Energy transfer by convection is primarily restricted to

Convective heat transfer occurs only in fluids, through the movement of groups of molecules that make up the medium.

Being this molecular movement, related to the properties of the fluid that change with temperature.

This means that convection occurs when the hottest parts of a fluid ascend to the coldest zones, generating a continuous circulation of the fluid (better known as convective current) transmitting heat to the cold zones by decreasing the density caused by the increase in temperature.

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