CLIMATE OF THE SANTA CATALINA MOUNTAINS. 



83 



It has been shown in the paper to which reference has been made 

 that the Desert Laboratory is situated well above the level of the 

 cold-air flow of the Santa Cruz Valley, 335 feet below it. The greatest 

 observed difference of minimum temperature in a single night between 

 the Laboratory and the Valley was 24, and the greatest difference 

 between the mean monthly minima of the two localities was 17.8 for 

 May. During the humid mid-summer the mean monthly difference 

 falls to 8 and 9 for these stations. 



TABLE 18. Minimum temperature records to show the operation of cold-air drainage in the 

 open vegetation of Soldier Canon and Bear Canon and its abeyance in the heavy forest of 

 Marshall Gulch. 



In each case one record is from the floor of the canon and the other from its slopes or rim. The 

 minus differences indicate a higher temperature on the floor and the absence of cold-air 

 drainage. 



The vigor of cold-air drainage is determined not only by the condi- 

 tions of cloudiness and wind but also by the size and nature of the area 

 from which the cold air is derived and by the character of the valley 

 bottom through which it moves. In the Santa Cruz Valley cold air 

 is derived from an area of more than 1,000 square miles, resulting in 

 the pronounced low temperatures shown in tables 16 and 18. The 

 broad level trough of the valley is conducive to a slow movement of 

 the air, and the nocturnal minimum is usually reached during the last 

 hours of darkness. The valleys of the Salt and Gila Rivers are larger 

 than the valley of the Santa Cruz, and they have their sources in still 

 higher mountains, but they do not seem to possess a well-marked 



