64 



TROPICAL CLIMATOLOGY 



rapidly, but when it is in the trade-wind belt, from November or 

 December to March or later, the climate is dry. 



It is the northern migration of the Equatorial Belt which brings 

 the heavy rains to the Uplands of Abyssinia, and causes the rise 

 of the Blue Nile and the Atbara, which produce the rise of the Nile. 



The same features are to be noted in the movement southwards; 

 thus rain comes on the pampas of Brazil in the months from October 

 to April, while the dry season lasts for the rest of the year. 



Some few places — e.g., Wady Haifa — show only one maximum 

 and one minimum temperature — i.e., the so-called tropical type 

 of temperature variation. 



II. Trade-wind Belts. — The lands which lie just outside the polar 

 boundaries of the equatorial or rain belt are situate some 20° to 

 35° north or south latitude, and are among the driest in the world, 

 except in India, where the south-west monsoon brings a little rain 

 into the dry regions of the Punjab and Sind. 



The worst places are the dry zones of California and other parts 

 of North America, the Sahara and Nubian Deserts, parts of Arabia 

 and Persia, Argentina, Eastern Patagonia, South-West Africa, and 

 the interior of Australia. The only rain these regions are likely to 

 get will come from the extension equatorially of the polar winds. 



We may, therefore, summarize the character of the trade-wind 

 belts as very regular annual and diurnal ranges of temperature, 

 with a complete absence of rain or with slight showers at infrequent 

 intervals. The range of temperature in the desert is often very 

 great; thus during the day the temperature may be very high, with 

 dry winds carrying dust and sand, and the nights, with the clear 

 sky free from cloud allowing active radiation, may be cool if not 

 cold, or even at times very cold. 



III. Monsoon Belts. — The word ' monsoon ' is believed to be 

 derived from the Arabic word Mansin, meaning ' a season.' 



The monsoons (Fig. 6) are classifiable into three groups: — 



1. North-East and South-West Monsoons. 



2. North- West and South-East Monsoons. 



3. West Monsoon. 



I. North-East and South-West Monsoons. — These are typically met with 

 in the Indian Ocean and its coasts. 



Dove's explanation of these monsoons is generally accepted — viz., that, 

 owing to heating of the great plains of Asia, where the air ascends in the 

 months of May, June, July, and August, the south-east trade-wind, which is 

 blowing south of the Equator, is drawn northwards, at the same time being 

 deflected to the west, thus forming the south-west monsoon. Conversely, 

 when the plains cool in November, December, January, February, and March, 

 there is a breeze from the north-east towards the Equator, which, though called 

 the north-east monsoon, is really a trade-wind. 



The interval between the two monsoons is characterized by changeable 

 winds, which blow alternately in opposite directions — north-east and south- 

 west. 



The south-west monsoon is laden with moisture, and on it Southern India 

 largely depends for rain, and a failure will mean a famine, because a large 

 portion of the population is agricultural, for the natives depend for food upon 



