Convection is the transportation of heat within a gas or fluid by the circulatory motion that occurs at a non-uniform temperature owing to the variation of their density and the action of gravity.

The law of gravity acts so that hot air rises because it is less dense than cold air, for example, a hot air balloon. Convection currents produce ventilation, which is known as passive ventilation because only heat and space are required for air movement to occur.

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