Illustration that depict how a hot-air balloon takes off and lands. The images are of particles in motion, a balloon valve, and hot-air balloons being launched and in the air. Particle Pressure: Particles in hot air move faster and are farther apart than those in cold air, so hot air is less dense than cold air. With lower density, heated air rises over colder air. Balloon Valve: The valve helps to control the balloon’s altitude. When the valve is opened, some hot air is released from the top of the balloon. The hot air is replaced by cold air flowing into the base of the balloon. This exchange of cold air for hot air increases the mass of the balloon, causing it to descend until the pilot closes the valve. Launching and piloting a hot-air balloon involves:

1. The crew uses fans to fill the envelope with cold air.
2. Gas burners are switched on to heat the air and inflate the balloon, which begins to rise.
3. Once up, the pilot maintains altitude by occasionally turning on the burner for a few seconds.
4. To lose altitude, the pilot opens a valve at the top of the envelope to let out some hot air.
5. The pilot selects a suitable place to land before opening the valve all the way.
6. The balloon slowly falls to the ground and collapses.
7. The crew gathers up the envelope.