In this region are included Arizona,
New Mexico, western Colorado, and the greater portion of both Utah
and Nevada. The amount of annual relative humidity in the remaining
portion of the elevated district, between the 100th meridian on the
east to the Sierra Nevada and the Cascades on the west, varies
between 55 and 65 per cent. In July, August, and September, the mean
values in the Southwest sink as low as 20 to 30 per cent, while
along the Pacific coast districts they continue about 80 per cent
the year round. In the Atlantic coast districts, and generally east
from the Mississippi River, the variation from month to month is not
great. April is probably the driest month of the year.
The air of the dry-farm territory, therefore, on the whole, contains
considerably less than two thirds the amount of moisture carried by
the air of the humid states. This means that evaporation from plant
leaves and soil surfaces will go on more rapidly in semiarid than in
humid regions. Against this danger, which cannot he controlled, the
dry-farmer must take special precautions.
Sunshine
The amount of sunshine in a dry-farm section is also of importance.
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