Moisture in the Air 



113 



total amount of water in the air, is generally of far less ecological 

 consequence, than the relative liiimidity. The ecologist therefore 

 focuses his attention on the relative humidity or the amount of mois- 

 ture in the air as a percentage of the amount which the air could hold 

 at saturation at the existing temperature. Since the capacity of air 

 for water vapor increases with temperature, the relative humidity of 

 the atmosphere is reduced in any situation in which an increase of 

 temperature occurs without an accompanying increase in the total 

 moisture content of the air. 



In the terrestrial environment the ecological effect of the water 

 factor is consequently strongly influenced by the temperature factor. 

 Of two regions having the same rainfall the warmer is the drier in 

 the ecological sense (Fig. 4.10). The climate of a locality with a 



-IS'-C -12 -7 

 llOrr 



■^ 70 



^ 60 



20 30 40 50 60 



Mean annual temperature 



Modified from McDougall, 1925 



Fig. 4.10. Schematic representation of the influence of rainfall and temperature 



on climate. 



