182 



ANALYSIS OF THE ENVIRONMENT 



one of these is known, the others can be 

 computed for the given conditions (see p. 

 165). Under ordinary ecological conditions, 

 the rate of evaporation depends upon the 

 steepness of the gradient between the vapor 

 tension at the evaporation surface (Eo) and 

 the air above (ei, e:, and so on), as well 

 as upon energy regulations at Eo (Fig. 40). 

 This part of the process is affected by the 

 relative humidity (or saturation deficit) of 

 the air and by its turbulence. 



morning temperature of 50° F. and a relative 

 humidity of 100 per cent, the vapor pressure 

 of the air would be .3626 inches of mercury. 

 If the temperature of the surface of a water 

 body were also 50 °F. the vapor pressures 

 would be the same and there would be no net 

 addition of water molecules to the air or the 

 water surface, and consequently neither evapo- 

 ration nor condensation. As the air temperature 

 rises to 60 °F., if moisture is neither added nor 

 abstracted, the vapor pressure will remain at 

 .3626 inches, the relative humidity will drop 



Fig. 40. Distribution of humidity in the laminar boundary layer and in adjacent turbulent air 

 over an evaporating surface. (Adapted from Leighly. ) 



Whenever the relative humidity and tem- 

 perature of the air produce a vapor pressure 

 that exceeds the vapor pressure at the ex- 

 posed water surface, condensation occurs 

 and water is added. Whenever the relative 

 humidity and temperature result in a vapor 

 pressure less than that at the evaporating 

 surface, evaporation results. The gradient of 

 vapor pressure is important both in its 

 steepness and its direction. It appears that 

 a more or less thin laminar layer of air 

 exists next to the evaporating surface in 

 which water movement occurs by diffusion 

 only; above that comes a layer, or a series 

 of layers, of turbulent air. The degree of 

 turbulence affects the rate of evaporation 

 and is in turn affected by wind action and 

 by convection currents produced by differ- 

 ences in heat. The mathematical relations 

 are developed by Leighly (1937). 



An illustration from Thornthwaite (1940, 

 p. 21) will help at this point. 



"There is a daily march of relative humidity 

 which accompanies the diurnal march of tem- 

 perature. On a summer day, with an early 



to 70 per cent, and a vapor pressure deficit of 

 .1594 inches will have developed. As the air 

 temperature rises to 70°F., 80°F., and 90°F., 

 the relative humidity vdll fall to 49, 35, and 26 

 per cent, and the vapor pressure deficit will 

 decrease to .3743, .6708, and 1.0608 inches 

 respectively. But as long as the water tem- 

 perature remains at 50°F., the vapor pressure 

 of the air and the water surface are the same 

 and there can be no evaporation. 



"Evaporation wall occur only when the vapor 

 pressure of the water surface exceeds that of 

 the air. With a rise in air temperature or vdth 

 direct absorption of radiant energy, the water 

 temperature will rise and the vapor pressure 

 of the water wiU become greater than that 

 of the air, more water molecules are emitted 

 from the water surface than are returned to it 

 and evaporation occurs. Also the moisture con- 

 centration and consequently the vapor pressure 

 of the air may be reduced. As the air increases 

 in temperature, turbulence due to convection 

 may set up, causing mixture of surface layers 

 with drier air from aloft. Similar dissipation of 

 moisture into the upper levels of the atmos- 

 phere may be caused by mechanical turbulence 

 due to wind movements. Wind therefore affects 

 evaporation simply through lowering the vapor 

 pressure of the air in relation to that of the 



