CLIMATE OF THE SANTA CATALINA MOUNTAINS. 79 
(0.63° C. per 100 m.), and for the latter 4.12° per 1,000 feet (0.75° C. 
per 100 m.). The gradient for Colfax is seen to be almost exactly 
coincident with the entire gradient for the Santa Catalinas. It is 
interesting to note, in this connection, that the fall of temperature in 
the free air has been determined at the Blue Hill Observatory to be 
2.5° F. per 1,000 feet, which is far more gradual than any of the moun- 
tain gradients that have been cited. The Blue Hill data apply only 
to low elevations, but are in substantial agreement with figures more 
recently secured in the free air at Avalon, California.* Seven balloon 
ascensions from Avalon to elevations of 18 km. and higher showed a 
mean gradient of fall in the first 3 km. (9,842 ft.) of 2.2° per 1,000 feet. 
These two determinations of the free-air gradient indicate a conserva- 
tism of temperature change in the lower atmosphere as compared 
with the changes on the slopes of mountains. 
While the normal temperature gradient is of profound interest from 
the standpoint of pure climatology, it is nevertheless of subsidiary 
value in the study of climate in relation to vegetation. Its chief value 
is as a basis with which to compare the differentiation of temperature 
conditions originating in the irregularities of topography and other 
causes. In later pages the subsidiary influences upon the temperature 
gradient will be discussed. 
THE ABSOLUTE MINIMUM OF WINTER. 
The absolute minimum temperature of the winter was secured at 
5,300 feet and at 7,600 feet for four winters, and during the winter of 
1912 and 1913 was secured at four stations, and during the succeeding 
winter at 10 stations, differing both in altitude and in topographic 
location. 
The winter of 1912 and 1913 was one of exceptional severity at 
Tucson—in fact throughout the extreme southwestern United States— 
while the winter of 1913 and 1914 was one of the customary modera- 
tion. The data for these two winters are calculated, therefore, to 
exhibit the extreme and the average conditions of winter temperature 
for stations in Arizona. 
The minimum temperature readings at the mountain stations are 
given in table 16; and in table 17 are given the minima for December, 
January, and February of the same years for a selected series of stations 
in Arizona. The lowest temperature recorded on the Santa Catalinas 
in 1912-13 was —6° at 6,000 feet, while the lowest temperature at 
the highest station, at 7,600 feet, was —2°. This figure should be con- 
trasted both with the absolute minimum at the Arizona Experiment 
Station, 6°, and with that at the office of the Desert Laboratory, 1°, 
as well as with that for Flagstaff, -—23°, situated in northern central 
* Blair, William R. Free-Air Data in Southern California, July and August, 1913. Mo. 
Weather Rev., 42: 410-426, 1914. 
