Abstract
Numerical computations are made to show how the vertical velocity, temperature, humidityβmixing ratio and liquidβwater content of a cloudy parcel will vary with height and time as it ascends from the condensation level, rises through, and mixes with, the surrounding environment whose temperature and humidity distributions are specified. The rate at which the parcel is diluted by mixing is formulated in two different ways which lead to essentially the same conclusions. This treatment does not attempt a detailed description of the mixing processes but arrives at rather critical values for a single parameter used to represent the rate of mixing. If this is sufficiently large to prevent the vertical motion undergoing large oscillations (such as are experienced by a closed parcel), the firstβformed cloud is small β usually only a few hundred metres deep over land and perhaps less than 100 m deep over the oceans. This implies that large clouds must evolve by successive thermals rising through air which has been warmed and moistened by their predecessors. Computations are made to demonstrate this stageβbyβstage growth. The formation and growth of the first visible cloud appears to depend rather critically upon the excess temperature and velocity with which the thermal reaches the condensation level. These conclusions, and also the computed vertical profiles of temperature, updraught speed and liquidβwater in these small clouds, appear to be consistent with observations.