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 Catalinas 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 
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Fie. 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- 
linas 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 of 
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). 
