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 "title": "Bergeron process",
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 "excerpt": "The Bergeron process, also called the Wegener–Bergeron–Findeisen process, is the mechanism by which ice crystals in cold clouds grow at the expense of supercooled water droplets.",
 "snippet": "The Bergeron process, also called the Wegener–Bergeron–Findeisen process, is the mechanism by which ice crystals in cold clouds grow at the expense of supercooled water droplets.",
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 "markdown": "# Bergeron process\n\nThe Bergeron process, more fully the Wegener–Bergeron–Findeisen (WBF) process and also called the ice-crystal process, is the mechanism by which ice crystals in a mixed-phase cloud grow at the expense of supercooled liquid water droplets, providing a route to precipitation in cold clouds<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup><sup> • </sup><sup>[2](https://forecast.weather.gov/glossary.php?word=Bergeron+process)</sup>. It rests on a single physical fact: at any subfreezing temperature, the equilibrium vapor pressure over ice is lower than that over liquid water, so air saturated with respect to liquid is supersaturated with respect to ice<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup>.\n\n| Key fact | Detail |\n|---|---|\n| Definition | Ice crystals grow by vapor deposition while surrounding supercooled droplets evaporate, transferring water from liquid to ice<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup><sup> • </sup><sup>[2](https://forecast.weather.gov/glossary.php?word=Bergeron+process)</sup> |\n| Physical basis | Saturation vapor pressure over ice is lower than over liquid water at the same subfreezing temperature<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup> |\n| Attribution | Proposed by Tor Bergeron in 1933, developed by Walter Findeisen; nucleation features suggested by Alfred Wegener in 1911<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup> |\n| Growth rate | Depositional growth is fastest at about −12°C, producing individual snow crystals in roughly 10 to 20 minutes<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup> |\n| Typical setting | Supercooled drops are common in clouds between about 0°C and −20°C or below; cold rain production is associated with cloud-top temperatures below about −15°C<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup><sup> • </sup><sup>[3](https://oit.tudelft.nl/Hydrology/main/04_precipitation/1_precipitation_processes.html)</sup> |\n| Limits | The WBF process is one of three possible growth regimes in mixed-phase clouds and occurs only under a limited range of conditions<sup>[4](https://journals.ametsoc.org/view/journals/atsc/64/9/jas4035.1.xml)</sup> |\n| Application | Glaciogenic cloud seeding with silver iodide or frozen CO₂ activates the process; under ideal conditions seeding may enhance precipitation by about 10%<sup>[5](https://www.ess.uci.edu/~yu/class/ess55/lecture.7.precipitation-cloud_files/slide0021.htm)</sup> |\n\n## Physical mechanism\n\nWater can remain liquid below 0°C as supercooled liquid, which requires ice nuclei to freeze, although at about −40°C liquid water freezes homogeneously without nuclei<sup>[6](https://courses.ems.psu.edu/meteo300/node/781)</sup>. When an ice particle coexists with supercooled droplets, the ambient vapor pressure, in equilibrium with the liquid, exceeds the saturation vapor pressure of the ice. [Water vapor](https://www.edgepedia.org/water-vapor) therefore deposits onto the ice crystal, lowering the ambient vapor pressure below the saturation point of the liquid drops, which then evaporate<sup>[6](https://courses.ems.psu.edu/meteo300/node/781)</sup>.