CLIMATE OF THE SANTA CATALINA MOUNTAINS. 85 



Catalinas. The data presented in table 18 for the rim of Marshall 

 Gulch at 8,000 feet, and for the Montane Garden at 7,600 feet, bear 

 out the results of the preliminary tests. The rim was 10 colder than 

 the bottom of the gulch in the over- winter period and 18 colder in 

 the period from May 18 to July 24. The 5 one-night readings and the 

 readings for the period from July 28 to October 11 all show an equality 

 or a slight difference, more often a difference, with the temperature 

 more commonly higher in the bottom of the Gulch than on the rim. 

 In other words, cold-air drainage is in abeyance at this locality. 

 Whether this is invariably the case can only be stated after further 

 instrumentation and after complete assurance that the readings have 

 not been influenced by the character of the weather during the nights 

 of lowest temperature. The heavy vegetation of the Forest region, 

 together with the high humus content of the soil and the litter of leaves 

 by which it is covered, all militate against the rapid terrestrial radia- 

 tion in which cold-air drainage has its origin. It will not be surprising, 

 therefore, to find that the phenomenon is either very weak or absent 

 above the elevation of 7,000 feet in such portions of the mountain as 

 are forested. The elimination of cold-air drainage by a forest cover 

 can take place only in small mountains which are forested to the sum- 

 mit. A large mountain mass, an extremely steep mountain side, or 

 an extensive area lying above timber line will cause a flow of cold air 

 down through forested areas below. This is exemplified at the San 

 Francisco Peaks, Arizona. 



The case mentioned in which cold-air drainage occasioned a differ- 

 ence of temperature at the same altitude, which was the equivalent of 

 nearly 3,500 feet, probably represents its maximum effect. The differ- 

 ence of 8 in Soldier Canon is the equivalent of an altitudinal difference 

 of about 2,200 feet. 



The influence which cold-air drainage exerts on vegetation is regis- 

 tered chiefly in the shortening of the season of vegetative activity on 

 the floor of a canon as contrasted with its sides. This effect has been 

 repeatedly observed in Bear Canon, where the oaks on the floor of 

 the canon are always far behind the individuals on the canon wall in 

 the advancement of their new foliage in the spring. Likewise in the 

 autumn the frost-killing of herbaceous perennials and of the leaves of 

 Prunus, Rhus, and Populus jamesii takes place in the floor of the canon, 

 while the herbaceous plants of the slopes are still green and active. 

 The plants on the canon floor are, in other words, subjected to a grow- 

 ing season similar to that usually found at a much higher altitude. 



The influence of cold-air drainage in determining the distribution of 

 plants is likewise marked. It is not wholly responsible for the fact 

 that mountain species extend down the canons to lower altitudes than 

 they assume on slopes or ridges, for the influence of ground water and 

 soil moisture is very potent in this connection. The occurrence of the 



