84 VEGETATION OF A DESERT MOUNTAIN RANGE. 
cold-air drainage, to judge by the minimum temperature records for 
the towns on the lower courses of these rivers, as Florence and Phoenix. 
It is possible that in traveling long distances at nearly the same altitude 
the cold air is gradually warmed by mixture with the warm air above it. 
During the winter of 1913-14 and the summer of 1914 thermometers 
were exposed in the Santa Catalinas so as to give a basis for comparing 
the cold-air drainages of the mountain with the drainage of the Santa 
Cruz Valley as investigated at the Desert Laboratory. Table 18 shows 
the readings of instruments placed so as to reveal the differences of 
temperature due to cold-air drainage, at three localities of different 
elevation. The first locality is in Soldier Cafion, in the open Encinal, 
where readings were taken on the floor of the cafion, at 4,900 feet and 
on its slope at 5,025 feet. The lowest minima of the winter were 
identical at the two stations, which can be accounted for only on the 
possibility of the stream of cold air having become so deep as to reach 
the upper thermometer, or else on the possibility that the lowest mini- 
mum of the winter occurred on a cloudy or very windy night. During 
three intervals in the arid fore-summer the depression of the temperature 
in the floor of the cafion was 6.5°, 8°, and 6° respectively, whereas through- 
out the humid mid-summer the depression was absent or negligible. 
The regular 7,000-foot station is located on the rim of Bear Cajion, 
and the data from it may be compared with those from an instrument 
placed in the floor of the cafion 1,000 feet below. This station may 
further be compared with the 6,000-foot station located on the summit 
of Manzanita Ridge. In spite of the difference of 1,000 feet in the 
elevation of the two thermometers in Bear Cajfion, the lowest tempera- 
ture of the winter was 6° in the Cafion and 12° on therim. A two-night 
interval in the autumn of 1913 gave a difference of 5° between these 
stations, due to air drainage, and during the arid fore-summer of 1914 
differences of 9° and 4° were obtained. During the three particularly 
cloudy and rainy nights in July there was an actual reversal of the 
conditions of cold-air drainage and a manifestation of the true tempera- 
ture conditions to be expected from altitude alone, the rim having a 
minimum 2° lower than the floor. Although the winter minimum of 
1913-14 was 6° in the floor of Bear Cafion at 6,000 feet, it was 18° on 
Manzanita Ridge at 6,000 feet. This difference of 12° between two 
stations at the same altitude is as great as should be expected, under 
the operation of the normal gradient of temperature, between localities 
3,468 vertical feet apart (12° -+3.46°, the rate of fall per 1,000 feet). 
Although there are some small bodies of forest on the walls of Bear 
Cafion and many scattered trees, nevertheless the surface of its sides 
is largely occupied by cliffs and boulders (see plate 21), and these are 
responsible for the acute operation of the drainage phenomenon. 
Several preliminary tests had shown an extremely weak manifesta- 
tion of cold-air drainage at the heavily forested elevations of the Santa 
