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The process of convection is the main means of heat transfer within a fluid that is exposed to a heat source. Usually, the fluid, which can be a liquid or a gas, is heated from below by a warm surface; the increase in temperature results in a decrease in density, causing the fluid to rise and cooler fluid to flow inward to replace it. As it rises, it loses heat to its surroundings, becoming denser and heavier than the fluid below. It cannot descend through this rising fluid, so it spreads horizontally before falling back toward the surface and being drawn toward its starting point by the rising fluid. This system is known as a convection cell and is a feature of fluid dynamics that can be observed in a variety of situations, from water that is heated in a pot to processes on a planetary or stellar scale.
The Earth’s atmosphere features convection cells on a vast scale: the equatorial regions receive more heat from the sun than the poles, causing warm air to rise then flow toward higher latitudes, where it descends to flow back toward the equator, forming a huge convection cell on either side. 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 descends 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 world’s great deserts. The subsequent movement of air back toward the equator is responsible for the trade winds.
Heat from the Earth’s core maintains the circulation of hot, fluid rock in the upper mantle, forming convection cells below the crust. The resulting movement of molten or semi-molten rock drives the process known as plate tectonics that is responsible for splitting the crust into continental “plates” that move relative to one another. This phenomenon is responsible for earthquakes and volcanic activity. Areas of the Earth’s surface that sit directly above a convection cell can split and move apart, forming new plates, such as in Africa’s Rift Valley. An existing plate, propelled by convection currents below, 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 consisting of bright, hot areas surrounded by darker, cooler boundaries. Each granule indicates the top of a convection cell formed by plasma that is heated from below and rises to the surface, cooling then spreading out and descending again at the boundary.