Abstract
The paper describes a relatively simple model of a nonβprecipitating cumulus cloud that grows by the ascent of successive spherical thermals through the residues of their predecessors, mixing with the surroundings being determined by the relative velocity and the radius of the thermal. The model appears to account for several important features of the cloud structure and the dropletβsize spectrum that are not produced in models of a single parcel or plume. In particular, it produces modest clouds in 30β40 min in which the vertical velocity, cloud depth and liquidβwater content agree quite well with observations. The computed droplet spectra closely resemble measured spectra and reproduce the bimodal structure observed by Warner. In model maritime clouds containing small concentrations of droplets, the spectra broaden quite rapidly and produce droplets of r = 25 ΞΌm by condensation on nuclei of m = 10^β11^g in concentrations of order 100 m^β3^ within half an hour, beyond which size they may continue to grow rapidly by coalescence to precipitation size. However, in continental clouds containing droplet concentrations of a higher order, it is difficult to produce a significant number of droplets of even 20ΞΌm radius.