The vegetation of a desert mountain range as conditioned by climatic factors . characterize southern Arizona: a low unequally-distributed rainfall, a short winter with frequent severe frost, and along summer with high maximum temperatures and low atmospherichumidity. The longitudinal position of the Santa Catalinas, betweenthe Pacific Coast and the southern Great Plains, gives to their climatealso some of the characteristics of both these regions, notably in respectof the incidence of the rainfall seasons. Both the winter rains of thePacific Coast and the summer rains which are prevalent on th
The vegetation of a desert mountain range as conditioned by climatic factors . characterize southern Arizona: a low unequally-distributed rainfall, a short winter with frequent severe frost, and along summer with high maximum temperatures and low atmospherichumidity. The longitudinal position of the Santa Catalinas, betweenthe Pacific Coast and the southern Great Plains, gives to their climatealso some of the characteristics of both these regions, notably in respectof the incidence of the rainfall seasons. Both the winter rains of thePacific Coast and the summer rains which are prevalent on the GreatPlains extend in attenuated form to Tucson and to the Santa Catalinas,giving them a short rainy season in July and August, often extendingover into September, and a longer less pronounced rainy season fromDecember to February or March.* Although the amount of rain in theseseasons increases with altitude, the duration of the seasons themselvesis 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 Tucsonand in the Santa Catalina Mountains, showing averaged limiting dates of rainfall seasonsfor 8 years and averaged limits of the frostless season for 1909, 1910, and 1911 (A A), andfor 1912, 1913, and 1914 (B B). The long frostless season characteristic of Tucson and the foothillsof the Santa Catalinas naturally decreases in length with altitude untilat 8,000 feet it is only one half as long. The curves of decreasing lengthof frostless season and a diagrammatic representation of the incidenceof the rainy seasons are shown in figure 2. The gentle rains and occasional snowfall of the winter season serveto replenish the moisture of the soil at all altitudes, but on the desert * See Shreve, :&-26, 1914. Rainfall as a Determinant of Soil Moisture. The Plant World, CLIMATE OF THE SANTA CATALINA MOUNTAINS. 47 thei
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