A model of an evaporating surface is proposed which consists of four levels: a lower boundary, a vapor source, an interface with the atmosphere, and an upper level. There are resistances to heat and vapor flow between these levels and energy balance and continuity equations can be used to solve for the various fluxes in the system. For any given set of boundary conditions the surface temperature of a given evaporating medium (leaf or soil) may vary within a range as a function of wind speed. It turns out that according to this model there is a discontinuity in the behavior of the system between very low internal resistances to vapor flow and no resistance at all (wet surface). When the atmospheric resistance is smaller than a critical value, then, if the internal resistances vanish, a sudden increase in the evaporation rate and a decrease in the surface temperature are predicted. Calculations for high values of the internal resistance indicate the existence of a maximum in the evaporation vs atmospheric resistance relationship.