Bergeron–Findeisen’s theory cannot explain the formation of all tropical rainfall, since ice crystals are often absent in tropical clouds. Langmuir’s coalescence theory (1974) suggests that the small droplets in clouds grow larger by coalescence until they are heavy enough to fall. As they fall, they collide with other droplets, growing more. (Not every collision results in coalescence.) The deeper the cloud, the more the drops grow; up to about 5 mm in diameter. See Xue and Wang (2007) J. Atmos. Scis 31, 15.