WINDS AND STORMS 



139 



General effect of unequal heating*. The effect of 

 heating the earth is to cause the air to rise from the 

 surface. Since all parts of the earth are not evenly 

 heated, the air does not rise evenly. It rises most 

 over the equator and in the tropics where the earth 

 receives the greatest amount of heat. This warm 

 upper air moves from equatorial regions toward the 

 poles. As it does so, it gradually becomes cooler 

 and consequently heavier, until in the regions of 30 

 degrees north and 

 south of the equator, 

 it settles down upon 

 the earth. This move- t 

 ment of the air estab- 

 lishes a region of 

 high pressure in each 

 hemisphere. From 

 each of these regions ^ 

 currents of air move 

 outward ; one toward 

 the nearest pole, and 

 one toward the equa- 

 tor. The elements of 

 the general circula- poles> 

 tion, then, are : 1. the rising of warm air in the tropics, 

 2. the moving poleward of this air, 3. its settling, 4. the 

 moving poleward of part of this air and the moving 

 equatorward of the remainder. 



If the earth did not rotate, the poleward moving 



FIG. 52. Diagram of the general 

 circulation ' of the air. The trade 

 winds blow toward the equator, the 

 prevailing westerlies blow toward the 



