Bergeron Process

Bergeron Process

The Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron and W. Findeisen), 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 saturation vapor pressure over ice. The saturation vapor pressure over water is greater than the saturation vapor pressure over ice (at the same temperature) creating a subsaturated environment for liquid water but a supersaturated environment for ice. This results 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.