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 Cafion 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 cafion as contrasted with its sides. This effect has been 
repeatedly observed in Bear Cafion, where the oaks on the floor of 
the cafion are always far behind the individuals on the cafion 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 jamesit takes place in the floor of the cafion, 
while the herbaceous plants of the slopes are still green and active. 
The plants on the cajion 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 cajions 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 
