Wegener–Bergeron–Findeisen process

The Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron and Walter Findeisen), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions where the ambient vapor pressure falls between the saturation vapor pressure over water and the lower saturation vapor pressure over ice. This is a subsaturated environment for liquid water but a supersaturated environment for ice resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition. If the number density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough.

The Bergeron process, if occurring at all, is much more efficient in producing large particles than is the growth of larger droplets at the expense of smaller ones, since the difference in saturation pressure between liquid water and ice is larger than the enhancement of saturation pressure over small droplets (for droplets large enough to considerably contribute to the total mass). For other processes affecting particle size, see rain and cloud physics.