Riding on Air
Warm air is less dense than cold air. So if enough warm air is confined to a lightweight container, the container can rise through the surrounding colder air. This is the principle that allows a hot-air balloon to get off and stay off the ground.
When air is heated, the particles in the air gain energy and move faster on average. The particles also move farther apart, so a given volume of hot air contains fewer particles and has less mass than the same volume of cold air. This difference in density produces an upward force. In a hot-air balloon, the force is very small, equivalent to lifting about one tenth of a gram for each liter of air. A large volume of air is needed to support the mass of the balloon and any passengers. That is why hot-air balloons need to be large.
To heat the air, the pilot burns propane gas, which is stored under pressure in tanks. The bottom of the balloon's envelope (the skirt) is treated so that it is not flammable.
As the balloon nears the chosen altitude, the pilot turns the burner off so that the balloon will stop rising. The pilot maintains the altitude of the balloon by turning the burner on and off and by opening a valve at the top of the balloon to let hot air escape.
The horizontal movement of the balloon is much harder to control. The wind speed and wind direction vary at different altitudes. The pilot uses the burner and the valve to change the altitude of the balloon and take advantage of favorable winds. A hot-air balloon cannot land at the same spot from which it took off. The ground crew must drive to the landing site to collect the balloon and the passengers.
Taking Off and Landing
Launching and piloting a hot-air balloon is an activity that takes skill and patience.