The principle of convection says that warm air always rises, and cool air always falls. Actually the principle extends to any fluid, but that’s a subject for another day. For now, let’s talk about air.
When air warms up, it expands. As it expands, it becomes less dense in relation to the cooler air around it. And because it’s less dense, it tends to “float,” that is, to rise. The most dramatic example of this phenomenon is a hot air balloon, which works because the burner heats the air up inside the balloon so that it becomes so light it floats in relation to the other air of the atmosphere. It is so light, in fact, that it floats even though it’s carrying a basket and one or more passengers with it.
When air eventually cools in relation to the air around it, the some principle says that it will fall, and the cycle will repeat itself.
Designing buildings that work is all about using the principles of convection so we’re not fighting against them but rather are using them to our advantage.