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 Cation and Bear Caton and its abeyance in the heavy forest of 
Marshall Gulch. 
In each case one record is from the floor of the cafion 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. Slope or Floor. | Difference. 
rim. 
Soldier Cafion, floor at 4,900 feet, slope at 5,025 feet: 
Sept. 27, 1913, to May 16, 1914..............-.054. 18 18 0 
May 27 t6: 19, 1914) oso s:ix 6 gs we ptrnevntntew nei. wlaridne 49 42.5 6.5 
MSY 205 LOVE orisc. coo cod dee chsk esta hoot Sarenevererereeurd wienre ae ase 52 44 8 
May 21 to July 22, 1914. .... 0... 00. cece eee eee 45 39 6 
July 23 to 28, 1914... . cee cee cee tet eaee 60 60 0 
July 29 to Aug. 8, 1914........... ccc ccc cee eee 63 62 1 
Aug. 9 to Oct. 10,1914. ....... 0. cece eee eee 40 40 0 
Bear Cajion, floor at 6,000 feet, rim at 7,000 feet: 
Sept. 24 to Sept. 26, 1913........... 0. eee eee eee 38 23 5 
Sept. 27, 1913, to May 17, 1914...............0006- 12 6 6 
May 18 to: 19; 1914. seisedewsaaeevawed sare scaas ans 47 38 9 
May 20 to July 23, 1914............ cee eee eee eee 38 34 4 
duly 24 ta 27, Wlave sans ceiccednnweessaaad44 Be Ode 52 54 — 2 
Marshall Gulch, bottom at 7,600 feet, rim at 8,000 feet: 
Sept: 26; LOS siicaiscwecadin dee ee eon tas ve ee aeOeS 34.5 33.5 1 
Nepts 26; TOUS od cesatiareseauaiaes dae he Vs ears Os Mien dee 30.5 31.5 -1 
Sept. 27, 1913, to May 17, 1914...............0000- 5 15.5 —10.5 
May 18 to July 24, 1914........ 0... cece eee eee 33.5 51.5 —18 
DULY? 25, LOVE cicinincrescatee Guo ueegr tet ek en aRaes 50.5 49.5 1 
DULY 26, LOMA, ois ciwinlene: duncocave Geet eG SAA RRR A ae Re 48.5 50.5 — 2 
WY! 27, V91G os ovat cewswscrmeiwve + Oye Nee eeu ee 51.5 51.5 0 
July 28 to Oct. 11, 1914... . 0... 00. ec eee 29.5 32.5 - 3 
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 
