METEOROLOGY 361 



monsoon, as has been proved by observations made with the 

 aid of kites. In consequence cloud is not formed, and the 

 uninterrupted sunshine maintains the high surface temperature 

 already mentioned, which is in itself a factor which tends to 

 prevent the formation of cloud and rain. There results a vicious 

 circle tending to maintain drought. 



Turning now to the question of the origin of the moisture in 

 the south-west monsoon, maps giving the mean pressure and 

 prevailing winds of the world in July show a large area of low 

 pressure over Asia and a high pressure area over the southern 

 part of the Indian Ocean between South Africa and Australia. 

 The air that reaches India forms part of the south-east trade 

 winds before crossing the Equator and then turns towards the 

 north-east to form the south-west monsoon, which is a part of 

 the general circulation of air round the Asiatic low pressure 

 system. This south-west wind is very humid, having traversed 

 over 4,000 miles of ocean. Now in May, before the rains have 

 begun, there is also a considerable inflow of air into the Indian 

 area, conditions, as judged from charts of India alone, appearing 

 to be very similar to those in July, except that the inflow is 

 a little weaker and the air that arrives from the ocean is some- 

 what drier. Pressure and wind-charts for the world show that 

 the south-east trades do not now cross the Equator, and that 

 northerly winds prevail over the Arabian Sea. The air has not 

 followed the very long course over the ocean that it takes in 

 July, and is consequently relatively dry. The explanation given 

 for the drought over the Indian desert during the rainy season 

 now applies for the country as a whole, and there is simultane- 

 ously great heat and a general absence of rain. It is noteworthy 

 that, when the rains fail, conditions generally resemble those of 

 May, except that in a failure of the rains the supply of air to 

 India is small because of a tendency for high pressure to develop 

 over Western India and not, as in May, because of the relatively 

 feeble development of the high pressure system over the southern 

 part of the Indian Ocean. 



To sum up, the primary cause of the south-west monsoon 

 is the high temperature over the land in the Northern Hemi- 

 sphere as a whole, which tends to lower the pressure there and 

 cause a south-west wind over India ; the connection between 

 temperature, pressure, and rainfall is, however, a complicated 

 one when considered in detail ; for instance, over India abnor- 

 mally high temperature is associated with a feeble development 

 of the south-west monsoon wind and a rainfall below the average. 



Het Vliegveldy of February 26, 1921, contains a paper on 

 Fog Banks," by A. C. Nell, in which observations of fog made 

 by Dr. Georgii, while in command of an air-squadron in Flanders 

 during the war, are summarised. 

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