Bergeron-Findeisen theory

(meteorology) The theoretical explanation that precipitation particles form within a mixed cloud (composed of both ice crystals and liquid water drops) because the equilibrium vapor pressure of water vapor with respect to ice is less than that with respect to liquid water at the same temperature. Also known as ice-crystal theory; Wedener-Bergeron process.

Proposed by T. Bergeron and subsequently modified by W. Findeisen, this is a theoretical explanation of the way droplets of precipitation form in a cloud composed both of ice crystals and liquid water drops, as minute droplets of water coalesce around ice crystals. The nub of the explanation lies in the differing saturation vapour pressures of ice and water; saturation vapour pressure is larger over supercooled water than over ice, so that when water droplets and ice crystals exist together in a cloud, the vapour between these particles cannot simultaneously be in equilibrium with respect to both states. If the vapour is in equilibrium with the ice crystals, it is too small for equilibrium with the water droplets, which will begin to evaporate. As a result, saturation vapour pressure is too big for equilibrium with the ice, so that vapour condenses onto the ice crystals, and the ice crystals grow at the expense of the water droplets. The crystals will have grown large enough to form precipitation when their fall-speed is greater than the upward movement of air currents. As they descend they may melt to form rain. Without the presence of ice crystals, water droplets can remain unfrozen at temperatures of - 20 °C. (Such droplets are supercooled.)

 This theory does not explain precipitation from tropical cumulus clouds, which can give rain when the cloud-top temperature is 5 °C or more. See also coalescence theory.