CLIMATE OF THE SANTA CATALINA MOUNTAINS. 



53 



exactly the same days covered by the Marshall Gulch record), and 

 14.86 inches at the mountain station. The general correspondence 

 between the dates of heavier rains at these stations, 5,000 vertical 

 feet apart, indicates the close relationship of the atmospheric factors 

 which determine the rainfall of all altitudes. 



Table 5. — Comparative daily incidence of rainfall at the Desert Laboratory {2,663 feet) and 

 at the Montane Garden in Marshall Gulch {7,600 feet), for June, July, and August 1911. 



Total rainfall: Desert Laboratory, 5.42 in.; Montane Garden, 14.86 in. Total number of 

 rainy days: Desert Laboratory, 15 (or 31, including days with T); Montane Garden, 19. 



Another comparison which it is possible to institute between the 

 summit of the Santa CataHnas and the desert is the summer rainfall 

 totals from 1907 to 1914 inclusive (see fig. 9). The directions of the 

 curves which show the march of the summer precipitation from year 

 to year indicate an almost complete lack of relationship between the 

 mountain and the plain. It is obvious that the curve of altitudinal 

 increase of rainfall determined in such a year as 1910 would be very 

 unlike the curve determined in 1911. 



It has been suggested by Smith * that there may be a relative 

 increase of rainfall at the higher altitudes as the summer advances, 

 which is to say that the gradient of increase of rainfall with altitude is 

 steeper for the late summer than it is for the early summer. In order 

 to test this possibility the series of ten readings taken in the humid mid- 

 summer of 1911 and the one set taken in the early arid after-summer 

 have been grouped into totals for five periods of approximately one 

 month each (table 6). An inspection of the table shows that the maxi- 

 mum rainfall occurred between July 18 and August 24 at 3,000, 4,000, 



* Smith, G. E. P. Groundwater Supply and Irrigation in the Rillito Valley. 

 Exper. Sta. BuU. 64, 1910. 



Ariz. Agric. 



