Convection is the primary means of heat transfer in fluids; it creates convection cells, observed in various situations. Earth’s atmosphere has large-scale convection cells, including Hadley cells, which cause trade winds and arid regions. Convection cells in the Earth’s mantle cause plate tectonics and volcanic activity. The sun also has convection cells, visible as granules on its surface.
The convection process is the primary means of heat transfer within a fluid exposed to a heat source. Usually the fluid, which can be a liquid or a gas, is heated from below by a hot surface; the increase in temperature results in a decrease in density, causing the fluid to rise and flow cooler fluid inward to replace it. As it rises, it loses heat to its surroundings, becoming denser and heavier than the underlying fluid. It cannot descend through this rising fluid, so it spreads horizontally before falling back to the surface and being pulled towards its starting point by the rising fluid. This system is known as a convection cell and is a fluid dynamics feature that can be observed in a variety of situations, from water heated in a cooking pot to processes on a planetary or stellar scale.
Earth’s atmosphere is characterized by large-scale cells of convection: the equatorial regions receive more heat from the sun than the poles, causing warm air to rise and then flow to higher latitudes, where it falls to flow back towards the equator, forming a huge convection cell on both sides. These are known as Hadley cells. Water vapor in the rising air condenses as the air cools at higher altitudes and can form towering cumulonimbus clouds that produce thunderstorms. The air generally drops about 30 degrees north and south of the equator, by which time it has lost most of its moisture; as a result, these regions are usually arid and contain some of the largest deserts in the world. The subsequent movement of air towards the equator is responsible for the trade winds.
Heat from the Earth’s core keeps the hot, fluid rock circulating in the upper mantle, forming convection cells under the crust. The resulting motion of the molten or semi-molten rock drives the process known as plate tectonics which is responsible for the splitting of the crust into continental “plates” that move relative to each other. This phenomenon is responsible for earthquakes and volcanic activity. Areas of the Earth’s surface directly above a convection cell can split and drift apart, forming new plates, as in the African Rift Valley. An existing plate, driven by underlying convective currents, can push into another plate, building mountain ranges such as the Himalayas.
Convection cells also exist in the sun. Images of the sun’s surface reveal a granular structure made up of bright, hot areas surrounded by darker, cooler boundaries. Each granule indicates the top of a convection cell formed by plasma which is heated from below and rises to the surface, then cools down, widening and descending to the boundary.
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