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{{Lead rewrite|date=October 2023}} File:Rayleigh–Bénard convection pattern in honey wine with cinnamon floating on top after 10 days of fermentation.jpg|thumb|6-gallon bucket of 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 currents, 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.weblink" title="archive.today/20120527090640weblink">Yu. A.Gaponenko and V. E. Zakhvataev,Nonboussinesq Thermal Convection in Microgravity under Nonuniform HeatingFluids are generalized as materials that exhibit the property of 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 slides).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 cells), 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,BOOK,weblink Civil engineers' pocket book: a reference-book for engineers, contractors, Albert Irvin Frye, 462, D. Van Nostrand Company, 1913, 2009-08-31, similar to hot air balloons.BOOK,weblink Ancient Chinese Inventions, Yikne Deng, Chinese International Press, 978-7-5085-0837-5, 2005, 112–13

water vapor in the rising packet of air to condensation>condense.FMI>YEAR=2007

TITLE=FOG AND STRATUS – METEOROLOGICAL PHYSICAL BACKGROUND

ACCESSDATE=2009-02-07, 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,HTTPS://ARCHIVE.ORG/DETAILS/STORMWORLDHURRIC00MOON >URL-ACCESS=REGISTRATION

AUTHOR=CHRIS C. MOONEY

20>ISBN=978-0-15-101287-9

YEAR=2007

Convective available potential energy>instability is present in the atmosphere, this process will continue long enough for 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'.BOOK, Extreme Weather, Michael H. Mogil, 2007, Black Dog & Leventhal Publisher, New York, 978-1-57912-743-5, 210–211, The average thunderstorm has a {{convert|24|km|mi|abbr=on}} diameter.BOOK, Peter Folger, Severe Thunderstorms and Tornadoes in the United States,weblink 10 April 2011, DIANE Publishing, 978-1-4379-8754-6, 16, Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.WEB,weblink A Severe Weather Primer: Questions and Answers about Thunderstorms, National Severe Storms Laboratory, National Oceanic and Atmospheric Administration, 2006-10-15, 2009-09-01,weblink" title="web.archive.org/web/20090825000832weblink">weblink 2009-08-25, dead, Adiabatic processes Heating caused by the compression of descending air is responsible for such winter phenomena as the chinook (as it is known in western North America) or the 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 granules, which are rising columns of superheated (5,800 Â°C) 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 Mountainnature.com — Chinook {{Webarchive|url=https://web.archive.org/web/20140214104142weblink |date=2014-02-14 }}