Wegener–Bergeron–Findeisen process

The Wegener–Bergeron–Findeisen process (named 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.

Source: Wikipedia — Wegener–Bergeron–Findeisen process (CC BY-SA 4.0)