86 



THE STUDY OF PLANT COMMUNITIES ' Chapter V 



Since the cup permits evaporation, it also will absorb water dur- 

 ing rainy spells. For field work, therefore, it is necessary to install 

 one of the various mercury traps designed to permit water to rise 

 in the tube but not to let it return to the reservoir. A simple but 

 effective trap consists of a drop of mercury in the lower end of 

 the tube, held in position between two plugs of pyrex glass wool. 





t r? 





FlG. 34. Two atmometers set up and in use in a study of grassland en- 

 vironment. The improvised shelter was used for max-min thermometers.— 

 Photo by R. B. Livingston. 



When temperatures fall below freezing, atmometers cannot be 

 used because of the danger of breakage. A summary of the devel- 

 opment, uses and limitations of atmometers is given by Living- 

 ston. 157 



Condensation of Atmospheric Moisture.— If air is sufficiently 

 cooled, its relative humidity increases to 100 percent, or saturation. 

 The slightest cooling beyond this point will result in condensation 

 of the vapor to form a liquid. The temperature at which condensa- 

 tion occurs (dew point) will, of course, vary with the moisture 

 content of the air. 



