THE MOISTURE OF THE AIR 253 



On the average, 30 to 40 inches of rain-water fall from the air 

 each year on land; that is, enough to make a layer 30 to 40 inches 

 deep if spread out over all the land. The amount of water evap- 

 orated each year must be about the same as the amount which is 

 precipitated. If the precipitation (rainfall and snowfall) on the 

 oceans is equal to that on the lands, and if all were taken from the 

 oceans and not returned, the oceans would be dried up in 3,000 or 

 4,000 years. If an amount of water equal to all the rainfall were 

 evaporated from the lakes of the earth, it would probably dry them 

 up in less than a year. 



Rate of evaporation. Fig. 219 shows the estimated amount of 

 evaporation, in inches of water, which there would be from surfaces 

 of water in various parts of the United States, if bodies of water 

 such as lakes were present. Thus in Mississippi a surface of water 

 would be lowered more than 50 inches in a year by evaporation; 

 at New York, about 40 inches; at Milwaukee, about 30 inches; 

 at Lake Superior, about 20 inches; at Denver, about 70 inches; 

 and in southern Arizona, 90 to 100 inches. 



Several conditions affect the rate of evaporation. The prin- 

 cipal ones are (1) the amount of water vapor already in the atmos- 

 phere, (2) the temperature of the water and of the space over it, and 

 (3) the strength of the wind. Fig. 219 shows the greatest evapora- 

 tion in dry parts of the country. It also shows that evaporation 

 is greater in the warmer latitudes of the United States than it is 

 in the cooler latitudes. 



Evaporation takes up heat. Evaporation cools the surface 

 from which it takes place. If the hand be moistened, it feels cool 

 as the water on it evaporates, and the faster the evaporation the 

 more distinct the cooling. Moist clothing seems cooler in the wind 

 than in still air, even when the temperature is the same, because 

 wind hastens evaporation. It takes about 1,000 times as much 

 heat to evaporate a pound of water as it would take to raise its 

 temperature 1 F. The evaporation from forested regions in moist 

 tropical lands is so great that the temperature there is often much 

 lower than would be expected from the insolation. The absence 

 of evaporation in dry regions is one reason why they are so hot 

 in the sunny days of summer. 