\n\nThe water transfer does not involve droplets colliding with the crystal. [Liquid drops](https://www.edgepedia.org/drop-liquid) evaporate to vapor, and that vapor diffuses to the ice and condenses there<sup>[6](https://courses.ems.psu.edu/meteo300/node/781)</sup>. The AMS Glossary notes that the process requires numerous small supercooled drops together with a small number of ice crystals; the vapor-pressure difference between the two phases is what drives the exchange<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup>.\n\nBecause the gap between the saturation vapor pressure curves over ice and over water varies with temperature, the depositional growth rate reaches its maximum at about −12°C, where individual snow crystals can form in some 10 to 20 minutes<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup>. Penn State course materials describe the process as a way that precipitation-sized particles can form in about 40 minutes in mixed-phase clouds<sup>[6](https://courses.ems.psu.edu/meteo300/node/781)</sup>.\n\n## History\n\nThe theory was first proposed by the Norwegian meteorologist [Tor Bergeron](https://www.edgepedia.org/tor-bergeron) in 1933, in a memoir presented to the Association de Météorologie of the U.G.G.I. in Lisbon, and published in 1935 as *On the Physics of Cloud and Precipitation*<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup><sup> • </sup><sup>[7](https://books.google.com/books/about/On_the_Physics_of_Cloud_and_Precipitatio.html?id=9zkaAQAAMAAJ)</sup>. The German military meteorologist Walter Findeisen (1909–1945) further developed it in 1938, and certain nucleation-related features had been suggested by [Alfred Wegener](https://www.edgepedia.org/alfred-wegener) as early as 1911<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup><sup> • </sup><sup>[8](https://repositorio.aemet.es/bitstream/20.500.11765/16538/4/Walter_Findeisen_CalMet25_ENGLISH.pdf)</sup>. A 1947 Caltech review described the Bergeron–Findeisen ice-crystal theory, advanced in the 1930s, as the explanation best accounting for the bulk of precipitation phenomena in middle-latitude storms<sup>[9](https://calteches.library.caltech.edu/1167/)</sup>.\n\n## Role in precipitation\n\nIn a cloud of sufficient vertical extent, the region between roughly −10°C and −23°C contains both supercooled droplets and ice crystals, the setting in which the Bergeron process operates<sup>[3](https://oit.tudelft.nl/Hydrology/main/04_precipitation/1_precipitation_processes.html)</sup>. Ice crystals grow by deposition, then accelerate as they fall and collide with other hydrometeors: collisions with other crystals produce snowflakes by aggregation, while collisions with supercooled droplets freeze onto the crystal as rime, eventually forming graupel or hail<sup>[3](https://oit.tudelft.nl/Hydrology/main/04_precipitation/1_precipitation_processes.html)</sup>. Once falling particles pass the 0°C isotherm, melting converts them to rain<sup>[3](https://oit.tudelft.nl/Hydrology/main/04_precipitation/1_precipitation_processes.html)</sup>.\n\nThe process complements rather than replaces collision–coalescence. [Irving Langmuir](https://www.edgepedia.org/irving-langmuir) proposed the collision–coalescence mechanism for rain formation on top of the Bergeron–Findeisen process in 1948, and the combination of the ice phase with collisional mechanisms is what explains heavier mid-latitude precipitation<sup>[10](https://www.cambridge.org/core/books/abs/physics-and-dynamics-of-clouds-and-precipitation/collision-coalescence-breakup-and-melting/CDD39411B4304B5DF972FD0254075BF8)</sup><sup> • </sup><sup>[11](https://ntrs.nasa.gov/api/citations/19770011718/downloads/19770011718.pdf)</sup>. In [cumulonimbus clouds](https://www.edgepedia.org/cumulonimbus-cloud), warm and cold precipitation processes contribute together, with the Bergeron–Findeisen process acting in the mixed-phase region and collision–coalescence in the warm region below<sup>[12](https://www.meted.ucar.edu/tropical/textbook_2nd_edition/navmenu.php?page=3.1.0&tab=6)</sup>.\n\n## Limits of the process\n\n**Three growth regimes.