TROPICAL CLIMATES 



63 



A locality may therefore lie entirely in the equatorial belt, or it 

 may lie entirely in the trade-wind belts, or it may be polewards of 

 these belts; but with the movement of the system polewards, as 

 indicated above, some of the places in a trade-wind belt must come 

 into the equatorial belt, while others usually situate polewards 

 to the trades must be included in these belts. 



Were the tropics flat this classification would sufiice, but there 

 are high mountain ranges in tropical lands, and therefore it is neces- 

 sary to make a division to include these, because it is possible in 

 the high lands of the tropics to pass through every degree of tem- 

 perature, if you ascend high enough, as if you proceeded from the 

 Equator to the Poles. 



There are therefore four divisions of warm climates — viz. : — 



I. The Equatorial Belt. 

 IT The Trade- Wind Belts, 

 in. The Monsoon Belts. 

 IV. Mountain Climates. 



I. The Equatorial Belt. — Localities situate within a few degrees 

 of the Equator are always more or less subject to rain and cloud, 

 because the heated air is full of aqueous vapour brought by the 

 trade winds from the sea; and as aqueous vapour is lighter than 

 air in the ratio of 0 623 to i-o, it can rise to high and cold altitudes, 

 and there be condensed into small droplets of water forming cloud 

 or larger droplets forming rain. 



As the capacity of air to hold water is doubled for every 27° F. 

 increase in temperature, and as in air saturated at 85° F. and then 

 cooled to 60° F. every cubic foot yields 7 grains of water, the exces- 

 sive violence of these equatorial and tropical rains is easily under- 

 stood. 



This belt is subject to alternate seasons of wind and calm — e.g., 

 in January it will be subject to the north-east trades, followed by a 

 period of calm ; and in July to the south-east trades, succeeded by 

 another period of calm. There are, therefore, two maximal and 

 two minimal temperatures after the two zenithal and solstitial 

 positions of the sun, and two short wet seasons and two short dry 

 seasons. 



Such places are Southern India, Ceylon and Java in Asia, Columbia 

 in South America, and in Africa parts of the Valley of the Nile, 

 and the Gold Coast. 



^yith the inclination of the North Pole towards the sun, which 

 begins on March 21 and ceases on September 22, the shifting of 

 calms and trades northwards begins some time after the first date 

 and lags behind the last date, and during this period a locality may 

 be in the equatorial belt for six months in the year and have six 

 months' drought and six months' rain. Such places are Bengal, 

 portions of the Nile Valley, Northern Australia, and Central America. 

 The Sudan receives its rains when the equatorial belt is passing 

 northwards — i.e.y from May to August— and its vegetation grows 



