HUMIDITY 193 



Humidity. — Several units are used for designating the humidity condi- 

 tions of an atmosphere. One of these is the actual vapor pressure of the 

 atmosphere. This should not be confused with the saturation vapor pressure 

 (Chap. XII). Since the rates of diffusion and evaporation are influenced di- 

 rectly by the vapor pressure of the atmosphere this is usually the most satis- 

 factory unit in which to express humidity values for physiological purposes. 



The vapor pressure deficit is also a common, and for many purposes a 

 valuable unit for the expression of humidity values (Anderson, 1936). The 

 vapor pressure deficit is the difference between the actual vapor pressure of 

 the atmosphere and the vapor pressure of a saturated atmosphere at the same 

 temperature. As an example, consider an atmosphere having a vapor pressure 

 of 10.51 mm. Hg at 25° C. The saturation vapor pressure of this at- 

 mosphere would be 23.76 mm. Hg (Table 23). Hence its vapor pres- 

 sure deficit is 13.25 mm. Hg (23.76 — 10.51). 



If an evaporating surface and the atmosphere are both at the same tem- 

 perature the vapor pressure deficit indicates directly the steepness of the vapor 

 pressure gradient between the atmosphere and the evaporating surface. The 

 term vapor pressure gradient as applied to water-vapor has the same signifi- 

 cance as the term diffusion pressure gradient which has been frequently used 

 in previous discussions. In fact the vapor pressure gradient of water vapor 

 is its diffusion pressure gradient. As long as the temperature of the evaporat- 

 ing surface does not differ materially from that of an atmosphere the rate of 

 evaporation from a saturated surface into that atmosphere will show a close 

 proportionality to its vapor pressure deficit. However, the temperature of 

 an evaporating surface is seldom exactly the same as that of the atmosphere, 

 and the deviation in temperature between the two is often very considerable. 

 There are therefore many situations in which rates of evaporation do not 

 show a close proportionality with the vapor pressure deficit of the surround- 

 ing atmosphere. 



The most generally employed of all units in expressing humidity values 

 is the relative humidity, which is the percentage saturation of an atmosphere 

 at a given temperature. For example, a saturated atmosphere at 30° C. will 

 have a vapor pressure of 31.82 mm. Hg. The relative humidity of this 

 atmosphere would be 100 per cent. If only half the amount of vapor is 

 present that would be present at saturation at this temperature {i.e. a vapor 

 pressure of 15.91 mm. Hg) then the relative humidity would be 50 per cent, 

 etc. (Table 23). The relative humidity depends both on the concentration 

 of water-vapor in the atmosphere and upon the temperature. Any increase 

 in temperature with no accompanying change in the amount of water-vapor 

 present always results in a decrease in relative humidity, because an increase 



