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Friday, June 22, 2007

How Hot Air Balloons Work

For practical purposes, hot air balloons really can't compete with airplanes and helicopters. They're extremely difficult to steer, and they can only move as quickly as the surrounding winds. But as a recreational activity, hot air ballooning can't be beat. You're outside, feeling the wind in your hair and the sun on your face, drifting serenely over the earth. If you've ever been in a hot air balloon, you know it's an experience you won't soon forget.

Hot air is less dense, and lighter, than cold air. Because it's lighter, it tends to rise. If you've ever taken a dive into a pool, you may have felt this effect in water: the surface water is warm, but as you move deeper, the water gets cooler. That's because warm water is lighter than cool water and tends to rise to the top, like air.

The air within a hot air balloon is heated, and as it rises, it takes the balloon—and anything attached to it—with it. A cubic foot of air within a balloon can lift about seven grams of weight. This isn't a lot of weight, which is why the balloon has to be so big.

The balloon itself is called an "envelope." It's usually made of nylon, which is the perfect fabric for hot air balloons: it's lightweight but sturdy, and it can withstand heat without melting easily.

Below the balloon, a burner is positioned to send a jet of flame upward into the envelope. The burner uses propane as a fuel. The fuel is stored as a highly compressed liquid in canisters attached to the burner with a fuel hose. Because it's so highly compressed, the liquid fuel flows quickly through the hose to the burner when the pilot turns it on. There, it's ignited by a pilot light. The flame heats up the surrounding metal of the burner, which turns the fuel to gas before it gets lit. This makes the flame stronger and the fuel usage more efficient.

If you look at a hot air balloon, you'll notice a big hole in the bottom where the burner is. So why doesn't the hot air escape through the hole at the lower end? Simple: buoyancy. Hot air is buoyant, which means it can only rise—it can't sink and drain out the bottom of the balloon. As long as the air stays hot, it will continue to rise.

It won't rise forever, though. The atmosphere thins as it goes up, and eventually the air within the envelope is too thin to support the weight of the balloon. The more air within the balloon, the greater the buoyancy—so bigger balloons can go higher than smaller ones.

A pilot can cause the balloon to rise by turning on the propane burner and heating the air within the envelope. To bring the balloon down, the pilot releases the parachute valve. That's a vent in the top of the envelope that allows some of the hot air to escape, cooling the temperature of the remaining air within. This causes the balloon to sink gradually.

There are no horizontal controls in a balloon, but pilots can still control the horizontal movement by raising and lowering the altitude of the craft. Winds at different altitudes move in different directions. If a pilot has a good working knowledge of the wind currents in the area, she can raise and lower the balloon to catch a current moving in the right direction.

Still, piloting a balloon isn't an exact science. It's impossible to target the precise location where you'll land. That's why it's usually necessary for someone on the ground to follow the balloon by car to meet the balloon where it lands and transport the equipment.

Launching and landing generally take more work than actually flying a balloon. The inflation process takes only about ten or fifteen minutes, and is done with a powerful fan. Once the balloon is full of air, it will still lie on the ground until the burner is fired, heating the air in the envelope and causing it to rise.

A balloon descends gradually, but can still land with a bit of a bump if the pilot isn't experienced enough to know how to bump the basket along the ground to lessen the impact. The wicker basket helps absorb the force of the landing. Wicker is lightweight and flexible, and absorbs the energy of landing better than a solid, less flexible basket would. This way, riders aren't jarred when the ride is over.

There's nothing like soaring over the earth in a balloon. Hot air balloons are exciting, fascinating, and pure fun. If you've never taken a ride in one, you're definitely missing out on an experience you'll never forget.

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