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 shght 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, the soil temperature fell only to 30°. This means that in the open 

 forest on the rim of Marshall Gulch the soil must have been very shghtly 

 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 shght 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. 



