CLIMATOLOGY 



It is essential that the climatic conditions of Arizona be under- 

 stood by farmers, since they are nt)t duplicated in any other part 

 of the United States, excepting in adjacent regions of bordering 

 states, and are subject to extreme variations in relatively short 

 distances. 



TEMPERATURE 



Arizona, in common with all of the region between the Rocky 

 :Mountains and the coast ranges, is subject to great variations be- 

 tween minimum and maximum temperatures, the former being rela- 

 tively low at a given altitude and latitude, and the latter relatively 

 high. The effects of extreme temperatures, however, are not so 

 pronounced as in a humid climate. The wide variation in tempera- 

 tures of night and day has a decided effect upon plants. Certain 

 sorghums, for example, wdiich w^ill mature in a given number of 

 days in a region having approximately the same maximum tempera- 

 ture as Snowflake fail to ripen at the latter place in a considerably 

 longer time, though they may not be at all injured by frost. Night 

 temperatures of the frost-free season in the higher altitudes of 

 Arizona, while not low enough to directly injure tender plants, are 

 too low to permit proper development throughout a large part of 

 the growing period. Extremely wide local variations are noted : for 

 example, in the Fort Valley Forest Experiment Station, west of 

 Flagstaff and directly at the base of San Francisco Mountain, lower 

 temperatures occur later in spring and earlier in autumn than one 

 thousand feet higher on the mountain side, and the same frost tem- 

 l)erature at each of the two points shows less eft'ect on vegetation on 

 the mountain slope than in the valley bottom. Likewise, in places 

 at the base of Cinder Mountain, east of Flagstaff', temperatures at 

 the same elevation vary decidedly. This inconsistency is due to lo- 

 calized atmospheric movement controlled by the abrupt topography. 

 Furthermore, there is a greater variation between average and 

 extreme dates of the last killing frost in spring and the first in 

 autumn than is ordinarily expected. See Table I. For this reason 

 it is more or less risky to plant crops which demand all of the ex- 

 pected frost-free season for their maturity. 



