As air is heated, it expands and becomes less dense than the air around it. As a result, it will rise. The greater the temperature differential between the heated air and the ambient (surrounding) air, the faster and/or higher the heated air will rise. This basic physical fact is the principal on which the lifting force of a hot-air balloon depends. The greater the temperature differential between the interior envelope air and the surrounding air, the greater the lifting capacity of the balloon.

Heated air provides the lift, and air currents provide propulsion. While a skilled pilot can control his altitude very precisely, only indirect control of the balloon direction is possible. The balloon will only travel at the speed and direction of the air around it. Fortunately, the wind will move in different directions at different altitudes, and the pilot can select his direction of travel by selecting an appropriate elevation. When the balloon is climbing, enroute to a specific altitude, the pilot can flare or "round out" to the selected altitude by gradually reducing the heat (and subsequently the rate of climb) as the balloon approaches the altitude.

Hot air is not just warm; the temperature at the crown of an ascending balloon is about 1OOC- the boiling point of water. To generate this heat, burners produce several million BTUs per hour. This is pure energy: none of it is absorbed by moving parts as is an engine's power. If the output of a 1O million BTU burner is converted to horsepower a staggering figure of over 4,000 bhp is achieved.