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Hot air balloons need less heat to fly when the air surrounding them is very cold. This is because hot air balloons fly using buoyancy. Buoyancy is the upward force exerted by gases and other fluids. Hot air is less dense than cold air and so hot air will be buoyant when surrounded by cold air. Hot air balloons can only fly when the air inside the balloon is much hotter than the air outside it. When the air outside is very cold, then the air inside the balloon doesn't have to be as hot to lift it up!