This classification emphasizes the great variation in distribution
of rainfall over the dry-farm territory of the country. West of the
Rocky Mountains the precipitation comes chiefly in winter and
spring, leaving the summers rainless; while east of the Rockies, the
winters are somewhat rainless and the precipitation comes chiefly in
spring and summer. The Arizona type stands midway between these
types. This variation in the distribution of the rainfall requires
that different methods be employed in storing and conserving the
rainfall for crop production. The adaptation of cultural methods to
the seasonal distribution of rainfall will be discussed hereafter.

Snowfall

Closely related to the distribution of the rainfall and the average
annual temperature is the snowfall. Wherever a relatively large
winter precipitation occurs, the dry-farmer is benefited if it comes
in the form of snow. The fall-planted seeds are better protected by
the snow; the evaporation is lower and it appears that the soil is
improved by the annual covering of snow. In any case, the methods of
culture are in a measure dependent upon the amount of snowfall and
the length of time that it lies upon the ground.

Snow falls over most of the dry-farm territory, excepting the
lowlands of California, the immediate Pacific coast, and other
districts where the average annual temperature is high. The heaviest
snowfall is in the intermountain district, from the west slope of
the Sierra Nevadas to the east slope of the Rockies. The degree of
snowfall on the agricultural lands is very variable and dependent
upon local conditions. Snow falls upon all the high mountain ranges.