what is bergeron-findeison theory?

precipitation - 降水量：暴雨、雪、凍雪或冰雹降落到地表的任何形式的水

268 K, temperature at which the Bergeron-Findeison enhances the precipitation

The Bergeron-Findeison theory of cloud physics, whereby ice particles grow at the expense of water droplets enabling a causal mechanism for the ultimate production of raindrops. This has, to some extent been validated, by the collection of samples of ice and droplets by research aircraft within clouds.


In cold clouds, which may contain supercooled water droplets, collision and coalescence is also an important process. However many cold clouds also contain ice crystals which grow by a number of mechanisms including the freezing of supercooled droplets, deposition, accretion (riming), aggregation, etc.

In mixed clouds, which contain both supercooled droplets and ice particles, there is an efficient mechanism for the formation of precipitation called the Bergeron-Findeison process. This relies on the fact that the relative humidity of air with respect to an ice surface is greater than with respect to a water surface. When an ice crystal is in the presence of supercooled water droplets the situation is unstable, because the water vapour is unsaturated with respect to water, but supersaturated with respect to ice. Water droplets evaporate and water vapour sublimes directly onto the ice crystals which grow at the expense of the supercooled water droplets.
http://ceos.cnes.fr:8100/cdrom/ceos1/science/dg/dg7.htm

Mechanisms of precipitation :

There are two complimentary theories to explain the physical processes involved :
(1) coalescence theory- the coalescence of falling drops of different sizes; and
(2) Bergeron-Findeison theory growth of ice crystals by vapour deposition .