44 DRY-FARMING 



districts. In the Plateau regions of the semiarid 

 country the average dail}' variation is from 30 to 

 35° F., while east of the ^lississippi it is only about 

 20° F. This greater daily range is chiefly due to the 

 clear skies and scant vegetation which facilitate 

 excessive warming l)v day and cooling by night. 



The important temperature cjuestion for the dry- 

 farmer is whether the gnnving season is sufficiently 

 warm and long to permit the maturing of crops. 

 There are few places, even at high altitudes in the 

 region considered, wh(>re the summer temperature 

 is so low as to retard the growth of jjlants. Like- 

 wise, the first and last killing fnjsts are ordinarily 

 so far apart as t(_) allow an amj^le growing season. 

 It must be remembei-ed that frosts are governed 

 very largely by local tojiographic features, and must 

 be known from a local p(.)int of view. It is a general 

 law that frosts are more likely to occur in valleys 

 than on hillsides, owing t(j the d(_)\vnward drainage 

 of the cooknl air. Further, the danger of frost in- 

 creases with the altituile. In general, the last 

 killing frost in spring over the dr}'-farm territory 

 varies from March 1.5 to May 29, and the first killing 

 frost in autumn from September 15 to November L3. 

 These limits permit of the maturing of all orchnary 

 farm crops, especiall}' the grain crops. 



