From AMS Glossary
The three-part change of the atmospheric boundary layer that typically occurs during fair weather over land on sunny days.
In the early morning, the mixed layer is shallow, slowly deepening, cool (in a potential temperature sense), and is capped by the remains of the stable boundary layer from the previous night. In mid- to late morning, the top of the mixed layer exhibits rapid rise as heating eliminates the nocturnal inversion, and the mixed layer grows through the residual layer. The third stage in late morning and afternoon is that of a deep (order of 1–2 km) convective boundary layer of relatively constant depth.