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, Cahfornia.* 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 Laboratorj^, 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. 



