J uitic high-pressure gives way 

 to a general low-pressure disiri, ution, the centre of which lies 

 over Persia and Baluchistan. At the same time a region of high- 

 pressure is found in the southern Indian Ocean between Mada- 

 gascar and Australia, and the air which moves from this "high" 

 area towards the "low" centre will in the Northern Hemisphere 

 be deflected to the right, as already explained, and so will have 

 a south-westerly direction instead of a southerly one as it ap- 

 proaches the Indian coast. This air has been travelling over a 

 wide expanse of Equatorial ocean, and therefore contains a vast 

 amount of moisture in the form of water-vapour. When the 

 wind arrives at the west coast of India it encounters there the 

 long ridge of mountains that runs all down the Malabar Coast, 

 and the mountains become a barrier which the wind has to sur- 

 mount before it can continue its course. The air is therefore 

 forced upwards, its pressure is decreased; thereupon it becomes 

 cooled and can no longer retain the water-vapour that was easily 

 held by the warmer air. The capacity of warm air for water- 

 vapour is very much greater than that of cooler air, and, if air 

 is cooled, any surplus of water-vapour is forced to condense out 

 as cloud, and eventually as rain. Thus it is that the S.W. mon- 

 soon wind brings torrential rain to the west coast of India. Simi- 

 larly the air streaming up the Bay of Bengal meets with moun- 

 tain barriers in southern Assam and along the Burmese coast, 

 and here again the rainfall is terrific. The moisture still remain- 

 ing in the air is brought down finally on the southern face of the 

 Himalayas, when the air encounters that mighty mountain-wall, 

 and this region also has heavy rain. It is believed that Cherra- 

 punji in southern Assam has the heaviest rainfall in the world, 

 nearly 500 inches falling in a year. 



VOL. Ill 14 



