CLIMATE OF THE SANTA CATALINA MOUNTAINS. 



71 



the frost dates which limited the growing season fell between the last 

 visit of autumn and the first one of spring. This has particularly been 

 the case with all of the lower stations. These circumstances have 

 made it necessary to resort to an indirect method of determining the 

 dates, which is as follows: A series of graphs was drawn showing the 

 march of the weekly absolute minimum temperatures at the Desert 

 Laboratory, as registered by thermograph, for the years covered by 

 the mountain records. Each reading of minimum temperature for a 

 given station was then compared with the minimum for the same 

 period at the Desert Laboratory, and the total number of such differ- 

 ences was averaged. In this manner it was possible to secure the 

 figures given in tables 10 and 11. 



TABLE 10. Altitudinal shortening of the frostless season in the Santa Catalina Mountains, 

 as shown by the dates of the last spring occurrence and the first autumn occurrence of a 

 temperature of 32 at 3 altitudes in 1908, 1909, and 1910. 



With a knowledge of the average difference between the minimum 

 temperature at the Desert Laboratory and at a given station, and with 

 the graph showing the march of minima at the Laboratory, it was 

 possible to locate the approximate date of the last and first occurrence 

 of 32 at the mountain station. Such a graph for 1911 is given in 

 figure 17, together with the graph of march of minimum temperatures 

 at the Montane Plantation in Marshall Gulch, at 7,600 feet. It will 

 be noted that the graph for the Laboratory rises by several pronounced 

 jumps during March, April, and May, and falls by precipitate stages 

 during September, October, and November. The relatively sudden 

 advent of summer and of winter is an invariable annual occurrence, 

 and it has helped to make more accurate the estimation of the limiting 

 dates for the mountain stations. In the cases in which a minimum 



