The effect of surface tension in causing the evaporation of the smaller droplets in clouds with simultaneous growth of the larger droplets is an important factor determining the early stages of cumulus cloud droplets. The process is too slow to account for the formation of raindrops. If larger droplets are produced near the top of the cloud by the melting of snowflakes (Bergeron-Findeisen) they may grow to raindrop size by accretion due to coalescence with cloud droplets. A quantitative theory is developed for the efficiency of the accretion process considering the trajectories of the small droplets moving near the surface of the larger falling drop.
Heavy rain consisting of large drops frequently forms in tropical clouds which lie wholly below the freezing level (‘warm’ clouds). Large summer cumulus clouds with tops above 23,000 ft, reaching above the freezing level (‘cool’ clouds), after seeding with dry ice have given heavy rain of large drops which reach the ground within fifteen to twenty minutes after seeding. It is considered that such effects are caused by a chain reaction in which a falling drop, of a size sufficient to give good collection efficiency, falls through the cloud until it grows so large that it breaks up giving numerous smaller drops which are carried aloft and grow to the break-up size before returning to the height at which they were formed. The critical conditions for maintaining such a chain reaction involve lower limits for the updraft velocity, the thickness and water content of the cloud, the diameters of the cloud droplets and of the fragments produced by the break-up of the falling drops. The production of heavy rain by dry ice seeding of warm clouds in the Hawaiian Islands by Leopold and Halstead are explained as being due to this chain reaction. The rapid formation of rain by seeding large cool cumulus clouds is explained. The rain is brought rapidly to the ground by the downdrafts caused by the localized weight of the falling rain. The seeding of rapidly developing warm cumulus clouds by introducing water drops or pellets of ordinary ice should thus under proper conditions lead to the production of heavy rain. The development of self propagating storms in unstable air masses is also discussed.