How does a balloon work?
Hot air balloons fly because the air inside of the balloon is hotter than the outside air. By using propane-fueled burners, the air is heated, and the balloon ascends. To descend, the air inside the balloon must be cooled by either letting the envelope cool on its own or by opening the vent at the top of the envelope to let some of the hot air out. Throughout a flight, the pilot will use a combination of heating and cooling the balloon to ascend and descend. And when it is time to land, the top of the envelope is opened completely to let all of the air out and deflate the balloon.
