FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 63 



for the most part, direct comparisons of the sites are only approxi- 

 mate because of the varying elevations of the exposed anemometers. 

 Evory such consideration would lead to the belief that the several 

 sites in the vicinity of Fremont would be quite closely comparable 

 as regards total wind movement 40 or 50 feet above the ground, 

 and therefore that the exposure of the growing tops of the trees, 

 the portion most sensitive to desiccating influences, is not essentially 

 different in different sites. The general air movement which char- 

 acterizes the locality, considered in connection with soil freezing, is 

 probably more important in fixing the types of forest than are the 

 specific velocities which have been measured. On the other hand, 

 high elevations doubtless experience much stronger winds than do 

 the middle elevations; the Plains region, slightly stronger; the 

 Wyoming locality, stronger winter winds; and the Wagon Wheel 

 Gap locality, winter winds of less importance, when like exposures 

 are compared. 



ATMOSPHERIC HUMIDITY. 



Atmospheric humidity is doubtless of some importance m any eco- 

 logical study, but nowhere more so than in a study of evergreen trees, 

 which are influenced by evaporation and transpiration during the 

 winter as well as the summer. In this relation, the saturation deficit 

 is a much more valuable criterion than either vapor pressure or rela- 

 tive humidity, as the " deficit" expresses at once the capacity of the 

 atmosphere for additional moisture, without further reference to tem- 

 perature. In other words, a lack of vapor in the atmosphere, amount- 

 ing, for example, to 0.100 inch (this being the difference between the 

 actual and possible pressures) should induce about the same rate of 

 evaporation whether the air temperature be 40° or 100°. The greater 

 deficits are, of course, likely to be encountered at high temperatures. 



The correct measurement of the moisture of the atmosphere is one 

 of the most difficult which a meteorologist is called upon to perform. 

 It is almost impossible to eliminate the personal element in securing 

 a depression of the wet bulb of the psychrometer, for there is con- 

 stantly a tendency to fail to secure the greatest possible depression. 

 To make the situation more difficult, there is no hygrograph, or auto- 

 matic instrument for this purpose, which is at all precise. One is 

 compelled to depend upon such psychrometer readings as can be 

 taken in estimating the mean moisture for whole days or longer 

 periods, and must never forget that even these observations are 

 fallible. In spite of the most conscientious effort in the field, there- 

 fore, the humidity data here presented can show only approximate 

 menu conditions. 



Although the ultimate interest is in vapor deficits, as related to 

 evaporation, it will be worth while to examine first the absolute- 

 humidity or vapor-pressure data, to determine whether there is any 

 essential difference between adjacent sites other than that induced 

 by variations in air temperature. Even this is difficult to determine, 

 as is realized when one notes the rapid changes in vapor pressure with 

 changes in temperature at a given point. However, as vapor is dis- 

 tributed through the atmosphere very rapidly, local excesses or 

 deficits tend to be nullified. Hence it is to be expected, at least for 

 the several sites immediately adjacent to the .Fremont station, that 

 there will be little variation in absolute humidity. 



