MEASUREMENT OF EVAPORATION RATES FOR 

 SHORT TIME INTERVALS 



EARL S. JOHNSTON and BURTON E. LIVINGSTON 

 The Johns Hopkins University, Baltimore, Md. 



One of the peculiarities of the various forms of atmometers^ 

 so far devised is that they measure rates of a process rather 

 than the conditions prevailing at any given moment. With the 

 barometer and thermometer it is different. Disregarding the lag 

 in the response of the last-named instruments to the conditions 

 measured, each of them furnishes a static measure of surroimding 

 conditions at the time a reading is taken. To obtain a summa- 

 tion or average from such instruments it is necessary to make 

 readings at frequent intervals and then subject the results to 

 mathematical treatment. Recording thermome;ers and barome- 

 ters simply record readings at very short time intervals, and the 

 integration and averaging must be accomplished after the record 

 is made. Atmometers, on the other hand, always give the mean 

 rate of the process of evaporation for the time period that has 

 elapsed since the last reading, and it has usually been practi- 

 cally impossible to determine the evaporation rate for very short 

 periods of time. It is clear that this marked difference is due 

 to the fact that themiometers and barometers measure a con- 

 dition (which may be stationary) while atmometers measure a 

 rate of change, and such a rate must involve a time period. 



Nevertheless, the rate of water-loss from an evaporating sur- 

 face is determined by a set of conditions that may be regarded 

 as static for any instant, just as static as is temperature or 

 barometric pressure. This set of conditions has been called the 



^ On atmometers, see: Livingston, B. E., Atmometry and the porous cup 

 atmometer. Plant World 18: 21-30, 51-74, 95-111, 143-149. 1915. Other litera- 

 ture is there cited. 



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