214 SCIENCE REMAKING THE WORLD 



This enormous amount of moisture given off into the 

 air by the forest, which may be compared to clouds of 

 exhaust steam thrown into the atmosphere, must play 

 an important part in the economy of nature and de- 

 servedly earns for the forests the name of the "oceans 

 of the continent." 



WIND PERIODICITY AND PRECIPITATION. In the 

 eastern part of the North American continent east of 

 the looth meridian the winds during the winter and 

 partly in the fall and in the early spring come from the 

 west and northwest. These prevailing winds bring 

 cold and comparatively dry air from the interior of the 

 continent. In summer the prevailing winds are from 

 the southeast in Texas, and farther north and east they 

 come from the south and southwest. There is a most 

 intimate relation between the prevailing southerly 

 winds and precipitation in the eastern half of the 

 United States. It is during the summer period, when 

 the entire eastern half of the United States is under 

 the influence of the southerly winds, that most of the 

 precipitation falls over it. On the plains east of the 

 Rocky Mountains the summer rainfall forms from three 

 fourths to four fifths of that of the entire year. In 

 winter, with the change in the direction of the wind, 

 there is a radical change in precipitation. 



The periodicity of the wind direction and its relation 

 to precipitation over the eastern half of the United 

 States is well illustrated on the two maps. The arrows 

 indicate the direction of the prevailing winds, and the 

 lines and figures show the mean precipitation for the 

 months of July and January. The map for the month 



