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The Bergeron–Findeisen process (after Tor Bergeron and W. Findeisen), also known as the cold rain or ice crystal process, is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal growth. The equilibrium vapor pressure over water is greater than the saturation vapor pressure over ice, at the same temperature. Therefore in a mixed phase cloud, the liquid water will be out of vapor pressure equilibrium and will evaporate to reach equilibrium. The water droplets will move toward the lower pressure over the ice and diffuse onto the ice crystals. The vapor will be condensed and freeze onto the ice crystal, causing it to grow larger.

Contents

Conditions needed

In order for the Bergeron Process to occur, supercooled water droplets and ice crystals must be present together in the cloud.

Formation of ice crystals

The most common way to form an ice crystal, starts with an ice nucleus in the cloud. Ice crystals can form from heterogeneous deposition, contact, immersion, or freezing after condensation. In heterogeneous deposition, an ice nucleus is simply coated with water. For contact, ice nuclei will collide with water droplets that freeze upon impact. During immersion, an ice nucleus will hit a water droplet and instantly freeze it. Water can also condense onto ice nuclei and then freeze.

Water will freeze at different temperatures depending upon the type of ice nuclei present. Ice nuclei cause water to freeze at higher temperatures then it would spontaneously. For pure water to freeze spontaneously, called homogenous nucleation, cloud temperatures would have to be -42 degrees Celsius.[1] Here are some examples of ice nuclei:

Ice Nuclei Temperature to Freeze(degrees C)
Bacteria -2.6
Kaolinite -4
Silver Iodide -7
Vaterite -9

Ice multiplication

Different ice crystals present together in a cloud.

As the ice crystals grow, they can bump into each other and splinter and fracture, resulting in many new ice crystals. There are many shapes of ice crystals to bump into each other. These shapes include hexagons, cubes, columns, and dendrites.

Aggregation

The process of ice crystals sticking together is called aggregation. This happens when ice crystals are slick or sticky at temperatures of -5 degrees Celsius and above, because of a coating of water surrounding the crystal. The different sizes and shapes of ice crystals fall at different terminal velocities and commonly collide and stick.

Accretion

When an ice crystal collides with supercooled water its called accretion. Droplets freeze upon impact and can form graupel.

Precipitation

Eventually this ice crystal will grow large enough to fall. It may even collide with other ice crystals and grow larger still through collision coalescence, aggregation, or accretion.

The Bergeron Process often results in precipitation. As the crystals grow and fall, they pass through the base of the cloud, which may be above freezing. This causes the crystals to melt and fall as rain. 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. Similarly, the layer of air below freezing may be at the surface, causing the precipitation to fall as freezing rain. The process may also result in no precipitation, evaporating before it reaches the ground, in the case of forming virga.

See also

References

  1. ^ Koop, T. (March 25, 2004). "Homogeneous ice nucleation in water and aqueous solutions". Zeitschrift für physikalische Chemie 218 (11): 1231–1258. http://cat.inist.fr/?aModele=afficheN&cpsidt=16233451. Retrieved 2008-04-07.  
  • Wallace, John M. and Peter V. Hobbs: Atmospheric Science, 2006. ISBN 0-12-732951-X
  • Yau, M.K. and Rodgers, R.R.: "A Short Course in Cloud Physics", 1989. ISBN 0-75-0632151

External links

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The Bergeron–Findeisen process (after Tor Bergeron and W. Findeisen), also known as the cold rain or ice crystal process, is the formation of precipitation in the cold clouds of the mid and upper latitudes by ice crystal growth. The equilibrium vapor pressure over water is greater than the saturation vapor pressure over ice, at the same temperature. Therefore in a mixed phase cloud, the liquid water will be out of vapor pressure equilibrium and will evaporate to reach equilibrium. The water droplets will move toward the lower pressure over the ice and diffuse onto the ice crystals. The vapor will be condensed and freeze onto the ice crystal, causing it to grow larger.

Contents

Conditions needed

In order for the Bergeron Process to occur, supercooled water droplets and ice crystals must be present together in the cloud.

Formation of ice crystals

The most common way to form an ice crystal, starts with an ice nucleus in the cloud. Ice crystals can form from heterogeneous deposition, contact, immersion, or freezing after condensation. In heterogeneous deposition, an ice nucleus is simply coated with water. For contact, ice nuclei will collide with water droplets that freeze upon impact. During immersion, an ice nucleus will hit a water droplet and instantly freeze it. Water can also condense onto ice nuclei and then freeze.

Water will freeze at different temperatures depending upon the type of ice nuclei present. Ice nuclei cause water to freeze at higher temperatures then it would spontaneously. For pure water to freeze spontaneously, called homogenous nucleation, cloud temperatures would have to be -42 degrees Celsius.[1] Here are some examples of ice nuclei:

Ice Nuclei Temperature to Freeze(degrees C)
Bacteria -2.6
Kaolinite -4
Silver Iodide -7
Vaterite -9

Ice multiplication

As the ice crystals grow, they can bump into each other and splinter and fracture, resulting in many new ice crystals.  There are many shapes of  ice crystals to bump into each other. These shapes include hexagons, cubes, columns, and dendrites.

Aggregation

The process of ice crystals sticking together is called aggregation. This happens when ice crystals are slick or sticky at temperatures of -5 degrees Celsius and above, because of a coating of water surrounding the crystal. The different sizes and shapes of ice crystals fall at different terminal velocities and commonly collide and stick.

Accretion

When an ice crystal collides with supercooled water its called accretion. Droplets freeze upon impact and can form graupel.

Precipitation

Eventually this ice crystal will grow large enough to fall. It may even collide with other ice crystals and grow larger still through collision coalescence, aggregation, or accretion.

The Bergeron Process often results in precipitation. As the crystals grow and fall, they pass through the base of the cloud, which may be above freezing. This causes the crystals to melt and fall as rain. 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. Similarly, the layer of air below freezing may be at the surface, causing the precipitation to fall as freezing rain. The process may also result in no precipitation, evaporating before it reaches the ground, in the case of forming virga.

See also

References

  1. ^ Koop, T. (March 25, 2004). "Homogeneous ice nucleation in water and aqueous solutions". Zeitschrift für physikalische Chemie 218 (11): 1231–1258. http://cat.inist.fr/?aModele=afficheN&cpsidt=16233451. Retrieved 2008-04-07. 

External links


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