Convection

CURRENTS MOVING IN fluids are convection. More specifically, fluids—liquids, gases, andrheids—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 [Page 283]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.

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