What makes heat rise?

What makes heat rise? — BN, Burlington, MA

Heat itself doesn’t rise—it’s a form of energy, not an object. But heated fluids often do rise. That’s because raising the temperature of a fluid usually causes that fluid to expand so that its density drops. Whenever a region of less density fluid is surrounded by more dense fluid, the less dense region experiences a net upward force. This result is a consequence of Archimedes’ principle that less dense materials float in more dense liquid. With a net force pushing it upward, the heated region floats upward and we say “heat rises.”

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