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The Bergeron Findeisen Process
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Ice crystals are in a minority because the freezing nuclei necessary for their formation are less abundant than condensation nuclei. Because relative humidity of air is greater above an ice surface than over water, water droplets evaporate and the resultant vapour condenses back onto ice crystals, they grow into hexagonal-shaped snowflakes. The flakes grow in size-either as a result of further condensation or by fusion as their numerous edges interlock on collision with other flakes. They also increase in number as ice splinters break off and form new nuclei.
The Bergeron Findeisen Process
If the air temperature rises above freezing point (0°C), as the snow falls to the ground, flakes melt into raindrops.
***The Bergeron-Findeisen theory is supported by evidence from temperate latitudes where rainclouds usually extend vertically above the freezing level. Radar and high-flying aircraft have reported snow at high altitudes when it is raining at sea-level. However, as clouds rarely reach freezing point in the tropics, the formation of ice crystals is unlikely in those latitudes.
Process
The basis of this process relies primarily on the fact that, at temperatures below 0°C, the equilibrium vapour pressure with respect to ice, is less than the vapour pressure with respect to liquid water. Therefore, in a cloud of mixed liquid droplets and ice crystals, the ice crystal tend to gain water molecules at the expense of the liquid droplet.
What is it ?
The Bergeron-Findeisen process or ice crystal mechanism refers to the rapid growth of ice crystals at the expense of surrounding cloud droplets, which frequently occurs in atmospheric mixed-phase clouds. The process is a result of the difference in saturation vapour pressures with respect to liquid and ice, and may in some circumstances lead to abrupt and complete cloud glaciation at temperatures between −40°C and 0°C in the Earth’s atmosphere. This theory was first proposed by T. Bergeron in 1933, and further developed by W. Findeisen.
Bergeron suggested that raindrop formation depends on ice crystal growth high in the cold clouds, the coexistence of water vapour, liquid cloud droplets and ice crystals. He saw freezing nuclei as important to initiate the freezing process. The ice crystals grow until they begin to fall and grow by accretion. As they approach the surface they may fall as ice forms or rain of temperatures are warm enough.