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 Cafion 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. 



Dates. 



Soldier Canon, floor at 4,900 feet, elope at 5,026 feet: 



Sept. 27, 1913, to May 16, 1914 



May 17 to 19. 1914 



May 20, 1914 



May 21 to July 22, 1914 



July 23 to 28, 1914 



July 29 to Aug. 8, 1914 



Aug. 9 to Oct. 10, 1914 



Bear Canon, floor at 6,000 feet, rim at 7,000 feet: 



Sept. 24 to Sept. 26, 1913 



Sept. 27, 1913, to May 17, 1914 



May 18 to 19, 1914 



May 20 to July 23, 1914 



July 24 to 27, 1914 



Marshall Gulch, bottom at 7,600 feet, rim at 8,000 feet 



Sept. 25, 1913 



Sept. 26, 1913 



Sept. 27, 1913, to May 17, 1914 



May 18 to July 24, 1914 



July 25, 1914 



July 26, 1914 



July 27, 1914 



July 28 to Oct. 11, 1914 



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 



