Blue Sky Science: How do hot air balloons work?
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Blue Sky Science: How do hot air balloons work?


Q How do hot air balloons work?

Rohan Kommuri, Madison, Wis.

A Jerome Teed, hot air balloon pilot and owner of Gentle Breezes LLC:

Directionally, you never know where a balloon is going to go because the wind controls it. A balloon operator can send a balloon higher or lower, but not steer it in specific directions. So before passengers are sent up in a hot air balloon, an operator sends up a small hydrogen balloon that tells the wind speed and direction.

If conditions are determined to be safe for flying, the operator starts to inflate the balloon using large fans. It is first filled with air that is the same temperature as outside, and once it’s mostly inflated, a burner is lighted 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 an operator must heat the balloon to make it fly.

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

In the spring, summer and fall, balloons 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 developing thermal currents. These currents are sending up more heat from the ground and can be unpredictable and dangerous to hot air balloons.

In the winter when the ground is covered in snow, these thermals don’t develop, so you could fly a balloon at any time of the day.

Blue Sky Science is a collaboration of the Wisconsin State Journal and the Morgridge Institute for Research.


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