Convection is the transfer of heat by physically moving the molecules from one place to another. Convection takes place when a fluid, such as gas or a liquid, is heated and moved from one place to another. When warm air in a room rises and forces the cooler air down, convection is taking place. For example, air, when heated, expands and rises. If this air movement is created mechanically by a floor register, fan, or the
wind, it is called forced convection. When the sun heats the warm air and it rises, causing the cold air to settle to create a convection loop, it is termed free convection. Free convection also occurs when the sun, shining through a window, heats the air inside a building.