Penetrative Downdraughts in Cumuli

Observations of cumuli have shown that the adiabatic model is quite inadequate, and that dry air must mix with the condensing upcurrent. Theories of the interaction between cumuli and their environment are discussed. The hypothesis that dry air enters these clouds chiefly from above is examined quantitatively. It is found that, as a result of evaporative cooling, such air could penetrate several kilometres into a growing cloud. This hypothesis accounts for the fine structure which is observed, and provides a natural and simple explanation for the fact that the lapse rate in cumuli is steeper than the wet adiabatic, and indeed approximates closely to that of the environment. Unlike alternative theories, it provides a means whereby the liquid water content may be automatically self-limiting, as the observations seem to require, for the motions which introduce dry air deep into the cloud depend for their energy supply on the presence of liquid water, and have velocities which are roughly proportional to its concentration.