THE TROPICS 



41 



Sphere this is due to the warm Equatorial Current (II.), and in the 

 Southern Hemisphere to the warming effects of the land and to the 

 warm Brazilian Current (IV.). 



With reference to the different effect of land and sea, it is neces- 

 sary to remember that though 25 per cent, of the heat of the sun's 

 rays which fall on the outer limit of our atmosphere are absorbed, 

 still these rays do not really warm the air to any appreciable extent. 

 The real warmth of the air is obtained from the dark heat radiated 

 from land and sea. 



Land not only absorbs the radiant heat from the sun more quickly 

 than water, but also more readily gives off the dark heat to the air ; 

 therefore the presence of a large area of land upon which the sun's 

 rays fall more or less vertically at noon all the year round will raise 

 the temperature of the air, and will tend to extend the area of the 

 warm climates. Hence the land may become extremely hot — in- 

 credibly high temperatures have been mentioned by authors — 

 while water never becomes very warm. The reason of this is that 

 water, being a liquid, by convection and by currents tends to keep 

 at a more even temperature than land, though, as already noted, 

 the currents, cold and warm, have a marked influence on the climate. 



Turning again to the map, and tracing the isotherm eastwards 

 towards the west coast of Africa, it will be noticed that again both 

 the northern and the southern isotherms approach the Equator, 

 and again this is due to cold currents — in the north to the North 

 Equatorial Current (V.), and in the south to the' cold Antarctic 

 Current (I.). Crossing the continent of Africa, the effect of land 

 is seen, and on passing into the Indian Ocean the effect of the 

 Mozambique and the South Equatorial Currents may be noted, 

 and finally, to the west and east of Australia the cold Antarctic 

 Currents produce effects. 



Thus the Tropical Zone is alternately decreased and increased in 

 area, the most marked diminutions being on the west coast of 

 America and the west coast of Africa. 



It will be observed that this zone includes Central America, a 

 large portion of South America, the West Indies, a large portion of 

 Africa, Madagascar, a portion of Arabia, India, Ceylon, Indo-China, 

 Sumatra, Java, Borneo, the Philippine Islands, New Guinea, and a 

 portion of Australia, and many other smaller islands. 



2. The Subtropical Zone. — The Subtropical Zone lies between the 

 isotherms of 68° F. (20° C.) for the temperature of the coldest month, 

 and the same isotherms for the mean temperature of the year. 



It is depicted in Fig. i, and will be noticed to include a portion of 

 North America and considerable portions of South America, Africa, 

 Asia Minor, Arabia, Persia, North India, China, and Australia. 



Another method of subdividing warm climates, based upon the 

 winds, rainfall, and altitude, will be given in the next chapter. 



