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How does a hot air balloon fly?

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Quick answer

A hot air balloon flies because the air inside its envelope is heated, making it less dense than the surrounding cooler air. This density difference creates an upward buoyant force. When that force is greater than the weight of the balloon, basket, and passengers, the balloon rises. Cooling the air or venting it makes the balloon descend.

A hot air balloon flies because heating the air inside its envelope makes that air less dense than the cooler air outside, so the balloon rises. Pilots control altitude by burning propane to heat the air or letting it cool.

Step by step

  1. The pilot opens the propane burner, which shoots a flame into the balloon's fabric envelope.
  2. The flame heats the air inside the envelope, causing the air molecules to move faster and spread apart.
  3. Because the heated air expands, the same volume of air inside the envelope now weighs less than the cooler air outside.
  4. The surrounding cooler, denser air pushes upward on the less dense hot air, creating a buoyant force.
  5. When the buoyant force exceeds the total weight of the balloon, basket, and passengers, the balloon rises.
  6. To descend, the pilot lets the air cool or pulls a cord to open a vent at the top, releasing hot air.

Key things to understand

  1. A propane burner heats the air trapped inside the balloon's fabric envelope.
  2. Heated air expands, so the same volume of air contains fewer molecules and becomes less dense.
  3. The less dense hot air inside creates an upward buoyant force greater than the weight of the balloon.
  4. When the buoyant force exceeds the total weight, the balloon accelerates upward.
  5. Venting hot air or letting it cool reduces buoyancy, causing the balloon to descend.

Labelled diagram

  1. Burner

    Produces a flame that heats the air inside the envelope.

  2. Envelope

    The large fabric bag that traps the heated air.

  3. Hot air

    Less dense than outside air, so it creates upward buoyancy.

  4. Cool air

    Denser surrounding air pushes up on the hot air inside.

  5. Basket

    Carries passengers and equipment; its weight is lifted by buoyancy.

How a hot air balloon flies

Real-world examples

  • The Montgolfier brothers' first manned flight in 1783 near Paris used a paper and fabric balloon heated by a fire.
  • Modern recreational balloons often carry two to four passengers in a wicker basket and fly for about an hour.
  • In Albuquerque, New Mexico, the annual International Balloon Fiesta fills the sky with hundreds of hot air balloons at sunrise.
  • In 1987, Richard Branson and Per Lindstrand made the first crossing of the Atlantic in a hot air balloon.

Common mistake

✗ Myth: Hot air balloons fly because the hot air pushes against the ground.

✓ Fact: The balloon rises because the hot air inside is less dense than the cooler air outside. The surrounding cooler air, being denser and heavier, sinks and pushes the lighter hot air upward, creating buoyancy.

People also ask

What makes a hot air balloon rise?
Heating the air inside the balloon's envelope makes it less dense than the surrounding cooler air. This density difference creates an upward buoyant force. When that force is greater than the total weight of the balloon, basket, and passengers, the balloon rises. The pilot controls ascent by burning propane to heat the air.
How do you steer a hot air balloon?
Pilots cannot steer directly because balloons have no engines or rudders. Instead, they change altitude to catch different wind directions. Winds at different heights often blow in different directions, so by rising or descending, the pilot can navigate. This is why balloon flights are carefully planned using weather forecasts.
Why do hot air balloons only fly at certain times?
Hot air balloons need calm, stable air to fly safely. Early morning and late evening usually have the lightest winds and most stable conditions. During the day, the sun heats the ground unevenly, creating thermals and gusty winds that make ballooning dangerous. Rain and thunderstorms also ground balloons because they can damage the envelope.
Why does hot air rise?
Hot air rises because heating makes air molecules move faster and spread apart, so the same volume of air weighs less. The surrounding cooler, denser air sinks under gravity and pushes the lighter hot air upward, creating an upward buoyant force.
How do pilots control the altitude of a hot air balloon?
Pilots burn propane to heat the air inside the envelope and rise, or they open a vent at the top to release hot air and descend. They also let the air cool naturally. By adjusting the temperature, they change the balloon's density and therefore its buoyancy.
Can a hot air balloon fly in any weather?
No. Hot air balloons need calm, stable air, so they usually fly in the early morning or late evening when winds are light. Strong winds, thunderstorms, and rain make flying dangerous because they can damage the envelope or make the balloon uncontrollable.
What keeps the balloon from spinning or tipping over?
The basket hangs below the envelope, so the heavy basket acts like a pendulum and keeps the balloon stable. The pilot can also use the burner to create a slight rotation, but the basic design naturally resists tipping because the center of gravity is low.

Part of the guide: Everyday Machines →