Question

Why hotter air is lighter than cool air

Answer 1

Hot air rises because when you heatair (or any other gas for that matter), it expands. When the air expands, it becomes less dense than the airaround it. The less dense hot air then floats in the more dense cold air much like wood floats on water because wood is less dense than water.

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Answer 2

actually it's lighter because it's less dense than cold air

as it is less dense it rises above cold air