How do hot air balloons fly?

A big flame warms the air inside. The balloon rises because warm air floats up. It lifts the basket into the sky. To come down, the air cools.

A hot air balloon has a big bag with air inside and a basket below. A burner makes a flame that heats the air in the bag. Warm air is lighter than the cooler air around it, so the big bag gets pushed upward, a bit like a beach ball popping up in water. The pilot adds heat to go up and lets air cool to come down.

The burner heats the air inside the balloon’s envelope, which is the big fabric bag. Heated air spreads out, so the air inside weighs less than the same amount of cooler outside air. The cooler air pushes up, making lift; this is called buoyancy. To rise, the pilot adds heat. To descend, the pilot lets air cool or lets some hot air out. Fun fact: the first hot-air balloon passengers were a sheep, a duck, and a rooster in 1783.