10 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



as oranges, although much farther north in CaUfornia these grow to perfection. Oddly 

 enough, the warmth of the winter in the intervals between cyclonic storms, when north winds 

 do not prevail, is almost as fatal to such northern fruits as apricots and peaches as is the 

 low temperature to oranges. Since this is a desert region of clear skies and slight humidity, 

 the daily extremes of temperature are naturally great, amounting often to 40°. In Febru- 

 ary, or even January, it is not uncommon for the mercury to rise to 70° F., and if the nights 

 are above the freezing-point the fruit trees are stimulated to open theh blossoms too early. 

 A bhghting wind from the north swoops down, and the flowers are nipped. 



In summer the conditions are the reverse of those of winter, except that the range of 

 temperature from day to night is still extreme. The whole continental interior becomes 

 greatly heated and in Tucson the temperature rises occasionally to 114° F., and at Yunia 

 still higher. Under such circumstances low barometric pressure must of necessity prevail, 

 and winds from the periphery of the continent tend to blow inward. This tendency is so 



5 10 15 



Fig. 1.— Rainfall of Arizona and New Mexico. The figures show annual rainfall in inches. 

 From two maps pubUshed in BuU. No.lSS, Bureau of Plant Industry, U. S. Dept. Agriculture. 



strong that toward the end of June the trade winds, which would normally be expected in 

 the district from Arizona southward, are entirely destroyed. They prevail in normal 

 fashion over the adjacent oceans, but on the continent they give place to somewhat irregular 

 winds whose prevailing direction is distinctly northward. They form, as it were, an inward 

 draft blowing from the Gulf of California and the Pacific Ocean on the one hand, and 

 from the Gulf of Mexico on the other, toward the continental center of low pressm-e. 

 In all essential respects they are like the monsoons of India, although less strong and 

 distinct because of the smaller size of the continent. As the American monsoons approach 

 the land, the fu-st tendency is for them to become heated and hence relatively dry, for the 

 land is hotter than the ocean, and in many places the height of the mountains is too sUght 

 to overcome the heating due to the land. The case is hke that of the plains of Sind at the 

 mouth of the Indus. As the winds blow inward, however, they are soon forced to rise by 

 the mountains, they reach more northerly and hence cooler latitudes, and they enter the 

 continental area of low pressure where the general tendency of atmospheric movements 



