160 Atmometric Units [358 



ATMOMETRIC UNITS 



BY BURTON" E. LIVINGSTON 



The increasing interest in atmometry 1 and the fact that 

 this subject is becoming recognized as of general and funda- 

 mental importance in many branches of scientific and practical 

 endeavor, make it desirable that there be some uniformity in 

 our conceptions as to the units employed in atmometric meas- 

 urements. To approach the subject it is first necessary that 

 the purpose of atmometric observations be clearly in mind; 

 much vagueness still prevails in this connection. The rate 

 of evaporation of water from any surface is dependent on 

 two sets of conditions. One set (internal ones) are effective 

 in or behind the surface and the other (external ones) are 

 effective in front of the surface, that is, in the gas phase of 

 the system. The internal conditions are the characteristics 

 of the evaporating surface and include such features as the 

 concentration of solutes in the liquid water, the influence ex- 

 erted by the presence of a solid in which the water is imbibed, 

 the shape and extent of the surface, its direction of exposure, 

 its ability to absorb or emit radiant energy, the heat-conduct- 

 ing capacity of the material back of the surface, etc. The 

 external conditions include primarily four characteristics of 

 the space in front of the evaporating surface : the temperature 

 of the gas phase, its moisture condition, the influence of move- 

 ment or circulation of the gas over the surface, and the effec- 

 tive intensity of impinging radiation. I have used the term 



1 1 employ the word as synonymous with and shorter than atmid- 

 ometry, just as I have adopted atmometer in place of its rival, atmid- 

 ometer. Both are etymologically correct, but the one formed from 

 the root atmo, besides being shorter, has received the sanction of an 

 international meteorological congress. Atmometer seems to have 

 been coined by Sir John Leslie, 1813. (See Livingston, B. E., 

 " Atmometry and the porous cup atmometer." Plant World 18: 21-30, 

 51-74, 95-111, 143-149. 1915. Other papers are there cited.) 



