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 



60 



SO 



30 



10 v* 



FEB. MAR. 



MAY JUNE JULY AUG. SEPT. OCT. 



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 world. The average 

 values for 17 extra-tropical mountains is 3.13 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 (Coif ax, 

 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. 



