Why Does Hot Air Lift a Balloon?

Step 1 of 5

Warm air spreads out.

Heat makes air molecules move faster and shove each other further apart, so the same air takes up more room than it did when it was cold.

Step 2 of 5

The bag cannot grow, so air leaves.

The balloon is already full and open at the bottom. When the air inside spreads out, the surplus simply spills out of the mouth and is gone.

Step 3 of 5

Now it holds fewer molecules than the same space outside.

The balloon's volume has not changed, but there is less air inside it than there would be if it were full of cold air. That difference is the entire lift.

Step 4 of 5

The burner is the only control you have.

There is no throttle and no rudder. Firing the burner makes it climb, letting the air cool makes it sink, and a vent at the top dumps hot air quickly.

Step 5 of 5

You steer by choosing a height.

The wind blows in different directions at different altitudes, so a pilot climbs or descends until they find a layer heading where they want to go.

A typical balloon holds about 2,800 cubic metres of air, and heating it to 100°C makes the whole thing roughly a quarter of a tonne lighter than the air it sits in.