FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 



87 



10 a.m., and a few as early as 7 a.m. The minimum soil temperature 

 occurs usually several hours later than the minimum air temperature, 

 but for most sites it occurs some time between 8 a.m. and noon. 

 It is, therefore, readily seen that the tendency has been to record, 

 for 1-foot temperatures, the minima rather than the mean tempera- 

 tures for the days of record. The conductivity of the iron pipe has 

 an influence in making these minima lower than they actually were 

 in the soil, unless the insolation of the current day has proceeded 

 long enough to be felt at the base of the pipe, as, for example, on an 

 easterly exposure. 



The mean temperature of the soil of a given site may be almost 

 constantly either higher or lower than the mean air temperature, 

 depending on the amount of insolation received. The air tempera- 

 ture in a large measure reflects the average soil temperature of all 

 the sites in a region of considerable extent. According to Moore 

 (15) j however, the more or less constant difference between the tem- 

 peratures of sites in the same locality disappears when great depths 

 are considered. Indeed, it is sometimes said that at a 4epth of 

 about 50 feet the soil temperature does not change and is, at any 

 time, an accurate index to the mean annual air temperatures above. 



At a depth of 4 feet the soil temperatures, although not subject to 

 rapid changes, appear to show in their year-long values the amount 

 and effectiveness of the insolation received locally. It is, therefore, 

 believed that a comparison of the 1-foot and 4-foot temperatures for 

 entire years is the simplest means of determining the probable ex- 

 tent to which the factors mentioned above — that is, the use of iron 

 pipes and the prevalence of morning observations — have tended to 

 give values for the 1-foot depth lower than the true means. 



Table 24 shows the probable mean error in the 1-foot soil tempera- 

 tures as determined by comparison with the 4-foot temperatures for 

 corresponding periods. It must be recognized that the mean cor- 

 rections so indicated are not equally applicable at all seasons, and 

 may not be absolutely right, even though relatively so for the several 

 stations. It should be noted also that the periods of comparison 

 here considered are not identical with those for which the average 

 temperatures for each station and depth have been computed. 



Table 24. — Probable mean error in 1-foot soil temperatures. 

 [Determined by comparison with 4-foot temperatures for corresponding periods.] 





Probable 





Probable 





depression 





depression 





of temper- 





of temper- 





ature in 





ature in 



Station. 



pipe below 



Station. 



pipe below 





the mean, 





the nuan, 





at hour of 





at hour of 





observa- 





observa- 





tion. 





tion. 





° F. 





° F. 



F-l 



12.92 

 1.01 

 0.71 

 0.34 

 1.40 

 1.17 

 1.10 

 0.94 

 1.19 

 0.78 



F-13 





F-2 



F-14 ... 



1.11 



F-3 



F-15... . .. 



0. '.17 



F-4 



F-1G ... 



1.38 



F-5 



H-2 



L.05 



F-6.... 



M-l 



0.64 



F-7-8 



W-A 1 



W-A2 



0. 38 



F-9 



»3.16 



F-ll 



W-D 



0.05 



F-12 



W-F 



0.34 









1 1916-17 only 





2 1914-1917. 





