CLIMATE OF THE SANTA CATALINA MOUNTAINS. 67 



and on opposed slopes without instrumental data for several series of 

 stations and covering several years. It is possible, nevertheless, to 

 state from the data presented that (a) the rate of evaporation through 

 the arid and humid summer seasons is about 33^ times as great on the 

 desert as it is at 8,000 feet; (6) the rates of evaporation are approxi- 

 mately half as great in the humid mid-summer as they are in the arid 

 fore-summer; (c) at the middle and higher altitudes the evaporation 

 on north slopes is less than on south slopes ; (d) the difference between 

 the amounts of evaporation on north and south slopes becomes greater 

 with increase of altitude, in proportion to the amounts of each. 



HUMIDITY. 



The prevalence of low atmospheric humidities is one of the most 

 pronounced features of desert climate and is an extremely potent factor 

 in causing the high rates of evaporation that have been shown to occur 

 at the lowest stations in the Santa Catalina Mountains. The relative 

 humidity is lowest in the arid fore-summer, although it is sometimes 

 nearly as low for brief periods in the arid after-summer. During the 

 two rainy seasons the humidity is extremely variable and may fluctuate 

 through a daily range of as much as 70 per cent. The daily curve of 

 humidity is extremely uniform during the cloudless days of April, May, 

 and June, falling rapidly during the early forenoon to mid-day values 

 as low as 5 and 10 per cent, and rising slowly through the late afternoon 

 and more rapidly during the night to a daily maximum of 20 to 30 per 

 cent just before sunrise. Cloudy days in the arid seasons cause a higher 

 minimum but seldom raise the maximum above 40 per cent unless 

 there is a trace of rainfall. 



The humidities of the mountain, varying with altitude and with the 

 seasons, possess their greatest importance for vegetation in their role 

 as joint determinants of the rate of evaporation. The altitudinal 

 gradient of humidity has, therefore, been most satisfactorily investi- 

 gated when it has been measured together with temperature and wind 

 in the collective effect of these climatic factors upon the evaporative 

 power of the air. It is not without interest, nevertheless, to know 

 something of the relative humidities which are prevalent at the moun- 

 tain altitudes and are partially responsible for the rates of evaporation 

 encountered there. 



In spite of the pronounced altitudinal changes of vegetation and of 

 climatic conditions which have been discussed (or are yet to be treated), 

 there are so many features of the Encinal and Forest vegetation that 

 strongly suggest the Desert (see p. 36) that it seemed particularly 

 desirable to secure readings of relative humidity at the forested alti- 

 tudes in the arid fore-sunamer. The few figures to be given here were 

 secured with a sHng psychrometer and converted to percentages by 

 the use of Marvin's tables. 



