This paper presents a diagnostic study of two models proposed by Betts (1973a). The first was a model for a well-mixed sub-cloud layer capped by a more stable transition layer. The downward heat flux at the base of this transition layer is here estimated from the thermal structure of the layer to be 15-25% of the surface sensible heat flux. The second model was a model for a two layer lapse-rate structure for the cumulus layer. A sensitivity analysis...