Ballooning, use of lighter-than-air craft known as balloons. Balloons consist of a large flexible bag containing either hot air or a gas that is lighter than air. The bag, known as an envelope, is made of varnished silk, rubber, or other suitable material. Piloted balloons carry one or more persons in a suspended gondola; unpiloted balloons are usually used for scientific research and carry instruments to measure and record a variety of physical phenomena.
Any gas that is lighter than air can be used to lift a balloon. Hydrogen, helium, methane, ammonia, natural gas, manufactured gas (gas made from soft coal or petroleum products), and heated air can and have been used to fly balloons. The earliest balloons were filled with hot air and often carried a brazier (metal container for burning coal or charcoal) to continuously heat the air. Modern balloons are usually filled with hydrogen or helium or air heated by a small gas burner. Helium has the great advantage of being nonflammable (difficult to burn or ignite), unlike hydrogen, which is flammable.
Balloons are used primarily for two purposes: sport or scientific research. Sport balloons mostly use hot air. They range from 10 to 20 m (33 to 66 ft) in size when the envelope or bag is inflated. Scientific balloons generally use hydrogen, helium, methane, or ammonia. They range in size from 30 to 200 m (100 to 660 ft) when fully inflated.