78 VEGETATION OF A DESERT MOUNTAIN RANGE, 
just as good data on which to base an estimation of the gradient as 
mean temperatures would be. Since the figures used in constructing 
the gradient were secured in all months from April to October, in 
several years, and in a wide diversity of localities—all outside the 
influence of cold-air drainage—it is probable that the gradient here 
presented, figure 16, is within one or two degrees of the same measure 
of accuracy that could be secured by a long series of consecutive read- 
ings of maximum and minimum. 
Using the elevation of the Desert Laboratory (2,663 feet) as a base, 
the actual fall of temperature between that locality and the 9,000-foot 
station on the Santa Catalinas is at the rate of 4.11° per 1,000 feet. 
The gradients between the several mountain stations are indicated in 
ce 
TS [ Fee. | mar, | apr. | may | sune | oury | auc. | sept] oct. | nov | ec. 
Fig. 17.—Graphs showing march of weekly minimum temperature at 
Desert Laboratory (upper) and weekly or other minimum tempera- 
ture at Marshall Gulch for 1911 (lower). 
figure 16, on which it will be seen that there is a negative gradient 
between the Desert Laboratory and the 4,000-foot station, and that 
the gradient between 4,000 and 5,000 feet is at the rate of 10° per 1,000 
feet. From 5,000 feet upward the gradients are more uniform and 
more nearly equal between the successive stations. 
Figures are given by Hann * for the gradients of temperature on a 
number of mountains in different parts of the wcrld. The average 
values for 17 extra-tropical mountains is 3.18° F. per 1,000 feet (0.57° C. 
per 100 m.). The only mountains in the western United States for 
which Hann gives figures are Pike’s Peak and Sierra Nevada (Colfax, 
Placer County). The gradient of the former is 3.46° per 1,000 feet 
* Hann, J. Handbook of Climatology. Translation by R. DeC. Ward, pp., 243-246. New 
York, 1903. 
