Bergeron process
Encyclopedia
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.
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. Here are some examples of ice nuclei:
to bump into each other. These shapes include hexagons, cubes, columns, and dendrites.
and commonly collide and stick.
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
.
Tor Bergeron
Tor Bergeron was the Swedish meteorologist who proposed a mechanism for the formation of precipitation in clouds. In the 1930s, Bergeron and W. Findeisen developed the concept that clouds contain both supercooled water and ice crystals...
and W. Findeisen), also known as the cold rain or ice crystal process, is the formation of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms 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
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. 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.
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 nucleusIce nucleus
An ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere.The presence of ice nuclei increase the temperature that ice will form in the atmosphere from around −42°C to about −10°C...
in the cloud. Ice crystals can form from heterogeneous deposition
Physical vapor deposition
Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...
, 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
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
, cloud temperatures would have to be -42 degrees Celsius. 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 crystalsIce
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
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 velocitiesTerminal velocity
In fluid dynamics an object is moving at its terminal velocity if its speed is constant due to the restraining force exerted by the fluid through which it is moving....
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 collisioncoalescence
Coalescence (meteorology)
Coalescence is the process by which two or more droplets, bubbles or particles merge during contact to form a single daughter droplet, bubble or particle. It can take place in many processes, ranging from meteorology to astrophysics. For example, it is both inve formation of raindrops as well as...
, 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
Ice pellets
Ice pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain...
. Similarly, the layer of air below freezing may be at the surface, causing the precipitation to fall as
freezing rain
Freezing rain
Freezing rain is the name given to rain that falls when surface temperatures are below freezing. The raindrops become supercooled while passing through a sub-freezing layer of air, many hundred feet , just above the surface, and then freeze upon impact with any object they encounter. The resulting...
. The process may also result in no precipitation, evaporating before it reaches the ground, in the case of forming virga
Virga
In meteorology, virga is an observable streak or shaft of precipitation that falls from a cloud but evaporates before reaching the ground. At high altitudes the precipitation falls mainly as ice crystals before melting and finally evaporating; this is usually due to compressional heating, because...
.
See also
- List of meteorology topics
- Precipitation (meteorology)Precipitation (meteorology)In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
- Coalescence (meteorology)Coalescence (meteorology)Coalescence is the process by which two or more droplets, bubbles or particles merge during contact to form a single daughter droplet, bubble or particle. It can take place in many processes, ranging from meteorology to astrophysics. For example, it is both inve formation of raindrops as well as...
- Ice nucleusIce nucleusAn ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere.The presence of ice nuclei increase the temperature that ice will form in the atmosphere from around −42°C to about −10°C...
- IceIceIce is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
- NucleationNucleationNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
- Physical vapor depositionPhysical vapor depositionPhysical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...
- Saturation vapor pressure