46 VEGETATION OF A DESERT MOUNTAIN RANGE. 
CLIMATE OF THE SANTA CATALINA MOUNTAINS. 
The latitude of the Santa Catalina Mountains and their position in 
the midst of a continental desert give to their lower slopes the climate 
which is well known to characterize southern Arizona: alow 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 eo southeastern Arizona. 
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Fie. 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. Rainfall as 1 Determinant of Soil Moisture. The Plant World, 
17: 9-26, 1914. 
