IMPORTANCE OF WINDS AND RAINS 191 



drops (precipitates it}. But in the summer months the 

 land is warmer than the sea, and the winds generally 

 pass on without giving up their moisture. So the coast 

 of California has distinct wet and dry seasons, just as is 

 common in the tropics. Farther to the north, in Oregon 

 and Washington, the rain is more abundant throughout 

 the year, because the land there remains comparatively 

 cool. 



Soon these eastward-blowing winds encounter rising 

 altitudes; they come to the first range of mountains. Since 

 temperature falls as altitude rises, they may continue to 

 deliver their rain. So there is plenty of rain and snow 

 west of the Sierras and the Cascades. But once these 

 mountain ranges are crossed, and the air begins to descend, 

 it becomes warmer, and so tends to retain what moisture 

 is left. Thus we see why the -windward side of high moun- 

 tains gets more rainfall than the leeward side, and we also 

 see why the Great Basin is so arid. 



When these winds after crossing the Great Basin come 

 to the Rocky Mountains they may again yield moisture, 

 but after they have crossed this second range, again they 

 become dry, and the land which they then traverse (west- 

 ern Kansas and Nebraska) is of little agricultural value. 

 But just beyond this, in the Mississippi basin, lies the 

 richest agricultural region in the world, a region where 

 rainfall is abundant and where farm lands are of great 

 value. Why is this? Whence comes this rain? Surely 

 there is not enough water left in the winds which have 

 already traversed more than half the continent. No, 

 the source of the moisture which makes the farms of our 

 Middle West so fertile is not the Pacific Ocean. It is the 

 Gulf of Mexico. But to understand how the winds blow 



