RAINFALL OF THE UNITED STATES 159 



slope decreases until, in southern California, there is al- 

 most no rain in summer and the entire rainfall for the 

 year .averages about 15 inches. By reference to the isobar 

 map of the world (page 141) it will be seen that the 

 high-pressure area of the dry tropical calm belt moves 

 sufficiently far north in summer to take this region out of 

 the influence of the wet westerlies and into that of the 

 drier belt. 



The western interior region, extending from the Cas- 

 cade and Sierra Nevada mountains to about the 100th 



EAST END OF THE ASSUAN DAM ACROSS THE NILE. 

 The greatest irrigation dam in the world. 



meridian, is dry over the larger part of its surface, since 

 the winds have deposited most of their moisture in pass- 

 ing over the mountains to the west. On the mountains 

 and high plateaus, however, there is a considerable fall of 

 rain, as th6 winds are cooled sufficiently in passing over 

 these to deposit their remaining moisture. In most of 

 this region, as in southern California, irrigation must be 



