Bergeron-Findeisen trial
The Bergeron-Findeisen process (after Tor Bergeron and Walter Findeisen ) describes the formation of large droplets in clouds . This requires high-reaching mixed clouds in which supercooled water droplets and ice crystals are next to each other in the temperature range from -10 ° C to -35 ° C.
With the same surface shape and temperature , the saturation vapor pressure above water is higher than above ice, so that the cloud air above the water droplets is unsaturated, which means that the supercooled droplets evaporate and the resulting water vapor is deposited on the ice crystals through resublimation . The ice crystals enlarge further through the freezing of supercooled water droplets and through wedging or electrostatic attraction of further ice crystals. Above a certain size, the ice crystals can no longer be held by the updraft and begin to fall.
When falling through the cloud or underlying clouds, further droplets are collected, which further increase the size and the speed of fall. When the 0 ° C limit (actually the melting point ) is exceeded, the crystals melt and reach the ground as large droplets of rain.
If the fall path is cold enough, especially in winter, but also in special weather conditions in summer, the melting process does not take place, so that the precipitation falls in the form of snow .