Abstract
A form of mixing length was deduced so that in neutral conditions it would give von Karman's mixing length and, near the ground, match KEYPS law. The system of equations with the assumption of horizontal uniformity was solved both in stationary and in diurnal conditions. The considerations of energy equations indicated that we should solve the problem with the time derivative terms, and the results of time integrations explained some features of the diurnal changes of the planetary boundary layer.
