In this chapter we will investigate the rate of mass convection (i.e. evaporation flux) from a flat water surface to air that is flowing over the water surface at a constant velocity. We will assume steady-state conditions for the convective mass transfer, constant water and air properties at 1 atm air pressure, and water surface and air temperatures which are specified later. Also, water vapor will be assumed to behave as an ideal gas in air. We will only investigate mass transfer through the water–air interface boundary layer by convective diffusion, and we will assume the water surface to be a flat plate. We will neglect all other types of mass diffusion exchange that can occur at the water–air interface due to radiation, condensation, conduction, and so on.