The volume of entrained air per unit area per unit time, which has units of velocity. It is a measure of dilution of an entity, such as a rising smoke plume, or of a whole layer, such as the atmospheric boundary layer. For a growing atmospheric mixed layer, the rate of rise of the top of the mixed layer zi equals the entrainment velocity we minus any large-scale subsidence ws that is imposed at the top of the boundary layer. During sunny days over land, the entrainment velocity is proportional to the heat flux from the surface divided by the potential temperature change across the entrainment zone (i.e., the strength of the capping inversion). On windy days, the entrainment velocity is proportional to the turbulence kinetic energy at the top of the boundary layer and is inversely proportional to the potential temperature change across the entrainment zone. See entrainment rate.