THE ENGINE OF THE AIR 



151 



loses heat and shrinks. The result is that at the equator there 

 is a greater height of air above the tropopause; and this has 

 a tendency to slide down hill toward the pole. At ground level 

 a compensating flow of cold air tends to flow south from the 

 Arctic. This is the prime motive of air circulation in the north- 

 ern hemisphere, but it is conditioned by other forces that 

 cause variations in the northern hemisphere wind system. 



In a previous chapter we have spoken of the effects of the 

 earth's rotation on the flow of water, and now we see related 



Diagram showing the heavy, irregular convections, accompanied by 

 cumulus and thunderstorm clouds, that would occur if the sun's heat 

 were applied uniformly everywhere and the earth did not rotate. 



NORTH POLE 



EQUATOR 



