igo ELEMENTARY SCIENCE 



Rain is needed, too, for man and beast as well as for the 

 crops. In cities rain flushes our streets, lays the dust, 

 freshens our lawns, and fills our cisterns. Though we may 

 not use rain-water for drinking, we usually need it for 

 washing. 



Snow as well as rain is important for agriculture. A 

 mantle of snow protects winter crops, like winter wheat, 

 which are planted in fall and harvested in late spring or 

 early summer. Both rain and snow are valuable aids to 

 health. They cleanse the air of dust and bacteria. Have 

 you never noticed how fresh and agreeable the air often is 

 after a summer shower? 



Variation in Amount of Rainfall. The amount of rain- 

 fall, as you have learned in geography, varies extremely 

 in different parts of the world. It depends, of course, 

 upon the winds, but it also depends upon the regions from 

 which the winds come, and upon the conditions which they 

 encounter when they arrive. The only way in which the 

 air obtains the water which falls as rani is by evaporation 

 from the land and water surfaces below. So, to bring 

 rain, a wind must come from a region where it could be- 

 come moisture-laden. Then, to give this moisture up as 

 rain, it must encounter conditions which will cause it to 

 condense. 



To illustrate this, let us take the case of the United 

 States. Our prevailing winds are, as you have noted, 

 from the southwest. They come to the California coast 

 laden with moisture from the Pacific. In the winter 

 months the California coast is cooler than the Pacific, 

 so in those months these winds bring rain. The cool- 

 ness which they encounter condenses their moisture into 



