From here: (bold mine)
Our perception in daily life is that heat rises up, but science tells us that this isn't the case. So how does get heat get transferred?
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Convection occurs when you take a heat source and put it in a movable medium, like air or water.
For example, if you put air near a hot object, the moving molecules of the hot object will pass heat energy to the air via conduction. But if that movable medium starts to absorb heat energy and its molecules start to move around, they will take up a little more space. That’s because the molecules move and bounce into other molecules and push them away. The effect ripples across the medium—in this example, air—and the result is that the air becomes less dense.
If a medium like air or water becomes less dense, then it rises through the medium. This is basically why air-filled objects like pool toys float on water. They’re less dense.
Colloquially, it can be said that in the case of convection, heat rises, but what really happens is that
the initial actual transfer of heat is caused by conduction. (Image: Semmick Photo/Shutterstock)
So in the case of, say, a room with a wood stove in it, what happens is that the air near the surface of the wood stove absorbs heat through conduction. The air then becomes less dense and rises through the remaining air and heads towards the ceiling. The colder ceiling encounters the warmer air and some of the moving molecules of the air cause the ceiling molecules to move, and thus the ceiling warms.
So, though colloquially it can be said that in the case of convection, heat rises, what really happens is that the air is heated by conduction.
The exchange of heat energy causes the air molecules to move faster, which causes them to move apart from one another and reduce the density. Colder and denser air falls to the bottom, which has the effect of causing the hotter and less dense air to rise, and the heat is carried away. But the initial actual transfer of heat to the air was caused by conduction.
Convection is a secondary process, which does, of course, carry heat from one place in a room to another.
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