Wegener–Bergeron–Findeisen process
The Wegener–Bergeron–Findeisen process (after Alfred Wegener, Tor Bergeron, and Walter Findeisen [de]), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of supercooled water and ice) in regions where the ambient vapor pressure falls between the saturation vapor pressure over water and the lower saturation vapor pressure over ice.
This is a subsaturated environment for liquid water but a supersaturated environment for ice, resulting in rapid evaporation of liquid water and rapid ice crystal growth through vapor deposition.
If the number density of ice is small compared to liquid water, the ice crystals can grow large enough to fall out of the cloud, melting into rain drops if lower level temperatures are warm enough.
Wegener theorized that if this process happened in clouds and the crystals grew large enough to fall out, it could be a viable precipitation mechanism.
While his work with ice crystal growth attracted some attention, it would take another 10 years before its application to precipitation would be recognized.
[1] In the winter of 1922, Tor Bergeron made a curious observation while walking through the woods.
Being familiar with Wegener's earlier work, Bergeron theorized that ice crystals on the tree branches were scavenging vapor from the supercooled stratus cloud, preventing it from reaching the ground.
In 1933, Bergeron was selected to attend the International Union of Geodesy and Geophysics meeting in Lisbon, Portugal, where he presented his ice crystal theory.
Alternatively, an adiabatic updraft has to be sufficiently fast so that high supersaturation causes spontaneous nucleation of many more droplets than cloud condensation nuclei are present.
The growth of the droplets would prevent the temperature from soon reaching the point of fast nucleation of ice crystals.
The larger supersaturation with respect to ice, once present, causes it to grow fast thus scavenging water from the vapor phase.
Korolev and Mazin[2] derived expressions for the critical updraft and downdraft speed: where η and χ are coefficients dependent on temperature and pressure,
These velocities can be easily produced by convection, waves or turbulence, indicating that it is not uncommon for both liquid water and ice to grow simultaneously.
Ice crystals can form from heterogeneous deposition, contact, immersion, or freezing after condensation.
For contact, ice nuclei will collide with water droplets that freeze upon impact.
In immersion freezing, the entire ice nucleus is covered in liquid water.
[4] Water will freeze at different temperatures depending upon the type of ice nuclei present.
For pure water to freeze spontaneously, called homogeneous nucleation, cloud temperatures would have to be −35 °C (−31 °F).
When an ice crystal collides with supercooled water droplets, it is called accretion (or riming).
It may even collide with other ice crystals and grow larger still through collision coalescence, aggregation, or accretion.
There also may be a layer of air below freezing below the cloud base, causing the precipitation to refreeze in the form of ice pellets.
The process may also result in no precipitation, evaporating before it reaches the ground, in the case of forming virga.
