HOW FORESTS FEED CLOUDS 215 



of July is typical for the summer period and the one for 

 the month of January is typical for the winter period. 

 The data represent more than twenty years of continu- 

 ous records. 



THE SIGNIFICANCE OF THE FACTS. The three facts 

 just discussed, namely, that vegetation from land con- 

 tributes more to the precipitation over land than 

 evaporation from the ocean, that forests evaporate 

 more water than free water surface or any other vege- 

 tation, and that transpiration of the eastern half of the 

 United States is intimately connected with the pre- 

 vailing south wind, throw an entirely new light on the 

 relationship between the forests of the coastal plain and 

 the Southern Appalachians and the humidity of the 

 central states and the prairie region. 



The central portion of the United States is distinctly 

 a continental region, particularly the prairie region, 

 which suffers from lack of precipitation. On the other 

 hand, large areas in the south and southeast because 

 of large swamps, suffer from too much humidity, which 

 is caused not only by excessive precipitation but 

 also by deficient evaporation. We have, therefore, 

 two extremes in the eastern half of the United States: 

 (i) in the states adjoining the Atlantic Ocean and the 

 Gulf of Mexico, there is an excess of moisture on the 

 ground, both on account of excessive precipitation and 

 slight evaporation; (2) in the vast interior of the Central 

 United States, on the other hand, there is a deficiency 

 of moisture both on account of scant precipitation and 

 of the intense evaporation. Is there not some con- 

 nection between these two extremes? Is it not possible 



