The Aerodynamic Approach

In the very thin layer next to the evaporating surface, diffusion is entirely molecular. The molecular diffusivity of water vapor is known to be approximately 0.25 square centimeter per second. If the air flow were laminar, the evaporation from a saturated surface would be determined entirely by the molecular diffusivity. However, the unevenness of the surface and differential heating lead to turbulent motion. Above the laminar sublayer, the turbulent diffusivity far exceeds the molecular diffusivity. At a height of one meter, the former may be as much as 5,000 times the latter. Unlike the molecular diffusivity, the turbulent diffusivity is highly variable in time and space. The various aerodynamic methods are designed to measure or to estimate the rate of water vapor diffusion, particularly that caused by turbulence.