Bergeron process (Bergeron-Findeisen process, ice-crystal theory)
The process by which precipitation is initiated in a mixed cloud with a temperature below freezing. Because the equilibrium vapour pressure of water vapour with respect to ice is less than that with respect to liquid water, ice crystals grow at the expense of supercooled water droplets. Eventually the crystals become sufficiently heavy to begin to fall, and may subsequently be modified by other processes such as accretion, melting, coalescence, and evaporation. The theory was introduced by Bergeron in 1933 and subsequently extended by W. Findeisen.