SUMMARY 



179 



high mountain than above a low one. The winds are 

 caused by changes in atmospheric pressure; their prevail- 

 ing direction is affected by the earth's rotation. Certain 

 winds common to all planets are called planetary winds; 

 when modified by cer- 

 tain peculiarities of the 

 earth they are called 

 terrestrial winds. Be- 

 cause of their constancy 

 and their aid to traffic, 

 some of these winds are 

 called trade winds. 

 South of Asia there are 

 winds called monsoons. 



When very moist air 

 cools, it cannot hold as 

 much moisture as when 

 it is warm, so this falls 

 as rain, hail, sleet or 

 snow. The rainfall 

 varies from nothing at 

 all in some places to 

 over fifty feet a year in 

 others. In the United States the north Pacific slope has 

 a rainfall of about seventy inches a year ; the south Pacific 

 slope about fifteen inches ; the eastern slope of the Rockies 

 is very dry ; and the Mississippi valley and the country to 

 the east of it have a rainfall of from thirty to sixty inches. 



Rainstorms when accompanied by thunder and light- 

 ning are called thunder-storms. Thunder and lightning are 

 caused by certain clouds having a higher charge of elec- 

 tricity than others. The higher charge bursts across to 

 the lower charge, making a flash of lightning and a roll 

 of thunder. 



A SOUTH SEA ISLANDER. 