** Alexei Korolev, a cloud physics researcher at Environment Canada, showed in 2007 that the WBF process is only one of three possible scenarios in a mixed-phase cloud: ice grows while liquid evaporates (WBF), both liquid and ice grow simultaneously, or both evaporate<sup>[4](https://journals.ametsoc.org/view/journals/atsc/64/9/jas4035.1.xml)</sup>. During simultaneous growth, liquid droplets compete for water vapor with the ice particle, slowing depositional growth of the ice instead of promoting it<sup>[4](https://journals.ametsoc.org/view/journals/atsc/64/9/jas4035.1.xml)</sup>. For typical values of the product of ice particle number concentration and mean radius in clouds, the vertical velocity required for simultaneous growth ranges from a few cm s⁻¹ to a few m s⁻¹, velocities that ordinary convection, waves, or turbulence can readily generate<sup>[4](https://journals.ametsoc.org/view/journals/atsc/64/9/jas4035.1.xml)</sup>.\n\n**How often it operates.** Cloud-resolving simulations of two types of Arctic mixed-phase clouds observed during the ISDAC and M-PACE field campaigns found that the WBF regime occupied about 50% of the mixed-phase cloud volume, with the other half in the both-grow regime and both-evaporating conditions negligible<sup>[13](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD015375)</sup>. In these simulations the WBF process occurred predominantly in downdrafts, while liquid and ice grew simultaneously in updrafts<sup>[13](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD015375)</sup>.\n\n**Entrainment and turbulence.** A 2020 modeling study found that an entrainment event generally accelerates glaciation through the WBF process by contributing to droplet evaporation, while small-scale processes associated with entrainment and mixing decelerate it<sup>[14](https://journals.ametsoc.org/view/journals/atsc/77/6/JAS-D-19-0289.1.xml)</sup>. Observations from the SOCRATES campaign over the [Southern Ocean](https://www.edgepedia.org/southern-ocean) found a positive relationship between droplet clustering, a proxy for entrainment-mixing, and concentrations of large ice particles, suggesting entrainment-mixing can enhance WBF process rates, though sufficiently strong mixing can offset this<sup>[15](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023JD038622)</sup>. A 2024 statistical modeling study found that small-scale turbulence does not significantly affect glaciation time on submetre scales, but that turbulence on larger spatial scales is likely to affect ice growth<sup>[16](https://arxiv.org/abs/2410.06724)</sup>.\n\n## Cloud seeding\n\nMuch cloud seeding is based on introducing artificial ice nuclei to supply more ice particles<sup>[1](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)</sup>. In glaciogenic static-mode seeding, silver iodide or frozen CO₂ is released into regions of a cumulus cloud containing supercooled water, activating the WBF process so that ice nucleates and grows rapidly to precipitation within the cloud's lifetime<sup>[17](https://journals.ametsoc.org/abstract/journals/amsm/59/1/amsmonographs-d-18-0004.1.xml)</sup>. [Dry ice](https://www.edgepedia.org/dry-ice) works by chilling the neighboring air below about −40°C so that ice needles form spontaneously, while silver iodide crystals, structurally similar to ice, serve as freezing nuclei at temperatures as high as 23°F<sup>[9](https://calteches.library.caltech.edu/1167/)</sup>. Under ideal conditions, seeding may enhance precipitation by about 10%<sup>[5](https://www.ess.uci.edu/~yu/class/ess55/lecture.7.precipitation-cloud_files/slide0021.htm)</sup>.\n\n## Representation in models\n\nWeather and climate models tend to overestimate WBF glaciation because cloud dynamics and microphysics are not accurately represented<sup>[16](https://arxiv.org/abs/2410.06724)</sup>. The process significantly affects cloud [optical depth](https://www.edgepedia.org/optical-depth), radiative forcing, precipitation rate, and cloud lifetime, but representing it in general circulation models is difficult because of subgrid variability of cloud properties and unresolved supersaturation<sup>[13](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD015375)</sup>. Applying the constraints identified by Korolev and Mazin to a general circulation model produced better agreement with observations<sup>[13](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD015375)</sup>.