CLIMATE OF THE SANTA CATALINA MOUNTAINS. 69 
flat altitudinal gradient of humidity. The difference between the 
observed humidities of 6 per cent in Sabino Basin and 12 per cent at 
Marshall Gulch is a very small one, and would doubtless register very 
small differences on the rate of evaporation under otherwise identical 
conditions. The differences in evaporation actually found to exist 
between the base and summit of the mountain are to be ascribed to the 
nocturnal humidities, which are higher in the Forest than on the Desert, 
to the greater frequency of cloudiness at higher elevations in the arid 
fore-summer, to the lower temperatures at higher altitudes (especially 
at night), and to the wind protection afforded by the forest cover itself. 
TEMPERATURE. 
The investigation of temperature on the Santa Catalinas has been 
carried on with a view to determining the decrease in length of the 
frostless season which accompanies increase of altitude, the normal 
decrease of temperature with increasing altitude, and the departures 
from the normal gradient of decrease which are due to the nature of 
the topographic relief and to other causes. The results secured afford 
an outline of the major temperature features which are capable of 
influencing the distribution or seasonal activities of the plants of the 
Desert, Encinal, and Forest regions. 
The character of the temperature conditions, and their relation to 
altitude and topography, in an isolated desert mountain is not without 
complexities which make it impossible to predict the conditions for 
vegetation in a given locality through a knowledge of its altitude 
and of general meteorological theory. The relative smallness of the 
entire mountain mass and its position in the midst of arid plains make 
its temperature conditions very different from those of extensive 
plateaus of the same elevation. The currents of warm air which ascend 
by day and the streams of cold air which descend by night serve to 
increase the diurnal amplitude of temperature in certain situations and 
to give striking differences within very short distances. Differences 
of slope exposure bring about differences of diurnal warming and 
nocturnal cooling of the soil, and these differences affect the general 
temperature conditions and also directly influence the vegetation. 
The differences of diurnal warming and nocturnal cooling which exist 
between the relatively bare soils of the Desert and Encinal regions 
and those of the Forest, with their heavy cover of vegetation, their 
litter of leaves and high humus content, are also considerable and tend 
to lessen the importance of topography at the higher elevations. 
Temperature readings have been secured at two localities, at ele- 
vations of 5,300 and 7,600 feet, respectively, since the early summer of 
1908. Since 1911 a series of thermometers has been exposed at the 
rainfall situations at 4,000, 6,000, and 7,000 feet, and during 1913 and 
1914 a complete series of thermometers was maintained at 1,000-foot 
