Editing Convection cell

{{Short description|Cyclic flow of convection currents in a fluid}}
{{Lead rewrite|date=October 2023}}
[[Image:Altocumulus15.jpg|thumb|Altocumulus cloud as seen from the Space Shuttle. Altocumulus clouds are formed by convective activity.]]
[[File:Rayleigh–Bénard convection pattern in honey wine with cinnamon floating on top after 10 days of fermentation.jpg|thumb|6-gallon bucket of [[Mead|honey wine]] after fermenting 10 days with cinnamon floating on top. Convection is caused by yeast releasing CO2.]]

In [[fluid dynamics]], a '''convection cell''' is the phenomenon that occurs when [[density]] differences exist within a body of [[liquid]] or [[gas]]. These density differences result in rising and/or falling [[convection current]]s, which are the key characteristics of a convection cell. When a volume of fluid is heated, it expands and becomes less dense and thus more buoyant than the surrounding fluid. The colder, denser part of the fluid descends to settle below the warmer, less-dense fluid, and this causes the warmer fluid to rise. Such movement is called [[convection]], and the moving body of liquid is referred to as a ''convection cell''. This particular type of convection, where a horizontal layer of fluid is heated from below, is known as [[Rayleigh–Bénard convection]]. Convection usually requires a gravitational field, but in microgravity experiments, thermal convection has been observed without gravitational effects.<ref>[https://archive.today/20120527090640/http://www.springerlink.com/content/nx7l8r7675471427/ Yu. A.Gaponenko and V. E. Zakhvataev,''Nonboussinesq Thermal Convection in Microgravity under Nonuniform Heating'']</ref>

Fluids are generalized as materials that exhibit the property of [[Fluid dynamics|flow]]; however, this behavior is not unique to liquids. Fluid properties can also be observed in gases and even in particulate solids (such as sand, gravel, and larger objects during [[rock slide]]s).

A convection cell is most notable in the formation of clouds with its release and transportation of energy. As air moves along the ground it absorbs heat, loses density and moves up into the atmosphere. When it is forced into the atmosphere, which has a lower air pressure, it cannot contain as much fluid as at a lower altitude, so it releases its moist air, producing rain. In this process the warm air is cooled; it gains density and falls towards the earth and the cell repeats the cycle.

Convection cells can form in any fluid, including the [[Earth's atmosphere]] (where they are called [[Hadley cell]]s), boiling water, soup (where the cells can be identified by the particles they transport, such as grains of rice), the ocean, or the surface of the [[Sun]]. The size of convection cells is largely determined by the fluid's properties. Convection cells can even occur when the heating of a fluid is uniform.

==Process==
A rising body of fluid typically loses heat when it encounters a cold surface when it exchanges heat with colder liquid through direct exchange, or in the example of the [[Earth's atmosphere]], when it radiates heat. At some point, the fluid becomes denser than the fluid beneath it, which is still rising. Since it cannot descend through the rising fluid, it moves to one side. At some distance, its downward force overcomes the rising force beneath it, and the fluid begins to descend. As it descends, it warms again through surface contact or conductivity and the cycle repeats.

