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 



