The production of showers by the coalescence of cloud droplets

The growth of unusually large cloud droplets by coalescence within isolated convective clouds is examined. It is found that if their initial radii are about 20 μ or more then growth into raindrops may readily occur. Raindrop nuclei of this size may be provided by spray in maritime air masses, but there is reason to suppose that in continental air masses droplets of the required initial size are not naturally introduced into the bases of convective clouds. Therefore the coalescence mechanism of precipitation release is probably effective only in maritime air masses. A condition for the production of a shower by the coalescence mechanism is obtained and applied to predict a minimum critical cloud depth of about 1.1/2 to 2 km. Showers may develop from clouds wholly below the 0°C isotherm if their base temperature exceeds about 8°C. A relation between the size of raindrops and the speed of the convective updraught is found, and an association is discovered between the non‐fulfilment of conditions for raindrop‐splitting within cloud and the absence of thunder.