Hot air balloons fly because hot air is lighter than cold air. Heat the air inside a big enough envelope and the balloon rises; let it cool and the balloon sinks. A pilot can't steer a balloon, but by climbing and descending they can find winds blowing in different directions. Here's how it all works.
When air is heated, its molecules spread out, so a cubic metre of hot air weighs less than a cubic metre of cold air. At sea level, a cubic metre of air at 15°C weighs about 1.2kg. Heat it to around 100°C, a typical temperature inside a balloon, and it weighs about 0.95kg. That difference, roughly a quarter of a kilo per cubic metre, is the balloon's lift.
A quarter of a kilo isn't much, which is why balloons are so big. A passenger balloon can hold several thousand cubic metres of air, enough to lift the envelope, basket, burners, fuel, pilot and passengers. The bigger the balloon, the more it can carry.
Outside temperature matters too. On a cold morning, the gap between the hot air inside and the air outside is bigger, so the balloon has more lift. On a hot afternoon there's less, which is one reason balloons fly in the cool of early morning and evening.
The envelope is laid out on the ground and a large petrol fan blows cold air into it until it's partly full. The pilot then fires the burners into the opening to heat the air, and the balloon slowly rises upright, lifting the basket off its side. Passengers can often help hold the envelope open during this stage.
You can't, at least not directly. A balloon moves with the wind, so it goes wherever the air around it is going. But the wind often blows in slightly different directions at different heights. By climbing or descending, a skilled pilot can pick the layer of air that's heading the way they want to go. They also use this to find a safe field to land in.
Because the balloon moves at exactly the speed of the wind, passengers don't feel any breeze in the basket. Even when you're travelling at 10mph, it feels completely still, which is why balloon flights are so peaceful.
Towards the end of the flight, the pilot looks for a suitable open field, away from livestock, crops and power lines. They let the balloon cool so it descends gently, then open the parachute valve as the basket touches down. The ground crew, who have followed by road, help pack the balloon away and drive everyone back to the launch site.
The best way to understand how a balloon flies is to go up in one. Read about the best time of year to fly, see our launch sites across England, or find answers in our hot air balloon FAQs.
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