Convection is heat transfer by mass motion of a fluid such as air or
water when the heated fluid is caused to move away from the source of
heat, carrying energy with it. Convection above a hot surface occurs
because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing
convection currents which transport energy. Convection is thought to play a major role in transporting energy from the center of the Sun to the surface, and in movements of the hot magma beneath the surface of the earth.
It is difficult to quantify the effects of convection since it inherently depends upon small nonuniformities in an otherwise fairly homogeneous medium. In modeling things like the cooling of the human body, we usually just lump it in with conduction.
Heat transfer concepts
Heat transfer examples