Gradients of temperature and humidity above water surfaces are analyzed in order to determine the dependence of the long-term average Bowen ratios β (the ratio of sensible to latent heat flux) on surface temperature. The least-squares fit that results from investigation of six such bodies of data, and which is supported by recent direct measurements of the fluxes by eddy correlation techniques, can be expressed as β = γ/s−0.15, where γ is the ratio of the specific heat of air at constant pressure to the latent heat of vaporization of water and s is the slope of the saturated specific humidity curve at the surface temperature. This expression forms the basis of a model which can be used to determine the average surface temperature from routine observations of air temperature and humidity at sea.