From AMS Glossary
(Commonly called the ice process of precipitation, and formerly, ice- crystal theory; also Bergeron–Findeisen–Wegener process or theory, and with the names in a different order.) A theoretical explanation of the process by which precipitation particles may form within a mixed cloud (composed of both ice crystals and liquid water drops).
The basis of this theory is the fact that the equilibrium vapor pressure of water vapor with respect to ice is less than that with respect to liquid water at the same subfreezing temperature. Thus, within an admixture of these particles, and provided that the total water content were sufficiently high, the ice crystals would gain mass by vapor deposition at the expense of the liquid drops that would lose mass by evaporation. Upon attaining sufficient weight, the ice crystals would fall as snow and very likely become further modified by accretion, melting, and/or evaporation before reaching the ground. This theory was first proposed by T. Bergeron in 1933, and further developed by W. Findeisen. Certain of its features related to nucleation had been suggested by A. Wegener as early as 1911. Operation of this process requires numerous small water drops that are supercooled, which is a common feature in clouds between about 0° and -20°C or below, along with a small number of ice crystals. The crystals grow by vapor deposition at a rate (maximum at about -12°C) to give individual snow crystals in some 10 to 20 minutes. Much cloud seeding is based upon the introduction of artificial ice nuclei to supply more of the ice particles.
Wegener, A. 1911. Thermodynamik der Atmosphäre. Barth, Leipzig.
Findeisen, W. 1938. Die kolloidmeteorologischen Vorgänge bei der Niederschlagsbildung (Colloidal meteorological processes in the formation of precipitation). Met. Z.. 55. p. 121.
Bergeron, T. 1935. On the physics of cloud and precipitation. Proc. 5th Assembly U.G.G.I. Lisbon. Vol. 2, . p. 156.