Heat energy is always on the move. It flows from a hotter object towards a cooler one until both are the same temperature. In liquids and gases, heat energy usually moves by convection. This means that the molecules nearest to the heat source begin to move more rapidly and spread apart, so that this area of the fluid is less dense. As the less dense part of the liquid or gas rises, denser parts sink to take its place. Convection, process by which heat is transferred by movement of a heated fluid such as air or water.
Natural convection results from the tendency of most fluids to expand when heated—i.e., to become less dense and to rise as a result of the increased buoyancy. Circulation caused by this effect accounts for the uniform heating of water in a kettle or air in a heated room: the heated molecules expand the space they move in through increased speed against one another, rise, and then cool and come closer together again, with increase in density and a resultant sinking.
Forced convection involves the transport of fluid by methods other than that resulting from variation of density with temperature. Movement of air by a fan or of water by a pump is examples of forced convection.
Atmospheric convection currents can be set up by local heating effects such as solar radiation (heating and rising) or contact with cold surface masses (cooling and sinking). Such convection currents primarily move vertically and account for many atmospheric phenomena, such as clouds and thunderstorms.
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