It is known that there is a sharp temperature gradient in a thin water layer at the air-water interface. Systematic measurements of this temperature difference and meteorological parameters were made at a cooling pond in East Mesa, CA. The water loss was measured directly, and the water temperatures were in the range of 30–46°C. Existing formulae which use the surface temperature difference and wind speeds to predict the net heat flux have been examined. Examination of the well-known Saunders’ equation shows that a parameter λ does not have a constant value. Nondimensional correlations have been developed to predict the net heat flux and the evaporative heat flux at the interface. A dimensional correlation which includes the effect of the surface temperature variation is found to give the most accurate prediction of the net heat flux.

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