From Glossary of Meteorology
(Also called mixed-layer height, mixed-layer top, mixing height.) The thickness, zi, of the mixed layer, defined as the location of a capping temperature inversion or statically stable layer of air.
Often associated with, or measured by, a sharp increase of potential temperature with height, a sharp decrease of water-vapor mixing ratio, a sharp decrease in turbulence intensity, a sharp decrease in pollutant concentration, a change of wind speed to geostrophic, a minimum of turbulent heat flux, and a maximum of signal intensity from remote sensors such as sodars and wind profilers. Quite variable in space and time, the mixed-layer depth typically increases during fair-weather daytime over land from tens of meters shortly after sunrise to 1–4 km before sunset, depending on the location and season.