The transfer of heat from a warm object to a cooler object through a fluid (or gas). Convection occurs when objects with different temperatures are in contact with a fluid (or gas). The fluid in contact with the warm object gains energy (through heat conduction) and usually expands. Because it is less dense than the surrounding fluid, it rises, carrying the energy it has gained with it. If the fluid encounters a cool object, energy from the fluid will be transferred. The fluid will become more dense and sink. See also heat conduction.

con-veck-shun A flow of liquid or gas caused by part of it being heated or cooled more than the rest.

the circulation of air or a liquid due to heating. As air is heated in one place, its particles speed up, pushing each other farther apart. Since now there are fewer air particles in the same space, warm air is less dense than cold air thus it rises. The cooler air moves in to replace the rising warm air. As the warm air rises, it cools because less heat is trapped higher up in the atmosphere. Eventually , this rising air begins to fall back down to the earth's surface and the cycle continues. This cycle sets up a convection current.