Bergeron Process

The Bergeron process, more fully the Wegener-Bergeron-Findeisen process, is the dominant precipitation-forming mechanism in mixed-phase clouds between 0 and -40 degrees Celsius. Saturation vapor pressure over ice is lower than over supercooled liquid water at the same subfreezing temperature, so an environment saturated for water is supersaturated for ice. Ice crystals grow by vapor deposition while the surrounding supercooled droplets evaporate to feed them. Because ice nuclei are scarce, vapor from many droplets concentrates onto a few crystals, which grow fast enough to fall, aggregate, and rime into snow or melt into rain. It explains most mid-latitude rainfall.