144 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



evaporation records, or possibly for comparing conditions locally 

 studied with stations for which there is no evaporation record, but 

 which maintain a complete record of wind movement, sunshine 

 duration, and humidity. In such an event it may be possible to 

 work out a fairly constant relation between the evaporation from 

 any given type of atmometer, and a combination of these other con- 

 ditions, properly integrated, in the general relation of: 



/• A) = (Wind movement plus saturation deficit) sunshine. 



The term "vapor pressure" expresses the weight per cubic foot, 

 or the pressure, in centimeters of mercury, of the vapor currently 

 in the atmosphere. The term " saturation deficit" expresses the 

 lack of vapor pressure, or the difference between the existing vapor 

 pressure and that which the atmosphere would contain at the current 

 t cinperature if the space were saturated with water vapor. The "dew 

 point" indicates the temperature at which the existing vapor would 

 condense; or, in other words, the temperature at which the existing 

 vapor would produce a condition of saturation. It is readily seen, 

 then, that the saturation deficit is the difference between saturation 

 pressure for the current dry-bulb temperature, and saturation pres- 

 sure for the temperature of the current dew point. The term "rela- 

 tive humidity" expresses, as a percentage, the relation between the 

 existing vapor and that which might be present if the space were 

 saturated at the current air temperature. 



The dew-point figure is used only incidentally in computing vapor 

 pressure, saturation deficit, or relative humidity. Of the three, 

 experience in a number of forest ecological studies has shown that 

 the saturation deficit is by far the most useful, giving, as it does 

 without further reference to temperature conditions, a direct measure 

 of the capacity of the atmosphere for more vapor, and hence, in 

 some degree, a measure of the rate at which evaporation will take 

 place. 



The psychrometer, consisting of a pair of thermometers mounted 

 on a frame in such manner as to be readily whirled in order to 

 accelerate evaporation, is the common instrument for determining 

 atmospheric humidity. One of the thermometers is covered with a 

 layer of cloth (preferably linen), which is dipped in clean water 

 before making the exposure. The evaporation of this water cools the 

 thermometer, or, as the expression is, causes "a depression of the 

 wet bulb"; and the maximum depression which it is possible to 

 produce by vigorous movement of the instrument through the air. 

 taken with the current temperature, is considered to give a measure 

 of the atmospheric humidity. Tables have been worked out. after 

 experiment, for almost all possible combinations of air temperatui 

 and wet-bulb depressions, showing the corresponding dew points 

 and relative humidities. Of these the best-known in' this country 



