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. Ata 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- 
