FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 



37 



such days are favorable to the growth of conifers, or whether, for the 

 Rocky Mountain species, they may not represent excessive heat, 

 might possibly be determined by growth studies; but nothing can be 

 said on the subject at present. 



No less interesting is the fact that January and March show not 

 only a number of days with mean air temperatures above freezing, 

 but the probability of a few days when the moan will be above 41°, 

 the minimum scarcely below 32°, with the possibility of very effec- 

 tive thawing of the soil on well-insolated sites, and of some snow 

 melting on any site at tins elevation. 



Ground temperature at control station. — The temperature of the at- 

 mosphere at the control station has been shown in considerable detail 

 for the purpose of depicting prevailing conditions at a middle eleva- 

 tion in the mountains. In addition to this the temperatures at the 

 control station are to serve as a base with which all other temperature 

 records will be compared for their respective periods. At a number 

 of the compared stations in various forest types only ground tem- 

 peratures have been recorded, and at practically all these stations 

 the elevation of the thermometers is considerably less than 20 feet. 

 It is desirable, therefore, to know how the ground temperatures at 

 the control station may compare with those of the higher aerial 

 position. 



As the control station is on a southerly exposure, though the gra- 

 dient is very slight, it is to be expected that the purely local tempera- 

 tures will be somewhat higher than the mean temperatures for the 

 locality, recorded 20 feet above the ground. This proves to be true. 

 In Table 6 are given, in condensed form, the results of a comparison 

 of the two for two years ending March 31, 1918. In this period the 

 maximum and minimum registering thermometers for the ground 

 location were placed on a shielded board, which did not entirely pro- 

 tect them from reflected light and heat, but allowed very free cooling 

 at night, as already described under "The Method of Study." As no 

 thermograph was employed, the ground temperatures are not syn- 

 chronous with those of the tower station, but they always represent 

 periods about 8 hours later. 



Table 6. — Departure of ground temperatures from temperatures 20 feet above ground 

 at the control station (with dissimilar exposure of instruments). 



Datum. 



Record by months in degrees Fahrenheit. 



Mean 



an- 



Mean 

 grow- 

 ing 

























nual. 



sea- 





Jan. 



Feb. 



Mar. 



Apr. 



May. June. 



July. 



Aug. 



Sept. 



Oct. 



iNov. 



Dec. 





son. 



Mean temper- 









1 



















ature 



+0.4 +0.5 



+1.3 



+2.3 



+1.5 



+0.9 



+0.8 



+ 1.1 



+0.9 



+1.5 



-0.1 



+1.1 



+ 1.0 - 



Mean maxima. 



-rl.7 



+2.1 



+3.0 



+5.3 



+ 4. 5 



+4.5 



+4.1 



+3.9 



+4.2 



+3. 4 



+2.2 



+2.6 



+3. 5 



+ 4.2 



Mean minima . 



- .9 



-1.2 



- .4 



- .7 



-1.4 



-2.6 



-2. 4 



-1.7 



-2.4 



- .4 



-2.3 



- .5 



-1.4 



-2.3 



Absolute min- 































ima 



-1.8 



-3.5 



-1.8 



-3.9 



-1.2 



-2.2 



-2.8 



-1.8 



-1.0 



- .7 



-1.2 



-2.1 



-2.0 



-2.2 



Daily ran pi'. .. 



+2.3 +3.3 



+3.4 +6.0 



+5.8 



+7.1 



+6.5 



+5.6 



+6.6 



+3.8 



+4.5 



+3.0 



+4.9 



+ 6.4 



See note, p. 29. 



It is seen that the maxima are always higher and the minima always 

 lower near the ground than at the 20-foot elevation. This results, of 

 course, in a much greater daily ran^e, and in a slightly higher mean 

 temperature close to the ground. The difference seems to be greatest 



