6 BULLETIN 1333, U. S. DEPARTMENT OF AGRICULTURE. 



was the lowest elevation at which maximum and minimum thermome- 

 ters could be conveniently operated without seriously disturbing the 

 snow blanket, and because it was thought adequate to represent 

 purely local conditions vi absorption and radiation. An elevation 

 of 44 to 6 feet, which is very convenient, may represent a good degree 

 of air mixing without nullifying local character, except to modify daily 

 extremes. With greater elevations, which have been used in a few 

 instances to measure the conditions affecting the crowns of trees, it 

 will be seen that the local character of the temperatures partly dis- 

 appears. 



In securing temperature records at these elevations the ordinary 

 method of exposing thermometers is not satisfactory where wind is 

 very light. Unless there is considerable wind to cause constant 

 change of air within a standard thermometer shelter, the temperatures 

 recorded are certain to reflect the heat absorption of the walls of the 

 shelter itself during the day, and at night the degree of heat retention 

 by the same walls, rather than the absorption and radiation of natural 

 objects. Although, because of the free air movement, this influence 

 is not very potent at considerable elevations above the ground, special 

 precautions must be taken in exposing thermometers for ground tem- 

 peratures. Direct insolation of the thermometers must of course be 

 prevented. At the risk, however, of some reflected radiation striking 

 the instruments, it has been found best to attach them to the north 

 faces of boards and to protect them from the sun above and on the 

 east and west, leaving them open to air currents on the north. There 

 is nothing below them to interfere with ground- and-air cooling at 

 night. Where standard shelters have been used on the ground, their 

 doors have been constantly open to the north except for short periods, 

 and usually the floors have been partly open. Although by no means 

 all obstacles to proper temperature recording have in this manner 

 been removed, it is believed that the records presented contain only 

 occasional errors due to improper exposure. 



Records of evaporation have been obtained with a variety of 

 instruments and methods, omitting evaporation from free-water 

 surfaces, but while an attempt has been made to compare the results 

 obtained by different methods, it must be recognized as a general 

 principle that such comparisons are of little value. Inasmuch 

 as the rate of evaporation at any time is dependent not only upon 

 the heat supplied by the sun and the air in contact with the evaporat- 

 ing surface, but also upon the rate of diffusion of the vapor as in- 

 fluenced by wind movement and atmospheric humidity, it is irref- 

 ragable thai two different evaporating instruments or surfaces, with 

 constant variations in each of these four contributing factors, can 

 not maintain constant relative evaporating rates (J). Furthermore, 

 it is hopeless to expect that the response of any instrument or 

 evaporating surface will bear a constant relation to the response 

 of any given plant, which must be differently affected by each of 

 the four factors. The problem is made even more intricate by the 

 variation in the capacity of different plants for sunlight, absorption 

 and for transpiration. For these reasons, the comparisons of types 

 arc based almost wholly on records of Type 4 evaponmeters, although 

 some of these records were not obtained until the early part of L92L 



Methods of soil study are so far from being standardized, imA the 

 methods used by agricultural investigators are in some respects so 



