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Blue Sky Science: How does a hot air balloon work?

Rohan Kommuri

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How does a hot air balloon work?

Directionally you never know where a balloon is going to go because the wind controls it, I don’t. I can send a balloon higher or lower, but not steer it in specific directions. So before we send passengers up in a hot air balloon, we send up a small hydrogen balloon that tells us the wind speed and direction.

If we’ve determined the conditions are safe for flying, we start to inflate the balloon using large fans. We first fill it with air that is the same temperature as outside, and once it’s mostly inflated we light a burner to heat the air. As the balloon is filled and heated, it will move from a horizontal position, lying on the ground, to a vertical one, standing straight up.
A balloon flies because of the difference between the inside air temperature and the outside air temperature. The hotter it is outside, the hotter I have to heat the balloon to make it fly.

Hotter air rises, so once we lift off I just add more heat to fly higher in the sky. If we want to go down or land, I use the burner less often and as the temperature of the balloon cools, it descends.

Spring, summer, and fall we are allowed to fly from sunrise to two hours after sunrise and from two hours before sunset up to sunset. So you’d only see balloons flying either early in the morning or later in the afternoon close to sunset.

This is for the safety of the balloon pilot and passengers. Once the sun has been up for two hours and is high in the sky, it’s starting to heat the ground and pavement and develops thermal currents. These currents are sending up more heat from the ground and can be unpredictable and dangerous to hot air balloons.

About Blue Sky Science

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research. The questions are primarily posed by visitors attending Discovery Building events.

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