The impact of the land surface on the potential for precipitating convection is examined. To do so, an analytical expression that relates boundary layer equivalent potential temperature to the surface evaporative fraction is derived by means of the slab model of the convective boundary layer. Analysis of the resulting expression shows that the equivalent potential temperature increases with the evaporative fraction under most circumstances, except for very dry atmospheres, and that this increase is enhanced in the presence of a low-level moist layer. ¿ 1997 American Geophysical Union