FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 



69 



On the whole, there is no indication that the humidity at those forest 

 stations near the ground is measurably different from that 20 feet 

 above the ground in the open position of the control station. 



Vapor pressures in central and southern Colorado. — The control sta- 

 tion may now be compared with the only other station in this series 

 for which dependable humidity records are available (Table 18) . The 

 stations are at similar elevations, and the record covers nearly seven 

 years. The later time of observations at Wagon Wheel Gap — 9 a. m. — 

 should tend to bring out higher vapor pressures than at the control 

 station. 



Table 18.- 



■Absolute and comparative humidity at W-Al (Wagon Wheel Gap 

 north-slope Douglas fir). 



Datum. 



January. 



February. 



March. 



April. 



May. 



June. 



July. 



Actual 9 a. m. vapor pressure. . . 

 Compared with 8 a. m. at con- 



0.0537 

 -.0066 



0.0562 



-.0050 



0.0734 



-.0049 



0.0995 



-.0133 



0. 1165 

 -.0345 



0.1849 

 -.0314 



0. 2792 

 — .0099 







Datum. August. 



Septem- 

 ber. 



October. 



Novem- 

 ber. 



Decem- 

 ber. 



Ypar Growing 

 w ' season. 



i 

 A rtua 1 9 a . m . vapor pressure ... 0. 2605 

 Compared with 8 a. m. at con- 

 trol station 1 -.0101 



0.1S94 



-.0140 



0. 1149 

 -.011S 



0.0735 



-.0059 



0.0506 

 -.0118 



0. 1294 

 -.0132 



0. 2363 

 -.016S 



The greatest difference between the two localities is seen to exist 

 during May and June, when their temperature difference is least. 

 This great deficit at Wagon Wheel Gap is due to the fact that the 

 rainless period in May or June, or both, is much more marked than 

 it is farther north and corresponds more closely to the conditions in 

 the Southwest. 



During other portions of the year the deficit at Wagon Wheel Gap 

 is relatively uniform. It should, however, be pointed out that this 

 deficit may not mean muchhigher evaporation rates at Wagon Wheel 

 Gap, because the general temperature is here lower than at Fremont. 



An examination of the different years at Wagon Wheel Gap, which 

 are comparable with whole years at Fremont, shows that the varia- 

 tions from year to year are of almost the same magnitude as those at 

 Fremont, but that the two series are by no means parallel. 



Saturation deficits. — The question of humidity in combination with 

 air temperatures may now be considered. It has been stated that 

 the most direct expression for humidity, to bring out the relation of 

 this condition to evaporation and transpiration, is the vapor deficit. 

 In the following tabulation the vapor deficit has been computed by 

 deducting the mean decade vapor pressure, determined by the dr. ily 

 observations at 8 a. m., from the saturation pressure, determined by 

 reference to the mean temperature for the decade. If applied to 

 single days, this method would be far from precise, because the 

 humidity might strongly increase or decrease during a whole day. 

 It is believed, however, that the 10 morning determinations of humid- 

 ity in a decade give a very close approximation to the true mean 

 humidity, and that the mean temperature is a sufficiently close 

 measure of the mean saturation pressure for the whole decade. Of 

 course, this method would err seriously if the temperature variations 

 in a decade were very great. 



