The production of rain by a chain reaction in cumulus clouds at temperatures above freezing

TLDR: In this article, 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.

Abstract: 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 mi...