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A convection cell is a phenomenon of fluid dynamics that occurs in situations where there are density differences within a body of liquid or gas. The density differences result in rising and/or falling currents, a key characteristic of a convection cell. The convection usually requires a gravitational field but in microgravity experiments, thermal convection has been observed without gravitational effects being needed.
Fluids are materials that exhibit the property of flow. Both gases and liquids have fluid properties, and in sufficient quantity, even particulate solids such as salt, grain, and gravel show some fluid properties. When a volume of fluid is heated, it expands and becomes less dense and thus more buoyant than the surrounding fluid. The colder, denser fluid settles underneath the warmer, less dense fluid and forces it 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 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 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, and they can even occur when the heating of a fluid is uniform.
A rising body of fluid typically loses heat because it encounters a cold surface; because it exchanges heat with colder liquid through direct exchange; or in the example of the Earth's atmosphere, because it radiates heat. At some point, the fluid becomes denser than the fluid underneath 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 itself.
Warm air has a lower density than cool air, so warm air rises within cooler air, similar to hot air balloons. 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 condense. When the moisture condenses, it releases energy known as latent heat of fusion which allows the rising packet of air to cool less than its surrounding air, continuing the cloud's ascension. If enough 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 airmass, and a lifting force (heat).
All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.
The Sun's photosphere is composed of convection cells called granules, rising columns of superheated (5800°C) plasma averaging about 1000 kilometres in diameter. The plasma cools as it rises and descends in the narrow spaces between the granules.