currents moving in fluids are convection. More specifically, fluids—liquids, gases, and rheids—undergo movements as convection currents. Convection plays a major role in heat transfers. In fluids, both movement of mass in the fluid, and the heat it contains, occurs in a random way, if Brownian motion occurs. However, in the process of advection, large motions develop in the fluid, which move not only its mass, but also the energy it contains. Convection means the transfer of mass and heat by diffusive and advective movements. Heat is the transfer of energy from one body to another. Heat is a cause of convection currents in fluids, because the fluid motion is initiated and continued by the energy transfers occurring in the fluid.

As a fluid is heated, it expands. If a part of the fluid is cooler than another part, then its density is greater, and it sinks in the fluid. However, the opposite occurs to a part of the fluid that is heated to an energy state that is greater than surrounding parts of the fluid. As a consequence, it acquires buoyancy and rises because gravity is causing the denser part(s) of the fluid to sink. The effect is called a convection current. The convection current allows the fluid to engage in convective heat transfers. Convection is a very common occurrence in nature. Convection currents cause movements in water, in the atmosphere, and in the mantle of the earth. It is familiar as rising warm air over a fire, or the steam rising off of a heated pan of water or soup. These familiar examples are very localized; however, in the atmosphere the same principles are at work on a grand scale.

The surface of the Earth is heated by solar energy. However, the sunlight that strikes the Earth does not do so in an even manner. Clouds block parts of the solar energy; it hits bright surfaces such as the ice caps and is reflected, or it hits dark areas and is absorbed so that the heating of the Earth's surface is very uneven. The energy that does hit is transferred to the atmosphere as radiant energy. The ground, trees, grasses, water, or other surfaces radiate energy, but very unevenly, so the air above these surfaces is heated unevenly. Local convection currents combine to form larger convection currents in the atmosphere over larger and larger areas. The results are winds or other atmospheric phenomena. As the heated air rises, its lighter density causes lower pressure at the surface from which it is rising to be lower than the surrounding, cooler, pressures. The denser air then moves in to fill the lower pressure area that now functions like a vacuum. The warmer air also displaces cooler air above it, so that as the warmer air rises, the cooler air sinks, contributing to convection currents and to heat transfer.

Very localized convection currents visible in desert areas are dust devils. These are thermal convection currents composed of rising hot air that has begun to spin, resembling a miniature tornado. All of the grand weather phenomena, such as thunderstorms, cyclones, hurricanes, and, ultimately global atmospheric circulation, are forged from convection. A convection cell is a single region of air that is alternately heated and cooled. Its convection currents can give rise to lateral movements that cause breezes, winds, and other weather phenomena.

The oceans are affected by the variable distribution of solar radiation they receive. Tropical waters receive tremendous amounts of direct sunlight. They warm in the open oceans and, in the summer months become quite warm, especially in relatively shallow areas such as the Gulf of Mexico. The warm water then moves toward the polar regions, and colder water moves toward the tropics in order to fill in the lower pressure areas that the warmer waters have created. However, convection can also occur without the involvement of heat. For example, a convention current that is not due to heat is the movement of air masses due to variation in the amount of water vapor they contain.

Buoyancy can be caused by variations in the density of substances apart from the application of heat. For example, ocean convention currents can be caused by variations in the salinity of ocean water. The ocean currents are powered by differences in the salinity of the ocean waters. The saline differences create thermohaline convection. The differences in the density of warm salty water and cooler, lower salinity water are such that the dense, warm salty waters sink, bringing up less dense, cooler water. At the same time, heat is transferred and transported.

Free convection is the natural heat convection found in nature. It is not the same as forced heat convection. In forced convection, the heat transfer is not due to natural buoyancy forces. Rather, it is the result of movements in fluid inserted by artificial means. Fans, pumps, convection ovens, and other forms of circulating hot air or fluid cause a disequilibrium in the system.

The resulting disequilibrium is applied to various tasks by the forced convection. The core of the Earth is heated under enormous pressures. Part of its heat is due to radioactive decay. Movements in the core are usually convection currents. The outer core has dense metals, mainly nickel and iron, that are a part of the Earth's magnetic field. As they move in a fluid way under enormous pressure, they give off electricity, which generates magnetic fields.

284 Cooperative Institute for Arctic Research

SEE ALSO: Cloud Feedback; Hurricanes and Typhoons; Sunlight; Weather.

BIBLIOGRApHY. E.M. Agee and T. Assai, eds., Cloud Dynamics (Springer-Verlag, 1982); Adrian Bejan, Convection Heat Transfer (John Wiley & Sons, 2004); B.M. Boubnov and G.S. Golitsyn, Convection in Rotating Fluids (Springer-Verlag, 1995); K.A. Emanuel, Atmospheric Convection (Oxford University Press, 1994); C.F. Kennel, Convection and Sub-storms: Paradigms of Magnetospheric Phenomenology (Oxford University Press, 1996).

Andrew J. Waskey Dalton State College

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