136 DRY-FARMING 



able. In New York the relative humidity in sum- 

 mer is about 73 per cent ; in Salt Lake City, about 

 35 per cent. At a high summer temperature evapora- 

 tion from the skin goes on slowly in New York and 

 rapidly in Salt Lake City, with the resulting discom- 

 fort or comfort. Similarly, evaporation from soils 

 goes on rapidly under a low and slowly under a high 

 percentage of relative humidity. 



Evaporation from water surfaces is hastened, there- 

 fore, by (1) an increase in the temperature, (2) an 

 increase in the air movements or winds, and (3) a 

 decrease in the relative humidity. The tempera- 

 ture is higher ; the relative humidity lower, and the 

 winds usually more abundant in arid than in humid 

 regions. The dry-farmer must consequently use all 

 possible precautions to prevent evaporation from the 

 soil. 



Conditions of evaporation from soils 



Evaporation does not alone occur from a surface 

 of free water. All wet or moist substances lose by 

 evaporation most of the water that they hold, pro- 

 viding the conditions of temperature and relative 

 humidity are favorable. Thus, from a wet soil, 

 evaporation is continually removing water. Yet, 

 under ordinary conditions, it is impossible to remove 

 all the water, for a small quantity is attracted so 

 strongly by the soil particles that only a tempera- 

 ture above the boiling point of water will drive it 



