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The Bergeron Findeisen Process is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal growth.
Summary
The equilibrium vapor pressure over water is greater than that over ice and therefore in a cold cloud, the water will be out of vapor pressure equilibrium and will evaporate to reach equilibrium. The ice will condense this vapor and grow into a larger ice crystal.
Eventually this ice crystal will grow large enough to fall. It may even collide with other ice crystals and grow larger still through collision coalescence.
"Bergeron process"