CLIMATE OF THE SANTA CATALINA MOUNTAINS. 87 
show that the soil temperatures at 6,000 feet were cooler in general, 
in terms of the air temperature, than were those of the 8,000-foot 
station. This difference is not to be attributed to the difference of 
elevation so much as to the naked and stony character of the soil at 
the 6,000-foot station and the relatively abundant humus and litter in 
the surface soil at 8,000 feet. In short, the radiation from the soil 
surfaces in the Encinal is greater than it is in the Forest, as has been 
already discovered from the difference in the behavior of cold-air 
drainage in these two regions. There is also a slight indication that 
the differences between the air and soil minima are least in the dry 
seasons of May and September, which is again in keeping with the 
greater radiation exhibited in dry soils as compared with wet ones. 
On the night of September 25, 1913, the difference between the air 
and soil temperatures was simultaneously determined on the rim of 
Marshall Gulch at the 8,000-foot station and in a thicket of young fir 
trees in the bottom of the gulch. The soil remained 6.5° warmer than 
the air on the rim of the gulch and 9° warmer in the fir thicket, showing 
the degree to which a heavy cover of vegetation retards radiation and 
conserves the warmth of the soil. On this night the air temperature 
in the bottom of the gulch was 1° lower than that on the rim (see 
table 18). 
One of the most striking features of the soil minima is the fact that 
although the air temperature at 8,000 feet fell to 5° in the winter of 
1913-14, thesoil temperature fell only to30°. Thismeansthat inthe open 
forest on the rim of Marshall Gulch the soil must havebeen very slightly 
if at all frozen in the winter in question, which was apparently a winter 
of about average severity. During the same winter a lower absolute 
minimum of the soil was recorded at 6,000 feet than at 8,000 feet. In 
shaded situations and on north slopes in the Fir Forest the soil undoubt- 
edly freezes to a slight depth. Inasmuch as no soil temperatures have 
been secured with the bulb of the thermometer in contact with the 
soil, and no readings have been secured at a greater depth than 3 cm., 
the further discussion of soil temperature conditions in these moun- 
tains should await further investigation. 