\n\n## References\n\n1. [Bergeron–Findeisen process, Glossary of Meteorology, American Meteorological Society](https://glossary.ametsoc.org/wiki/bergeron-findeisen-process/)\n2. [Bergeron Process, NOAA National Weather Service Glossary](https://forecast.weather.gov/glossary.php?word=Bergeron+process)\n3. [4.1. Precipitation processes, TU Delft Hydrology](https://oit.tudelft.nl/Hydrology/main/04_precipitation/1_precipitation_processes.html)\n4. [Korolev, A. (2007). Limitations of the Wegener–Bergeron–Findeisen Mechanism in the Evolution of Mixed-Phase Clouds. Journal of the Atmospheric Sciences, 64, 3372–3375.](https://journals.ametsoc.org/view/journals/atsc/64/9/jas4035.1.xml)\n5. [Cloud Seeding, ESS 55 lecture notes, University of California, Irvine (Prof. Jin-Yi Yu)](https://www.ess.uci.edu/~yu/class/ess55/lecture.7.precipitation-cloud_files/slide0021.htm)\n6. [5.9 An Unusual Way to Make Precipitation in Mixed-Phase Clouds, METEO 300, Penn State University](https://courses.ems.psu.edu/meteo300/node/781)\n7. [Bergeron, T. (1935). On the Physics of Cloud and Precipitation. Imprimerie Paul Dupont.](https://books.google.com/books/about/On_the_Physics_of_Cloud_and_Precipitatio.html?id=9zkaAQAAMAAJ)\n8. [Walter Findeisen, CalMet25 contribution, AEMET repository](https://repositorio.aemet.es/bitstream/20.500.11765/16538/4/Walter_Findeisen_CalMet25_ENGLISH.pdf)\n9. [Cloud Seeding—A New Technology, Caltech Magazine (1947)](https://calteches.library.caltech.edu/1167/)\n10. [Wang, P. K. Physics and Dynamics of Clouds and Precipitation, Chapter 10, Cambridge University Press](https://www.cambridge.org/core/books/abs/physics-and-dynamics-of-clouds-and-precipitation/collision-coalescence-breakup-and-melting/CDD39411B4304B5DF972FD0254075BF8)\n11. [Cloud Physics Processes—Cold Clouds, NASA Technical Reports Server](https://ntrs.nasa.gov/api/citations/19770011718/downloads/19770011718.pdf)\n12. [Introduction to Tropical Meteorology, Ch. 5, UCAR/COMET](https://www.meted.ucar.edu/tropical/textbook_2nd_edition/navmenu.php?page=3.1.0&tab=6)\n13. [Fan, J. et al. (2011). Representation of Arctic mixed-phase clouds and the Wegener–Bergeron–Findeisen process in climate models. Journal of Geophysical Research: Atmospheres.](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD015375)\n14. [Effects of Entrainment and Mixing on the Wegener–Bergeron–Findeisen Process. Journal of the Atmospheric Sciences, 77 (2020).](https://journals.ametsoc.org/view/journals/atsc/77/6/JAS-D-19-0289.1.xml)\n15. [D'Alessandro, J. J. et al. (2023). Impacts of Drop Clustering and Entrainment-Mixing on Mixed Phase Shallow Cloud Properties Over the Southern Ocean. Journal of Geophysical Research: Atmospheres.](https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023JD038622)\n16. [Sarnitsky, G. et al. (2024). Does small-scale turbulence matter for ice growth in mixed-phase clouds? arXiv:2410.06724.](https://arxiv.org/abs/2410.06724)\n17. [100 Years of Progress in Applied Meteorology. Part I: Basic Applications, AMS Monographs, Vol. 59](https://journals.ametsoc.org/abstract/journals/amsm/59/1/amsmonographs-d-18-0004.1.xml)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Earth sciences › Climate and weather › Meteorology and atmospheric science › Clouds › Cloud physics and formation*\n\n*Initially written Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n",
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 "speakable": "The Bergeron process, also called the Wegener–Bergeron–Findeisen process, is the mechanism by which ice crystals in cold clouds grow at the expense of supercooled water droplets."
}