The Fomation of Rain by Coalescence

Abstract

It is generally acknowledged that drizzle or light rain can fall from clouds which do not reach freezing level and cases have recently bean described in which moderate to heavy rain has been observed to fall from such clouds. A simple theory is developed to account for the phenomenon, based on the initial growth of cloud droplets by condensation followed by the growth of a small fraction of their number by coalescence. These grow in their ascent through the cloud until they are large enough to remain in suspension in the upward air current, after which they fall as min. It is shown that for a given set of cloud conditions the maximum height reached by the drops increases with increasing vertical air velocity and that the size of the drops emerging from the base of the cloud is nearly a linear function of the height attained. The time for the precipitation to appear, on the other hand, is an inverse function of the upward air velocity. Experimental observations of rain from non-freezing clouds have distinguished two main types. The first of these shows an increase in drop diameter or rainfall intensity downward through the cloud, as would be expected if the drops followed a variety of trajectories within the cloud. The second type is one in which the drop trajectories tend to coincide, in which case there would be a maximum in the raindrop density and the rain water content at some defined height within the cloud. This has been verified qualitatively by radar observations and flight experiments.