==Within the Earth's troposphere==

===Thunderstorms===
[[File:Thunderstorm formation.jpg|thumb|500px|Stages of a thunderstorm's life.]]
{{see also|Cloud|Thunderstorm}}
Warm air has a lower density than cool air, so warm air rises within cooler air,<ref>{{cite book|url=https://archive.org/details/civilengineersp00fryegoog |title=Civil engineers' pocket book: a reference-book for engineers, contractors|author=Albert Irvin Frye|page=[https://archive.org/details/civilengineersp00fryegoog/page/n510 462]|publisher=D. Van Nostrand Company|year=1913|accessdate=2009-08-31}}</ref> similar to [[hot air balloon]]s.<ref>{{cite book
| url = https://books.google.com/books?id=ssO_19TRQ9AC&dq=Kongming+balloon&pg=PA112
| title = Ancient Chinese Inventions
| author =  Yikne Deng
| publisher = Chinese International Press
| isbn=978-7-5085-0837-5
| year=2005
| pages = 112–13
| accessdate = 2009-06-18}}</ref>  Clouds form as relatively warmer air carrying moisture rises within cooler air. As the moist air rises, it cools, causing some of the [[water vapor]] in the rising packet of air to [[condensation|condense]].<ref>{{cite web|author=FMI|year=2007|url=http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?/docu/Manual/SatManu/CMs/FgStr/backgr.htm|title=Fog And Stratus – Meteorological Physical Background|publisher=Zentralanstalt für Meteorologie und Geodynamik|accessdate=2009-02-07}}</ref>  When the moisture condenses, it releases energy known as the [[latent heat]] of vaporisation, which allows the rising packet of air to cool less than its surrounding air,<ref>{{cite book|url=https://archive.org/details/stormworldhurric00moon |url-access=registration |title=Storm world: hurricanes, politics, and the battle over global warming|author=Chris C. Mooney|page=[https://archive.org/details/stormworldhurric00moon/page/20 20]|isbn=978-0-15-101287-9|publisher=Houghton Mifflin Harcourt|year=2007|accessdate=2009-08-31}}</ref> continuing the cloud's ascension. If enough [[Convective available potential energy|instability]] is present in the atmosphere, this process will continue long enough for [[Cumulonimbus|cumulonimbus clouds]] to form, which support lightning and thunder.  Generally, thunderstorms require three conditions to form:  moisture, an unstable air mass, and a lifting force (heat).

All thunderstorms, regardless of type, go through three stages: a 'developing stage', a 'mature stage', and a 'dissipating stage'.<ref name="Extreme Weather">{{cite book |title=Extreme Weather |author=Michael H. Mogil |year=2007 |publisher=Black Dog & Leventhal Publisher |location=New York |isbn=978-1-57912-743-5 |pages=210–211 }}<!--|accessdate=2009-08-08--></ref>  The average thunderstorm has a {{convert|24|km|mi|abbr=on}} diameter.<ref name="Folger2011">{{cite book |author=Peter Folger |title=Severe Thunderstorms and Tornadoes in the United States |url=https://books.google.com/books?id=PB0U26iYawwC&pg=PA16 |date=10 April 2011 |publisher=DIANE Publishing |isbn=978-1-4379-8754-6| page=16}}</ref> Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.<ref name="tsbasics">{{cite web|url=http://www.nssl.noaa.gov/primer/tstorm/tst_basics.html|title=A Severe Weather Primer: Questions and Answers about Thunderstorms|author=National Severe Storms Laboratory|publisher=[[National Oceanic and Atmospheric Administration]]|date=2006-10-15|accessdate=2009-09-01|archive-url=https://web.archive.org/web/20090825000832/http://www.nssl.noaa.gov/primer/tstorm/tst_basics.html|archive-date=2009-08-25|url-status=dead}}</ref>

===Adiabatic processes===
Heating caused by the compression of descending air is responsible for such winter phenomena as the [[chinook wind|chinook]] (as it is known in western North America) or the [[Foehn wind|Föhn]] (in the Alps).
[[File:Granulation Quiet Sun SST 25May2017.webm|left|thumb|Movie of the solar photosphere observed with the Swedish 1-m Solar Telescope (SST) on La Palma, Spain. The movie shows solar granulation which is a result of convective motions of bubbles of hot gas that rise from the solar interior. When these bubbles reach the surface, the gas cools and flows down again in the darker lanes between the bright cells. In these so-called intergranular lanes, we can also see small bright points and more extended bright elongated structures. These are regions with strong magnetic fields.]]

== Within the Sun ==
The Sun's [[photosphere]] is composed of convection cells called ''[[Granule (solar physics)|granule]]s'', which are rising columns of superheated (5,800&nbsp;°C) [[Plasma (physics)|plasma]] averaging about 1,000 kilometres in diameter. The plasma cools as it rises and descends in the narrow spaces between the granules.

==References==
{{reflist}}

==External links==
* [http://www.mountainnature.com/Climate/Chinook.htm Mountainnature.com — Chinook] {{Webarchive|url=https://web.archive.org/web/20140214104142/http://www.mountainnature.com/Climate/Chinook.htm |date=2014-02-14 }}

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[[Category:Convection]]