46 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



CLIMATE OF THE SANTA CATALINA MOUNTAINS. 



The latitude of the Santa Catahna Mountains and their position in 

 the midst of a continental desert give to their lower slopes the cHmate 

 which is well known to characterize southern Arizona : a low unequally- 

 distributed rainfall, a short winter with frequent severe frost, and a 

 long summer with high maximum temperatures and low atmospheric 

 humidity. The longitudinal position of the Santa Catalinas, between 

 the Pacific Coast and the southern Great Plains, gives to their climate 

 also some of the characteristics of both these regions, notably in respect 

 of the incidence of the rainfall seasons. Both the winter rains of the 

 Pacific Coast and the summer rains which are prevalent on the Great 

 Plains extend in attenuated form to Tucson and to the Santa Catalinas, 

 giving them a short rainy season in July and August, often extending 

 over into September, and a longer less pronounced rainy season from 

 December to February or March.* Although the amount of rain in these 

 seasons increases with altitude, the duration of the seasons themselves 

 is essentially the same from Tucson to the summit of Mount Lemmon, 

 and in fact throughout southeastern Arizona. 



mm^:'t. 



m 



._L. 



M 



Fig. 2. — Schematic representation of rainfall seasons and length of frostless season at Tucson 

 and in the Santa Catalina Mountains, showing averaged limiting dates of rainfall seasons 

 for 8 years and averaged limits of the frostless season for 1909, 1910, and 1911 (A A), and 

 for 1912, 1913, and 1914 (B B). 



The long frostless season characteristic of Tucson and the foothills 

 of the Santa Catalinas naturally decreases in length with altitude until 

 at 8,000 feet it is only one half as long. The curves of decreasing length 

 of frostless season and a diagrammatic representation of the incidence 

 of the rainy seasons are shown in figure 2. 



The gentle rains and occasional snowfall of the winter season serve 

 to replenish the moisture of the soil at all altitudes, but on the desert 



* See Shreve, Forrest. 

 17:&-26, 1914. 



Rainfall as a Determinant of Soil Moisture. The Plant World, 



