WegenerBergeronFindeisen

Wegener–Bergeron–Findeisen process is a meteorological microphysical mechanism in which ice crystals grow at the expense of surrounding supercooled liquid water droplets within mixed-phase clouds. It operates because the saturation vapor pressure over ice is lower than that over liquid water at the same subfreezing temperature, causing water vapor to diffuse from liquid droplets to ice crystals.

In a cloud containing both supercooled liquid droplets and ice crystals, water vapor moves toward the colder ice phase. The liquid droplets evaporate as vapor is lost, while the ice crystals grow by deposition. This vapor-phase exchange can continue for a range of subfreezing temperatures, leading to rapid growth of ice crystals and depletion of liquid water droplets. As the crystals grow, they may collide and aggregate, producing snowflakes or other icy precipitation. If the cloud layer passes through warmer air and the crystals melt, the resulting precipitation may become rain.

Conditions for the process are typical of mid- to high-latitude, mixed-phase clouds where temperatures are below freezing, often in the range of about -10 to -40 C. The efficiency of the Wegener–Bergeron–Findeisen mechanism depends on the degree of supercooling, the concentration and size distribution of droplets, and cloud dynamics. It is a fundamental concept in cloud microphysics and plays a key role in determining precipitation type and rate in many weather systems.

The process is named after Alfred Wegener, Tor Bergeron, and W. Findeisen, who contributed to its description and understanding in the early 20th century.