SUMMARY 245 



the earth they are called terrestrial winds. Because of 

 their constancy and their aid to traffic, some of these winds 

 are called trade winds. 



In middle latitudes there is a belt of irregular winds that 

 have a prevailing tendency to move from west to east. This 

 constant eastward drift of air is frequently interrupted by 

 great rotary air movements having a diameter of from 500 

 to 1000 miles. These are called cyclones and anti-cyclones. 

 The cyclone is an area of storm, and the anti-cyclone is an 

 area of clear sky. These eastward-moving cyclones are 

 responsible for most of the various changes in our weather. 

 The two chief factors that enter into the forecasting of 

 weather in middle latitudes are the direction of movement 

 and the rapidity of movement of cyclonic areas. 



Brief rainstorms accompanied by lightning are called 

 thunderstorms. They are caused by local updrafts of air 

 over hot, moist areas. When these local updrafts become 

 exceedingly violent and of small diameter, tornadoes and 

 waterspouts result. 



{ When moist air cools, it cannot hold as much moisture 

 as when it is warm, and so the excess falls as rain, hail, 

 snow, or sleet. 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. 



The average succession of weather changes throughout 

 the year considered for a long period of years, constitutes 

 the climate. The climate of any section depends not only 

 on latitude, but also upon altitude, nearness to large bodies 



