A theory, proposed around 1930 by T. Bergeron, and subsequently developed by W. Findeisen, that provides a mechanism for the growth of raindrops in ice/water (mixed) cloud. It is based on the differential values for saturation vapour pressure over ice and supercooled-water surfaces. At cloud temperatures of −12 to −30 °C air can be saturated over ice but not over water particles. Consequently, water evaporates from liquid droplets and accumulates on ice crystals by deposition. When they are large enough, the ice particles can fall through the cloud, melting to form rain drops as they pass through lower, warmer air. The process depends on there being a mixture of ice and water, and so may operate in mid- and high-latitude cloud but not in all clouds, e.g. not in tropical clouds which are at temperatures above freezing throughout. See also collision theory; ice nucleus.