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 Phcenix. 

 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 Canon, in the open Encinal, 

 where readings were taken on the floor of the canon, 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 canon 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 Canon, 

 and the data from it may be compared with those from an instrument 

 placed in the floor of the canon 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 Canon, the lowest tempera- 

 ture of the winter was 6 in the Canon and 12 on the rim. 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 Canon 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 -f- 3.46, the rate of fall per 1,000 feet). 



Although there are some small bodies of forest on the walls of Bear 

 Canon 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 



