To advance the understanding of cloud top entrainment in stratus clouds, laboratory-based fluid dynamics experiments have been conducted. In both the atmosphere and the laboratory, thermal convection is driven at the interface between two fluid layers that are separated by a sharp, stable stratification. Convection causes the formation of entrainment cusps along this interface. A new physical model is proposed in which the spacing between cusps is governed by a buoyancy instability timescale and a heating timescale. When applied to actual stratiform clouds, the physical model implies cusp spacing of about 100 m. Entrainment rates measured in the laboratory have also been compared with those measured in the atmosphere, and the dependence of entrainment on thermal diffusivity has been investigated