THE DOMAIN FOR DRY FARMING 47 



in process of filling. In the higher elevations, and 

 especially in "proximity to the mountains, summer frosts 

 do more or less harm, but the injury that would other- 

 wise result is much neutralized by the dryness of the 

 atmosphere. Hail also is to be reckoned with, but usu- 

 ally the area covered by hailstorms is not very exten- 

 sive. Certain quick growing grain crops may be matured 

 in the latitude of Denver, Col., at an altitude of fully 

 7,000 feet. 



The influence of soils is also marked on production 

 and on the relative amounts of rainfall called for to make 

 production safe. For the further discussion of this ques- 

 tion see chapter IV. From what has been said, it will 

 be abundantly apparent that the adaptation of the various 

 areas of the semi-arid country for the growing of crops 

 cannot by any means be judged of correctly on the basis 

 of the amount of the precipitation alone. 



Arid and semi-arid America. The area comprised as 

 arid and semi-arid in the United States is not easily de- 

 fined owing to the great variation in the conditions that 

 lead to aridity or the opposite in the various states of the 

 west. For instance, the rainfall in certain areas of a state 

 usually considered arid may be such as to make these 

 areas come within the sub-humid class. In those por- 

 tions of Colorado where the normal annual rainfall runs 

 between 15 and 20 inches, is Gunnison county, where an 

 annual precipitation of 50 inches is reported. These 

 marked variations may be found in all or nearly all of 

 the arid and semi-arid states. They are caused in part at 

 least by a difference in the elevation. The cooling of 

 the moisture-laden air through expansion as it rises over 

 a range of hills or mountains leads to precipitation. This 

 explains in part at least why the precipitation is so much 

 greater west than east of the 'Cascades. But this is not 

 the sole explanation of the difference in the precipita- 

 tion, as marked differences occur in the normal rainfall 



