There's nothing quite like the sight of hot air balloons drifting over a valley at sunrise. They rise slowly and silently, except for the occasional roar of a burner. No wings, no engine, no propeller. So what keeps a basket full of people floating thousands of feet above the ground?
The short answer is that hot air is lighter than cold air. When the air inside the balloon is heated, it becomes less dense than the cooler air outside. The surrounding air pushes up on the balloon with a buoyant force, the same force that makes a rubber duck float in the bathtub. If that push is greater than the weight of the balloon, basket, and passengers, up it goes.
Archimedes in the Sky
The idea goes back to Archimedes' principle, which describes the upward push on any object sitting in a fluid. We usually think of water, but air is a fluid too. A balloon floats in air much the way a boat floats in water.
The amount of lift depends mainly on the difference between the temperature inside the balloon and the temperature outside. Hotter air inside means more lift.
The numbers are impressive. A large balloon holding about 100,000 cubic feet of air, heated to about 210 degrees Fahrenheit, can lift roughly 1,600 pounds. The fabric sets a limit, though. Most nylon envelopes can only handle inside temperatures up to about 250 degrees.
How the Pieces Work
The big colorful bag is called the envelope, usually made of nylon. Below it hangs a wicker basket that carries the passengers and the burner, which typically burns liquid propane. The bottom of the envelope stays open, since the air inside is at about the same pressure as the air outside. The part closest to the flame is made of fire-resistant material.
To climb, the pilot fires the burner and heats the air. To come down, the pilot lets the air cool, turns down the flame, or opens a flap at the top to release hot air.
Steering With the Wind
Here's the surprising part: hot air balloons can't really be steered. Pilots get some control by changing altitude to catch winds blowing in different directions at different heights. It's a skillful game of riding the right breeze. In the Northern Hemisphere, winds tend to turn toward the east as you climb higher, something experienced pilots keep in mind. Real steering only arrived in the 1960s with thermal airships, hot-air blimps fitted with tail fins and a rudder.
From Paris to Around the World
The hot air balloon was the first successful technology to carry people into the sky. French brothers Joseph-Michel and Jacques-Étienne Montgolfier developed it in 1783. On November 21 that year, Jean-François Pilâtre de Rozier and the Marquis d'Arlandes made the first free flight with people aboard, over Paris. King Louis XVI had wanted condemned criminals to go first, but the two men asked for the honor themselves.
Balloons have gone far since then. In 2005, Vijaypat Singhania flew a hot air balloon to a record 68,986 feet over India. In 1999, Bertrand Piccard and Brian Jones made the first nonstop trip around the world by balloon.
So the next time you see one drifting overhead, wave hello. It's riding on one of the oldest and simplest ideas in science: warm air rises.
