Science Behind Hot Air Balloon Buoyancy (Video)

This cool time-lapse sequence was captured at the 41st annual International Balloon Fiesta in Albuquerque, N.M., which is the largest hot air balloon festival in the world. The science behind what makes this colorful spectacle possible is fascinating.

Since hot air is lighter and less dense than the cool air around the balloon, the heated air from a burner causes the whole balloon to rise. The gas burner is heated to about 212 degrees Fahrenheit (100 degrees Celsius) and typically fuelled by propane gas. When the air inside the balloon structure, called the envelope, cools down, or when the hot air is let out, the balloon goes down. By letting out the air or firing the burner, a hot-air balloon pilot can decrease or increase the altitude.

The hot air won't escape from the hole at the bottom of the balloon because buoyancy, the tendency to remain afloat in a gas, keeps it moving up.

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Nina Sen is a frequent contributor to Live Science’s Life’s Little Mysteries series: an exploration and explanation of our world’s phenomena, both natural and man-made. She also writes astronomy photo stories for Live Science's sister site Space.com.