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 314 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 réle 
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-summer. The few figures to be given here were 
secured with a sling psychrometer and converted to percentages by 
the use of Marvin’s tables. 
