110 



BULLETIN 12.33, U. S. DEPABTMEXT OF AGRICULTURE. 



only to determine the temperature of some object lying on the surface, 

 exposed to the same insolation as the surface and presumably ab- 

 sorbing somewhat the same amount of the radiant energy. In 

 this study the Freiz soil thermograph has been used, the bulb (1 

 inch in diameter by about 12 inches long") being laid up and down 

 the slope and being about half imbedded in the surface material. 

 Corrections for the thermograph readings have been secured by 

 exposing mercurial thermometers in the same way the thermograph 

 bulbs were exposed. Frequent readings have been made when both 

 thermograph and thermometer would be affected mainly by the soil 

 with which they were in contact. The temperatures, therefore, at 

 correction points, are considerably below the maxima secured in sun- 

 light, and this introduces another possible source of error when the 

 maxima of the thermographs are used. The observations in 1920 

 extended from May to September, inclusive; but, as the July tem- 

 peratures were greatly in excess of those for other months, only 

 these need be considered. (Table 31.) It is possible, with the cor- 

 rections obtained through the 5 months' period, to estimate the 

 maxima from the thermograph traces very closely, except in the 

 record for Station F-5, where the instrument behaved very errati- 

 cally. 



Table 31. — Soil temperatures in July, 1920, in degrees Fahrenheit. 

 [Numbers in parenthesis indicate the decade or day of month in which maxima occurred.] 



F-12 

 F-4. 

 F-6. 

 F-5. 



Western yellow pine ridge .. . 



Pine-fir east slope 



Limber pine northwest slope. 

 Spruce bottom 



Monthly 



Highest 



mean. 



decade. 





(1) 



112. 57 



119. 97 





(1) 



116. 6S 



124. 75 





(1) 



94.45 



9S.81 





(2) 



84. 24 



S9.71 



(22) 



130.6 

 (23) 



142.7 

 (2) 



136.5 

 (2) 



110.5 



Even though these data do not cover a great variety of conditions, 

 it i^ possible to make certain generalizations, which at least indicate 

 the importance of the maximum temperatures attained by the soil 

 surface 4 . 



(1) Tlie extreme temperatures which may be expected at the 

 surface of the soil vary, as between differently insolated sites, by 

 three to four times as wide a margin as do the corresponding 1-foot 

 soil temperatures. For example, the mean Julv temperature for 

 Station F-5 (Table 27) is 49.0°, and for Station F-12 is 58.3°, a 

 difference of 9.3°;. but the difference in their surface temperatures 



v3°. It i- thus Been that the surface temperatures are needed 

 to srive an adequate idea of the possible effects of direct insolation. 



(2) In this Locality it seems that an east exposure may be liable 

 in greater extremes of temperature than a flat or slightly southerly 

 exposure, simply because of the normal tendency of clouds to accu- 

 mulate as the day advances. Thus tin 4 month in question irave 8,336 

 minutes of sunshine before noon, and only 3.027 after noon, the 

 hour- from 7 to 10 a. m. being about 88 per cent clear. Likewise, 



