The production of showers by the growth of ice particles

A model of cumulus‐cloud structure is described, and used, together with the available evidence on the temperatures of ice‐crystal formation and a calculation of the speed of growth of crystals into precipitation elements, to estimate the summit‐level temperatures at which the formation of showers becomes likely. It appears that in warm weather showers may be expected with summit temperatures of about −10°C, but in cold weather not until they fall to about −20°C. Observational evidence is shown to support this view although emphasizing large casual variations, some of whose causes are suggested.