CLIMATIC FACTORS : THE AIR 79 



midity of 80 percent may mean "dryness" if the temperature is 

 high or "wetness" if the temperature is low. 



It is desirable then to have a term indicating the amount of water 

 that air can take up before it becomes saturated. Vapor pressure 

 is a measure of the quantity of water vapor present, the tempera- 

 ture being constant, and is usually expressed in units of pressure 

 (inches or mm. of Hg). Therefore, vapor pressure deficit is the 

 difference between the amount of water vapor actually present 

 and the amount that could exist without condensation at the same 

 temperature. It is a direct indication of atmospheric moisture, 

 quite independent of temperature and, therefore, compared to 

 relative humidity, its values are much more indicative of the po- 

 tential rate of evaporation. 



When the relative humidity is 100 percent at 68° E, the vapor 

 pressure is 17.54 mm. of mercury. If the relative humidity were 70 

 percent, the vapor pressure would equal 12.28 mm. (0.70 x 17.54), 

 and the deficit would be 5.26 mm. (17.54-12.28). If the relative 

 humidity were the same (70%) at 59° E, the vapor pressure would 

 be 8.95 mm. (0.70 x 12.79) and the deficit would be only 3.84 mm. 

 (12.79—8.95). Tables of saturation pressures (vapor pressures) are 

 usually available in handbooks of chemistry, and it is possible to 

 transform relative humidities to vapor pressure deficits quickly 

 when the temperature is also known. The relationships are shown 

 in Table 6. 



Greater general use of the vapor pressure deficit in ecological 

 work seems desirable, for in spite of certain limitations, its ac- 

 curacy is much greater than that of relative humidity. The poten- 

 tial rate of evaporation cannot be indicated with a single simple 

 expression of atmospheric moisture since the rate depends upon 

 the vapor pressure gradient between evaporating surface and at- 

 mosphere. The gradient can be determined only when the tem- 

 perature and vapor pressure of the liquid are known as well as 

 those of the atmosphere. Vapor pressure deficit is directly related 

 to evaporation only when the temperatures of the air and of the 

 evaporating surface are equal. 252 Ecologists more often than not 

 measure evaporation directly, but when evaporation is not known, 

 in spite of the above, vapor pressure deficits could well be used in- 

 stead of relative humidities. 



