The Study of Evaporation from Small Surfaces by the Direct Measurement of Water Vapour Pressure Gradients

The construction, maintenance and calibration of a sensitive instrument capable of making numerous vapour-pressure measurements within humidity gradients by the dew-point method is described. Coefficients of diffusion of water vapour in air, calculated from observed vapour-pressure gradients and measured rates of evaporation agree with theoretical and other experimental values in still air. Apparent coefficients in wind speeds between 10 and 100 cm/s were significantly lower than those in still air. This finding, together with the performance of the dew-point probe, is discussed in relation to its possible use in the study of evaporation from animals and plants."