FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 



67 



Table 15 indicates that there is an increase in the vapor pressure, 

 corresponding to the rise in temperature from 7 a. m. to 1 p. m., and 

 amounting on the average to 0.0284 inch, or 0.0047 inch per hour, and 

 that most of the increase has been lost by 7 p. m. There is every rea- 

 son for supposing that the lowest vapor pressure of the day would be 

 recorded with or shortly following the minimum air temperature, for, 

 if this is lower than the dew point, some of the atmospheric moisture 

 is certain to be precipitated as dew or frost. The humidity at 7 a. m. 

 is, then, probably less than the mean humidity for the day, as the tem- 

 perature at that hour is slightly below the mean temperature. The 8 

 a. m. humidity, however, can not miss the mean for the day by a very 

 wide margin. 



To estimate the probable increase from 7 a. m. until 1 p. m., at 

 any other station, it is best to use the average figure, 0.0284 inch, 

 representing the whole year; for, though October and January do 

 not agree in the amount of this change, October, 1910, and October, 

 1911, were by no means uniform in this respect. 



Vapor pressures at other stations near the control station. — Stations 

 F-2 and F-3, which are situated only a few hundred feet from the 

 control station, but represent very different sites, were observed 

 daily during the first two years of this project and only a few minutes 

 later than the control station. Hence their humidities are compara- 

 ble, with little or no correction for time. The variations from the 

 control station, by months, are shown in Table 16. 



Table 16. — Departure of vapor pressures at two near-by stations from those at 



control station, 1910-1912, in inches of mercury. 



[All records obtained 20 feet above ground.] 



Station 

 No. 



Type. 



January. 



Febru- 

 ary. 



March. 



April. 



May. 



June. 



July. 



F-2A 



F-3A 



Yellow pine, south 



slope. 

 Canyon spruce 



-0.0015 

 +.0045 



-0.0021 

 +.0031 



-0.0007 

 +.0063 



-0.0054 

 +.0193 



-0.0056 

 -.0036 



-0.0080 

 -.0195 



-0.0071 

 ' -.0070 



Station 

 No. 



Type. 



August. 



Septem- 

 ber. 



October. 



Novem- 

 ber. 



Decem- 

 ber. 



Year. 



Growing 

 season, i 



F-2A 



F-3A 



Yellow pine, south 



slope. 

 Canyon spruce 



-0.0097 

 -.0186 



-0.0085 

 +.0043 



-0.0158 

 -.0102 



+0.0007 

 -.0005 



-0.0008 

 +.0021 



-0.0053 

 -.0016 



-0.0082 

 -.0158 



^ased on arithmetic means of 10 decades. 



It is indicated that the atmospheric humidity over the south-slope 

 station is always less than at the control station, with the possible 

 exception of November. The same is true of the canyon site which 

 has developed spruce, if the year as a whole is taken; but from 

 December to April this site consistently shows higher humidity than 

 the control station. The amount of excess is generally more than 

 could possibly be accounted for by the difference in tune between 

 the observations. 



For the growing season at least, it is safe to say that the vapor 

 pressure 20 feet from the ground in the pine stand is less than at 

 the control station, and in the spruce stand less than in the pine. 

 Whether the forest growth has any direct relation to this fact may 

 be seriously questioned. It is believed that marked deficits during 



