78 THE STUDY OF PLANT COMMUNITIES • Chapter V 



to be at the "dew point!' If the cooling continues, the vapor be- 

 comes a liquid, which may condense on objects near the surface 

 of the earth as dew or frost or, if condensation takes place in the 

 air, may result in precipitation. 



Terminology of Atmospheric Moisture.— Several expressions 

 are used to describe the moisture content of the air. Absolute hu- 

 midity is commonly interpreted as the amount of water vapor per 

 unit volume of air and can be expressed as grams per cubic meter 

 or any other units of mass and volume. In itself the absolute hu- 

 midity has little bearing on the life of a plant, for it is not the total 

 atmospheric moisture that determines evaporation and transpira- 

 tion, but rather the difference between the weight of vapor pres- 

 ent and the maximum weight the air could hold at the time. Thus 

 the relative humidity, which is an expression of percentage of sat- 

 uration, is more nearly related to the rate of water loss from a free 

 water surface or from a plant. Relative humidity depends upon 

 temperature as well as the amount of moisture present, and, as a 

 consequence, identical relative humidities do not indicate identical 

 moisture conditions unless the temperatures are also the same. This 

 means that every shift in temperature results in a change in rela- 

 tive humidity, regardless of moisture present, and a consequent 

 change in rate of evaporation or transpiration. 



Several authors have emphasized that, when considered inde- 

 pendently of other factors, the actual amount of water vapor in 

 the air has little if any influence upon evaporation. One illustra- 

 tion 7 especially serves to emphasize the ecological significance of 

 this fact. Death Valley, California, is probably the most arid region 

 in the United States, yet its "dry" atmosphere contains on the aver- 

 age in July almost exactly the same amount of water vapor per 

 unit volume as does the "moist" atmosphere of Duluth, Minnesota, 

 at the same time of the year. 



An atmosphere 70 percent saturated at 60° F. will contain much 

 less water vapor than an atmosphere 70 percent saturated at 80° E, 

 and the capacity to hold more water will be less in the first than 

 the second case. Evaporation will, therefore, normally be more 

 rapid at 80° E even though the relative humidities are the same. It 

 can be seen that a statement of relative humidity alone gives little 

 indication of atmospheric moisture conditions since a relative hu- 



