ATMOMETRY AND THE POROUS CUP ATMOMEf ER 



BURTON EDWARD LIVINGSTON 

 The Johns Hopkins University, Baltimore, Md. 



IV 



THE RADIO-ATMOMETER 



The Instrument and its Use. The porous cup atmometer, as 

 has been seen, exposes an imbibed porous evaporating surface 

 in much the same way as the evaporating surfaces of organisms 

 are exposed. Our discussion of the instrument has thus far dealt 

 with it as a means for studying the evaporating power of the air 

 as this influences water loss from plants and animals. The term 

 evaporating power of the air implies the power of the air to re- 

 move water from some arbitrarily adopted standard surface 

 and it is calculated to include the effects of air temperature, air 

 humidity and air movement. As has been mentioned, however, 

 the temperature of the surface plays an important role in de- 

 termining the rate of water loss, and this surface temperature 

 may not always depend merely upon air temperature; may be 

 strongly influenced by conditions effective within the instrument, 

 as by the temperature of the contained water mass. It is very 

 difficult to distinguish between the influences exerted by these 

 two sources of heat in evaporation phenomena and no serious 

 attempt has yet been made in this direction. 



There is, however, another important influence frequently 

 exerted upon the evaporation rate from any surface, which may 

 or may not be related to the surface temperature but which is 

 distinctly not related to air temperature; nor is it directly related 

 to any other atmospheric condition. This is the intensity of 

 radiant heat absorption by the evaporating surface. This ab- 

 sorption is of course dependent upon the intensity of the imping- 

 ing rays and upon the absorptive power for these rays, possessed 



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