Evaporation can be determined by the aid of the first law of thermodynamics in such a way that wind velocity need not enter the calculation. Air temperature and humidity enter only as terms in a correction which can have a relatively small average value under typical conditions. The complete equation is $E=\frac{\mathrm{}(H-S-C)\mathrm{}}{L(1+R)\mathrm{}}$ where $E$ is the evaporation, $H$ the difference between the incoming and outgoing radiation, $S$ the heat stored in a column of water having unit cross-section, $C$ a correction for heat carried by flowing water and leakage of heat through the walls of the vessel, $L$ the latent heat of water, and $R$ is Bowen's ratio. A method of finding the difference between the incoming and outgoing radiation, by means of observations on a well insulated pan is described, and also a method of finding the ratio of sensible heat to latent heat transmitted through the air-water surface. Bowen's theoretical conclusions respecting this ratio were found to be consistent with observations.

The above formula can be used in estimating evaporation from an actual lake whenever the requisite data can be obtained. Although from a physical standpoint the quantity of sensible heat passing through the air-water surface is not strictly negligible, nevertheless is most meteorological and engineering applications, the error caused by neglecting this quantity will be negligible in comparison with other errors that enter the main problem of which evaporation is a part.

• Received 16 May 1927

DOI:https://doi.org/10.1103/PhysRev.30.527