FOREST TYPES IN CENTRAL ROCKY MOUNTA] 5 



the heaviest downpours in a given locality likely to affect only 

 small areas. Temperatures are generally not so directly affected by 

 local conditions. Thus the month of December, 1917, was an un- 

 usually warm month not only at the Fremont Forest Experiment 

 Station, but over most of the western part of the Unitea States. 

 January, 1918, likewise, was generally cold to an unusual dej 

 It seems safe, therefore, to compare the records of any two neai 

 stations for short periods, whatever the factors under consideration, 

 and an increasing period will be demanded as the distance between 

 stations increases. 



Fortunately the dozen or more stations located in the vicinity of 

 Fremont all come under the same general influences. This is true of 

 the entire area from the plains to the summit of Pikes Peak, with the 

 exception that summer rains frequently fall in one part of the area 

 and not in others. Winter snows may also be so localized, but usually 

 in conformity with altitudinal zones. It is true that two stations not 

 100 yards apart may on a given day have temperatures varying !> 

 F. in one direction, and on the next day varying in the opposite direc- 

 tion. Such variations from a consistent relation are, however, always 

 small, and there is every reason to believe that the means of a single 

 month usually express essentially the normal temperature relation 

 between two stations for that month of any year. 



Therefore the method of presenting results in this bulletin is to 

 compare each condition at any outlying station, for whatever period 

 observations may have been taken, with the corresponding condition 

 recorded during the same period at the so-called ''control station." 

 The latter is merely a single point, near the headquarters of the Fre- 

 mont Forest Experiment Station, at which all of the factors to be 

 considered have been measured almost continuously from 1910 to 

 1921, or later, with the important exception of the evaporation factor 

 for which satisfactory measurements were not obtained until 1916. 



In the study of air temperatures, several attempts have been made 

 to cover the somewhat distinct sets of conditions which may exist at 

 any station — that is, to measure both the very local air temperatures 

 near the ground and those well above the ground, which may repre- 

 sent the mean for the locality, obtained by the mixing of the warmer 

 and. cooler strata of air. On a breezy day, of course, this mixing is 

 constant and effective and even brings together the warm air near the 

 earth and the much cooler air of high strata in the atmosphere. On 

 quiet days such mixing may be very incomplete, so that even at 20 

 feet above the ground temperature records imxy reflect local rather 

 than general conditions. Sudden changes of temperature are then 

 likely to be recorded when a stronger movement of air begins. 



The temperature of the air close to the ground is a condition in 

 which foresters are greatly interested because of its relation to seed- 

 lings and young growth. A number of observations have been made 

 at an elevation of 1 foot from the ground surface, and these will here- 

 after be spoken of as "ground temperatures'' in contrast with tem- 

 peratures in the soil, on the one hand, and the temperature of mixed 

 air at higher levels, on the other. At the "ground" elevation the 

 rate of air movement is low, and, unless brisk breezes are blowing, is 

 such as to permit the air to attain almost the temperature of the 

 ground and of the vegetation with which it comes in contact. An 

 elevation of 1 foot was adopted for ground temperatures, because it 



