From Flat to Fabulous: Inflating a Hot Air Balloon Explained The science behind hot air balloon flight comes down to air density and buoyancy. When the envelope is laid out on its side, cold air is first blown in to give it shape, because the fabric is too limp to stand on its own. Once the pilot heats the air with the propane burner, the air inside the envelope becomes less dense than the cooler air outside. This density difference creates lift, allowing the balloon to rise. The envelope itself