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How does a hot air balloon fly?
Hot air balloons fly when the air inside the hot air balloon is less dense than the air surrounding it. Hot air is less dense than cool air; the heated air causes the balloon to rise simply because it is lighter than an equal volume of cold air. Buoyancy is an upward force that the air exerts, and it helps hot-air balloons and blimps stay in the air.
A hot air balloon uses a burner to heat up the air inside the balloon. The burner is usually fueled by liquid propane. The burner mixes the fuel with air and ignites the mixture, which creates a flame directly underneath the bottom opening of the hot air balloon. Once the air inside the balloon begins to get warm, the balloon will rise. The pilot controls the balloon’s altitude by releasing hot air from a vent at the top of the balloon, releasing the warmer air causes the balloon to descend.