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The Wegener–Bergeron–Findeisen process (after
Alfred Wegener Alfred Lothar Wegener (; ; 1 November 1880 – November 1930) was a German climatologist, geologist, geophysicist, meteorologist, and polar researcher. During his lifetime he was primarily known for his achievements in meteorology and ...
,
Tor Bergeron Tor Bergeron (15 August 1891 – 13 June 1977) was a 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 w ...
and ), (or "cold-rain process") is a process of ice crystal growth that occurs in mixed phase clouds (containing a mixture of
supercooled water Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...
and ice) in regions where the ambient vapor pressure falls between the
saturation vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...
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. The Bergeron process, if occurring at all, is much more efficient in producing large particles than is the growth of larger droplets at the expense of smaller ones, since the difference in saturation pressure between liquid water and ice is larger than the enhancement of saturation pressure over small droplets (for droplets large enough to considerably contribute to the total mass). For other processes affecting particle size, see
rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water fo ...
and
cloud physics Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest p ...
.


History

The principle of ice growth through vapor deposition on ice crystals at the expense of water was first theorized by the German scientist Alfred Wegener in 1911 while studying
hoarfrost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a gas ...
formation. Wegener theorized that if this process happened in clouds and the crystals grew large enough to fall out, that 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. In the winter of 1922, Tor Bergeron made a curious observation while walking through the woods. He noticed that on days when the temperature was below freezing, the stratus deck that typically covered the hillside stopped at the top of the
canopy Canopy may refer to: Plants * Canopy (biology), aboveground portion of plant community or crop (including forests) * Canopy (grape), aboveground portion of grapes Religion and ceremonies * Baldachin or canopy of state, typically placed over an a ...
instead of extending to the ground as it did on days when the temperature was above freezing. 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. In his paper, he stated that if the ice crystal population was significantly small compared to the liquid water droplets, that the ice crystals could grow large enough to fall out (Wegener's original hypothesis). Bergeron theorized that this process could be responsible for all rain, even in tropical climates; a statement that caused quite a bit of disagreement between tropical and mid-latitude scientists. In the late 1930s, German meteorologist Walter Findeisen extended and refined Bergeron's work through both theoretical and experimental work.


Required conditions

The condition that the number of droplets should be much larger than the number of ice crystals depends on the fraction of
cloud condensation nuclei Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2  µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This ...
that would later (higher in the cloud) act as
ice nuclei An ice nucleus, also known as an ice nucleating particle (INP), is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere. Ice Nucleation Mechanisms There are a number of mechanisms of ice nucleation in the atmo ...
. Alternatively, an adiabatic
updraft In meteorology, an updraft is a small-scale current of rising air, often within a cloud. Overview Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding region, ...
has to be sufficiently fast so that high supersaturation causes spontaneous nucleation of many more droplets than cloud condensation nuclei are present. In either case, this should happen not far below the freezing point as this would cause direct nucleation of ice. 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. If the vapor pressure p drops below the saturation pressure with respect to liquid water p_w, the droplets will cease to grow. This may not occur if p_w itself is dropping rapidly, depending on the slope of the saturation curve, the
lapse rate The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
, and the speed of the updraft, or if the drop of p is slow, depending on the number and size of the ice crystals. If the updraft is too fast, all the droplets would finally freeze rather than evaporate. A similar limit is encountered in a downdraft. Liquid water evaporates causing the vapor pressure p to rise, but if the saturation pressure with respect to ice p_i is rising too fast in the downdraft, all ice would melt before large ice crystals have formed. Korolev and Mazin derived expressions for the critical updraft and downdraft speed: :u_ = \frac \eta N_i \bar \, :u_ = \frac \chi N_w \bar \, where ''η'' and ''χ'' are coefficients dependent on temperature and pressure, N_i and N_w are the number densities of ice and liquid particles (respectively), and \bar and \bar are the mean radius of ice and liquid particles (respectively). For values of N_i \bar typical of clouds, u_ ranges from a few cm/s to a few m/s. 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. In comparison, for typical values of N_w \bar, downdraft velocities in excess of a few m s^ are required for both liquid and ice to shrink simultaneously. These velocities are common in convective downdrafts, but are not typical for stratus clouds.


Formation of ice crystals

The most common way to form an ice crystal starts with an
ice nucleus An ice nucleus, also known as an ice nucleating particle (INP), is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere. Ice Nucleation Mechanisms There are a number of mechanisms of ice nucleation in the atmo ...
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. In immersion freezing, the entire ice nucleus is covered in liquid water.Ice Nucleation in Mixed-Phase Clouds Thomas F. Whale University of Leeds, Leeds, United Kingdom, CHAPTER 2,1.1 Modes of Heterogeneous Ice Nucleation. Water will freeze at different temperatures depending upon the type of ice nuclei present. Ice nuclei cause water to freeze at higher temperatures than it would spontaneously. For pure water to freeze spontaneously, called homogeneous nucleation, cloud temperatures would have to be . Here are some examples of ice nuclei:


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. This process is referred to as "ice enhancement" by atmospheric physicists and chemists.Microphysics of clouds and precipitation. Pruppacher, Hans R., Klett, James, 1965.


Aggregation

The process of ice crystals sticking together is called aggregation. This happens when ice crystals are slick or sticky at temperatures of 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 droplets it is called accretion (or riming). Droplets freeze upon impact and can form
graupel Graupel (; ), also called soft hail, hominy snow, or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming balls of crisp, opaque rime. Graupel is distinct from ...
. If the graupel formed is reintroduced into the cloud by wind, it may continue to grow larger and more dense, eventually forming
hail Hail is a form of solid precipitation. It is distinct from ice pellets (American English "sleet"), though the two are often confused. It consists of balls or irregular lumps of ice, each of which is called a hailstone. Ice pellets generally fal ...
.


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 Coalescence may refer to: * Coalescence (chemistry), the process by which two or more separate masses of miscible substances seem to "pull" each other together should they make the slightest contact * Coalescence (computer science), the merging o ...
, 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 are a form of precipitation consisting of small, hard, translucent balls of ice. Ice pellets are different from graupel ("soft hail") which is made of frosty white opaque rime, and from a mixture of rain and snow which is a slushy l ...
. Similarly, the layer of air below freezing may be at the surface, causing the precipitation to fall as
freezing rain Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The raind ...
. The process may also result in no precipitation, evaporating before it reaches the ground, in the case of forming
virga In meteorology, a virga, also called a dry storm, is an observable streak or shaft of precipitation falling from a cloud that evaporates or sublimates before reaching the ground. A shaft of precipitation that does not evaporate before re ...
.


See also

*
List of meteorology topics This is a list of meteorology topics. The terms relate to meteorology, the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. (see also: List of meteorological phenomena) A * advection * a ...
*
Precipitation (meteorology) In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, Rain and snow mixed, sleet, snow, ice pellets, ...
*
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 ...
*
Ice nucleus An ice nucleus, also known as an ice nucleating particle (INP), is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere. Ice Nucleation Mechanisms There are a number of mechanisms of ice nucleation in the atmo ...
*
Ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
*
Nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
*
Physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...
*
Saturation vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...


References

*Wallace, John M. and Peter V. Hobbs: ''Atmospheric Science'', 2006, . *Yau, M.K. and Rodgers, R.R.: "A Short Course in Cloud Physics", 1989, .


External links


Demonstration of the Bergeron Process
{{DEFAULTSORT:Wegener-Bergeron-Findeisen process Cloud and fog physics