Volume 13 Number 4 



The Plant World 



A Magazine of General Botany 

 APRIL, 1910 



A RAIN-CORRECTING ATMOMETER FOR ECOLOGICAL 



INSTRUMENTATION. 



By Burton Edward Livingston. 



Up to the present time there has appeared to be an insur- 

 mountable difficulty in the operation of the porous cup atmo- 

 meter, * in the fact that rain falling upon the exposed cup 

 penetrates, to some extent, into the reservoir below, thus intro- 

 ducing an unknown and practically indeterminate factor of 

 negative evaporation. Screening the cups in any way so as to 

 shield them from falling rain always alters the exposure to air 

 currents or sunlight, or both, and thus diminishes the value of 

 the instrument as a measure of the atmospheric environment 

 of plants. It is possible to determine empirically the rate at 

 which water will enter the cup when the latter is completely 

 covered by a water film. This rate depends upon the porosity and 

 superficial area of the cup and upon the length of the water column 

 below. It occurs, however, that during the average rainstorm 

 the entire surface is not by any means continuously wet. Gusts 

 of wind blow the rain drops against one or the other side of the 

 vertical cylinder, with a constant breeze one side is usually dry 

 and may even sometimes be actually losing water, and only when 

 the rain falls vertically is the cup uniformly covered with a 

 water film. Also, slight showers often fail to furnish water 

 at a sufficiently rapid rate to keep the porcelain surface wet. 

 Therefore it is wide of the mark to assume complete wetting and 

 to determine the rate of penetration and the duration of pre- 

 cipitation periods, calculating the absorption of water by means 

 of such data. 



•Publication SO. Camegie Institution 1906; Science. N. S.. 28: 319. 1908. 



