288 ENVIRONMENTAL CONDITIONS. 



maximum vapor-pressures corresponding to the respective air-tempera- 

 tures, and p and p' the corresponding vapor-pressures of water actually 

 in the air at those times, then 



E P-p 



E'~P'-p' 



Under such conditions P — p and P' — p' are measures of the respective 

 forces tending to drive water-vapor off from the surface into the air. 

 To determine the values of p and p' , we may measure the absolute 

 humidity by chemical methods, we may resort to the sling psychrom- 

 eter or any form of wet and dry bulb thermometer with constant and 

 rapid air-movement, or we may employ the Regnault dew-point 

 apparatus. It is clear that the values of p and p' depend upon the 

 absolute humidity and upon the air-temperature and barometric 

 pressure, while the values P and P' depend only upon the temperature 

 and barometric pressure. Since the influence of barometric pressure 

 is relatively small under natural conditions, it need not be seriously 

 considered here. The vapor-tension deficit is seen to include the air- 

 temperature influence. 



Relative humidity. — The vapor-pressure deficit is not one of the 

 climatic features usually determined by climatologists, w^ho have 

 rather uniformly followed earlier workers in the employment of the 

 concept of relative humidity in its stead. Relative humidity is the 

 vapor-pressure of the water- vapor actually present in the air expressed 

 as percentage of the maximum vapor pressure for the given temper- 

 ature and pressure ; it is simply the percentage of water-saturation of 

 the air. This bears no quantitative relation to atmospheric evaporat- 

 ing power, even with wind and barometric effects left out of considera - 

 tion, for it is obvious that air with a given relative humidity must be 

 more effective in promoting evaporation at a higher temperature than 

 at a lower. It is not the percentage of the maximum vapor-pressure 

 actually present, but the difference between the maximum pressure and 

 the actual, which measures this influence upon evaporation-rate. 

 Since the maximum increases with temperature (though not propor- 

 tionally), a given percentage of deficit must represent a larger actual 

 deficit as the temperature rises. 



If H and H' represent the relative humidity of the air at different 

 times, the remaining symbols being the same as above, then 



H p/P 

 H'~p'/P' 



From this it is clear that, if the air-temperature is known in each case, 

 thus furnishing the values of P and P' , the vapor-pressure deficits 

 may be found; from this equation and the one for E/E' , given above, 

 it follows that ^ pf-i _ii\ 



E'^P'{l-H') 



