As shown in Chapter IV,
the relative humidity becomes smaller as the rainfall decreases. The
lower the relative humidity is at a given temperature, the more
rapidly will water evaporate into the air. There is no more striking
confirmation of this law than the fact that at a temperature of 90
deg sunstrokes and similar ailments are reported in great number
from New York, while the people of Salt Lake City are perfectly
comfortable. In New York the relative humidity in summer is about 73
per cent; in Salt Lake City, about 35 per cent. At a high summer
temperature evaporation from the skin goes on slowly in New York and
rapidly in Salt Lake City, with the resulting discomfort 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, therefore, 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 temperature
is higher; the relative humidity lower, and the winds usually more
abundant in arid than in humid regions.
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