THE WATER VAPOR OF THE AIR 



213 



and the soil. (1) The evaporation from water surfaces ranges 

 from a very small amount per day in polar regions, in winter, 

 to as much as Y$ in. per day, on some days, over tropical 

 oceans and over inland lakes in hot dry regions like south- 

 western United States (Fig. 149). (2) All plants give off 

 water vapor through their leaves. (3) Evaporation from 

 the soil varies with the kind of soil, the amount of water 

 in the soil, the method of surface tillage, the dryness of the 



FIG. 149. Mean annual evaporation in inches, from a water surface. 



air and the velocity of the wind. Wet soil, dry air, or 

 strong winds increase evaporation. Evaporation is greater 

 from porous soils than from compact soils. Rolling a field 

 checks evaporation from the soil. Tillage that forms a "dust 

 mulch" an inch or so thick over the surface is an effective 

 check and keeps the water in the soil (see Arts. 547 to 551). 

 Both rolling and dust mulch tillage are used in "dry farming" 

 regions of the plains, and are valuable to agriculture in dry 

 seasons anywhere. 



