A hot-air balloon floats because the warm air inside the balloon is lighter that the cooler air outside it. On an average day with an average load, the air inside the envelope needs to be one hundred degrees hotter that the air around it. The first manned flight of a hot-air balloon took place two-hundred years ago near Paris, France. The trip covered five miles and lasted twenty-five minutes. The heat came from burning straw and wood. Now we have propane tanks that provide fuel for the burners. This way is much more efficiant. The skirt's job is to protect the burners from strong winds. Balloonists also benefit from an instrument panel in the gondola, which measures altitude, rate of climb, and envelope air temperature.