44 DRY-FARMING 
districts. In the Plateau regions of the semiarid 
country the average daily variation is from 30 to 
35° F., while east of the Mississippi it is only about 
20° 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. Like- 
wise, 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 in- 
creases 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. 
