Temperature

With the exceptions of portions of California, Arizona, and Texas
the average annual surface temperature of the dry-farm territory of
the United States ranges from 40 deg to 55 deg F. The average is not
far from 45 deg F. This places most of the dry-farm territory in the
class of cold regions, though a small area on the extreme east
border may be classed as temperate, and parts of California and
Arizona as warm. The range in temperature from the highest in summer
to the lowest in winter is considerable, but not widely different
from other similar parts of the United States. The range is greatest
in the interior mountainous districts, and lowest along the
seacoast. The daily range of the highest and lowest temperatures for
any one day is generally higher over dry-farm sections than over
humid districts. In the Plateau regions of the semiarid country the
average daily variation is from 30 to 35 deg F., while east of the
Mississippi it is only about 20 deg F. This greater daily range is
chiefly due to the clear skies and scant vegetation which facilitate
excessive warming by day and cooling by night.

The important temperature question for the dry-farmer is whether the
growing season is sufficiently warm and long to permit the maturing
of crops. There are few places, even at high altitudes in the region
considered, where the summer temperature is so low as to retard the
growth of plants. Likewise, the first and last killing frosts are
ordinarily so far apart as to allow an ample growing season. It must
be remembered that frosts are governed very largely by local
topographic features, and must be known from a local point of view.
It is a general law that frosts are more likely to occur in valleys