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
than on hillsides, owing to the downward drainage of the cooled air.
Further, the danger of frost increases with the altitude. In
general, the last killing frost in spring over the dry-farm
territory varies from March 15 to May 29, and the first killing
frost in autumn from September 15 to November 15. These limits
permit of the maturing of all ordinary farm crops, especially the
grain crops.
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