The increase with time of an integral measure of depth of the stable boundary layer. Often the SBL has a structure with a poorly defined top. Potential temperature gradient and turbulence intensity gradually decrease to zero at the top of the SBL. Because there is no well- defined top, often an integral or e-folding measure of the the SBL depth is used instead. The growth of such a bulk measure of SBL depth can be used to specify the full structure of the SBL evolution.