• Hot air balloons fly on the principle that warm air (weighing less than cooler air) rises. The balloon (known as the envelope) captures that warmer air. With a large enough volume of warmer air inside the envelope compared to the surrounding air, the entire balloon begins to rise.  To keep the air warmer inside the envelope, the pilot activates a burner (fueled by propane) which propels a hot flame upward into the envelope.