Convection

CONVECTION (from Latin convectio—carriage, transport), the movement of any property associated with the movement of the substrate itself. This name is most commonly used to denote the transfer of heat caused by the movement of heated matter (liquid or gas). A liquid heated from below becomes lighter and rises into the remaining, still cold mass. Conversely, a liquid cooled from above forms descending currents. In the case of water, the phenomenon is complicated by the fact that water has its greatest density at 4°. Thus, in the interval from 0° to 4°, phenomena in water proceed in a reversed manner. Particularly important are those closed vortices that arise when two adjacent columns of liquid (or gas) have a stable temperature difference. Then an upward current forms in the warm column, supplemented by the downward current of the cold column until a complete circulation is achieved. Such a stable circular convection is used in some of the simplest types of water heating and ventilation. Convection plays a major role in the regime of deep water basins; even more important is its role as a primary factor determining air circulation between areas of lower and higher temperature. This is the origin of breezes, monsoons, and trade winds. Convection is often complicated by the intervention of other factors that can affect the density of the liquid. For example, warm salt water of tropical currents may be heavier than cold fresh water of polar currents, which determines the nature of their interlayering. Skillful use of convection helps in combating waves of poisonous gases. In addition to thermal convection, the term "convection" is applied to electrical phenomena. This name is given to the current formed by the movement of charged bodies. Thus, a rapidly rotating charged disk creates something like a circular current. In this sense, the current in electrolytes is sometimes called convection current, since it is realized there through the movement of charged particles—ions.