Why Would Heating the Gas in an Air Balloon Make the Balloon Rise?

Heating the gas in an air balloon makes the balloon rise because the gas in the balloon becomes lighter than natural air. The heated gas has less mass per unit of volume than that of the cool air surrounding the balloon.

A hot air balloon consists of three main parts: the envelope, the burner and the basket. The burner, which is usually fueled by liquid propane, is located below the envelope; it is responsible for heating the air in the envelope. The basket holds the pilot, the passengers and the propane tanks.

To move the balloon upwards, the pilot firsts opens the propane valve to release the liquid propane into the envelope. The mixture of propane and natural air ignites under the envelope, heating the air in the envelope. The balloon then starts rising. The pilot must keep reheating the air for the balloon to stay airborne.

To ascend higher, the pilot opens the valve for more propane to be released. To lower the balloon, the pilot gradually releases the hot air from the envelope through the parachute valve at the top of the balloon.

A hot air balloon doesn't land in a predetermined location. The pilot lands it where there is a safe, open space to land.