GENERAL CIRCULATION OF THE ATMOSPHERE 273 



tudes is always warmer than that in high latitudes, 1 the upper air 

 should always be moving from the equatorial zone to the polar 

 zones in both hemispheres. These poleward movements of the 



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90 C 



Fig. 230. The movement of air indicated in Fig. 229 would result in the 

 further movement shown by the lower arrows in this figure. 



upper air lessen the pressure at the bottom of the atmosphere in 

 low latitudes, because air has moved away from that zone. After 

 air has moved from the equatorial region toward the poles (Fig. 229) , 

 there is more air over a given spot in high latitudes than in low. A 

 barometric gradient is thus established toward the equator at the 

 bottom of the atmosphere (Fig. 230). Air then moves from higher 

 latitudes to lower latitudes at the bottom of the air. 



Here, then, we have the elements of a general circulation, a 

 pole-ward movement in the 

 upper air, and an equator- 

 ward movement in the lower 

 air, and the unequal heating 

 which generates these move- 

 ments is in operation all the 

 time. 



If the earth did not rotate, 

 these movements of air would 

 tend to follow meridians. The 

 poleward-moving air should 

 blow north in the northern 

 hemisphere, and south in the 

 southern, while the air moving 

 toward the equator would blow 

 south in the northern hemi- 



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Fig. 231. Generalized diagram of wind 

 directions at the bottom of the at- 

 mosphere. 



sphere, and north in the south- 

 ern. Rotation turns the air currents to the right in the northern 



1 High latitudes sometimes receive more heat per day than low lati- 

 tudes (see p. 230), but the air of high latitudes is never so effectively heated, 

 because of the abundance of ice, snow, ice-cold water, and frozen ground. 



