CLIMATE OF THE SANTA CATALINA MOUNTAINS. 



73 



sideration of the two sets of curves is more fruitful than the possession 

 of their average, as it shows the extent of a fluctuation which must 

 be a common and normal feature of the climatology of the mountain, 

 just as it is of every locality, regardless of its topographic location. 



The average length of the frostless season at the Desert Laboratory 

 is about 38 weeks, at 5,000 feet in the Santa CataUnas it is about 

 30 weeks, while at 8,000 feet it is about 17 weeks. The period of safe 

 plant activity is, therefore, less than half as long in the Forest region 

 of the mountain as it is in the Desert of the Santa Cruz Valley. The 

 altitudinal abbreviation of the frostless season is of primary importance 

 to vegetation, especially as it is accompanied by a series of inseparable 

 features of temperature, such as the lower range of the entire daily 

 curve of temperature, the attainment of lower minima, the more 



Fig. 15. — Schematic representation of length of frostless season at different altitudes in Arizona, 

 together with curves for limits of growing season at successive altitudes in the Santa Cata- 

 Unas for 1909 to 1914 inclusive. 



frequent occurrence, and the longer duration of freezing temperatures. 

 One of the cardinal features of importance in the altitudinal shortening 

 of the growing season is the concomitant shortening of the arid fore- 

 summer. The rising temperatures of spring call the vegetation of the 

 Desert into activity at a time of the year when extremely arid condi- 

 tions are bound to prevail for from 14 to 16 weeks. At the summit ot 

 the mountain, however, the advent of spring is only 5 or 6 weeks in 

 advance of the earliest of the mid-summer rains. In other words, the 

 most trying season of the year is shortened on the mountain by the 

 inhibitory effects of low temperatures, so that the arid fore-summer is 

 only one-third as long in the Forest as in the Desert. These relations 

 are graphically represented in figure 2, which shows both the curves 

 of the frost dates and the incidence of the rainy seasons. More will 

 be said in regard to this subject in a succeeding section (see p. 93). 



