12 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



Tadlk 1a. — Summer and Winter Rain- 

 fall at Tucson, (and Fort Lowell) 

 from 1868 to 1912. 



[See Figures 3 and 4]. 



they do not come till mid-July. In exceptionally warm years, however, they may begin 

 unusually early because of the more rapid heating of the continent. Thus in 1910 the mean 

 temperature of the month of May at Tucson was 6° F. above the average of the preceding 

 four years; and showers, light on the plains, but heavy on the mountains, began early in 

 June. Farther north, where the showers begin later, they also end earlier, and instead of 

 lasting into September, terminate in August. Everywhere they are accompanied by vivid 

 lightning and the rainfall is torrential. 



The summer rains are more abundant and less vari- 

 able than those of the winter. At Tucson they average 

 7.14 inches for the six months from INIay to October inclu- 

 sive. During the 45 j^ears embraced in the records at 

 Tucson and Fort Lowell the minimum was 3.01 inches 

 in 1885 and 3.03 in 1900, while the maximum was 14.2 

 inches in 1876. Fifteen sunamers had a rainfall of less 

 than 5 inches, 12 from 5 to 7.5 inches, 9 from 7.5 to 10 

 inches, and 9 over 10 inches. The summer showers are so 

 sudden and the rain falls so rapidly that a large part of 

 the water runs off in great floods, serving no useful pur- 

 pose. Nevertheless, the showers support considerable 

 vegetation, and from the earliest times have enabled the 

 inhabitants to cultivate quickly growing crops like corn 

 and beans, but neither these nor any other crops can be 

 grown without irrigation except in a few places at great 

 altitudes; yet in these places there is always danger of 

 faiku'e. Even with the aid of irrigation the arable area 

 is at best extremely limited. The division of the rainfall 

 into two seasons has, as we shall see, a beneficial effect 

 upon native vegetation, but it can scarcely be considered 

 particularly advantageous to agriculture, especially to the 

 type brought by the inhabitants of Europe to the better- 

 watered parts of the United States. 



From a theoretical standpoint the rainfall of Arizona 

 and New Mexico is peculiarly interesting because the 

 winter rains possess the characteristics of the temperate 

 zone and the summer rains those of equatorial regions. 

 It is not possible to enter into any detailed discussion of 

 the subject at this time, but one or two matters may be 

 pointed out as especially deserving of study. In the fu-st 

 place, at high elevations among the mountains the sea- 

 sonal distribution of precipitation is not the same as in 

 the lowlands. For instance, on the Santa Catalina Mountains, over 9,000 feet in eleva- 

 tion, Dr. MacDougal has found not only that the rainfall is two or thi-ee times as large 

 as down below, as might be expected, but also that the winter rains are heavier than those 

 of summer. This, of course, is the reverse of what prevails in the lowlands. It seems to 

 mean that the climate of the mountains approximates to that of regions farther north, not 

 only in temperature, but in the character of its storms. In other words, it seems as if 

 westerly winds and their attendant conditions prevailed for a longer time, or else during 

 the same time, but more completely at high levels than at low, while in the districts of 

 lower altitude equatorial conditions are predominant so far as precipitation is concerned. 

 To put the matter in another way, it may be that the heating up of the continent in 

 summer disturbs the equiUbrium in the upper air so much less than in the lower that 



