A hot air balloon flies for one reason: the air inside the envelope is hotter, and therefore lighter, than the air outside it. The cool, denser air around the balloon sinks and shoves the lighter envelope upward, the same way water shoves a submerged cork to the surface. Archimedes worked out the rule more than 2,200 years ago: anything immersed in a fluid is pushed up with a force equal to the weight of the fluid it displaces. A balloon is a ship, and the ocean it floats in is the atmosphere.
Warm air is thin air
Heat air and its molecules speed up, spread out and take up more room. Since the envelope is open at the bottom, the surplus simply spills out, and the air that remains inside weighs less. At everyday temperatures a cubic metre of air weighs about 1.2 kilograms; heat it to around 100 °C and about a fifth of that mass has left the envelope.
The weight of the air that left is your lifting budget. Everything the balloon carries, fabric, basket, fuel, pilot and you, has to weigh less than that missing air.
The maths of a four-passenger flight
Here is the only sum you need. Air heated to about 100 °C lifts roughly 250 to 300 grams per cubic metre, depending on the day. Now add up a typical load: four passengers and a pilot (around 400 kg), basket, burners and full fuel tanks (perhaps 250 kg), plus some 200 kg of envelope fabric and tape. Call it 850 to 900 kg. At 270 grams per cubic metre you need well over 3,000 cubic metres of heated air, which is exactly why four-seat ride balloons run 3,000 to 4,000 cubic metres. Double the payload and you need roughly double the envelope.
Why cold mornings beat hot afternoons
Lift comes from the temperature gap between inside and outside, and only one side of that gap is adjustable. The envelope has a hard ceiling: pilots keep the fabric below about 120 °C, because hot nylon ages fast (see the life of a balloon envelope).
The outside air is another story. On a 0 °C winter dawn, a 100 °C envelope enjoys a gap of a full hundred degrees and every cubic metre pulls hard. On a 35 °C afternoon the same envelope buys far less lift, so the balloon must carry fewer people or less fuel. That, along with calmer air, is why flights leave at first light: more in the best time to fly.
How high can this go?
Most ride flights cruise between 500 and 2,000 feet above the ground, low enough to hear dogs bark and smell cut grass. The physics does not stop there, but the air thins with altitude, so each cubic metre of envelope lifts less, and a balloon basket is open to the sky, with no pressurization, no heating and no oxygen supply. Pilots stay low because that is where the air is rich and the view is best.
The outer limit is remarkable all the same. In 2005, Indian aviator Vijaypat Singhania rode a pressurized capsule under an enormous envelope to 21,027 metres, about 21 kilometres up, a hot air balloon altitude record that still stands. His story and others live on our records and adventures page.
Fly when the physics helps you
Cold, calm and clear beats hot and gusty every time. Find the season and the hour that give your flight the most lift and the smoothest air.
Balloon lift FAQ
How hot is the air inside a hot air balloon?
Typically between 90 and 110 °C in cruise, adjusted burn by burn. Pilots keep the envelope below roughly 120 °C to protect the nylon, so on hot days a balloon lifts less and carries lighter loads.
Why can't a balloon just keep climbing forever?
Air gets thinner with altitude, so each cubic metre of envelope displaces less weight and generates less lift, while the envelope temperature stays capped. At some altitude lift equals weight and the climb stops on its own.
Do passengers need oxygen in a hot air balloon?
Not on a normal ride. Sightseeing flights stay a few hundred to a couple of thousand feet above the ground, far below the altitudes where supplemental oxygen becomes necessary. Only record attempts and mountain crossings carry oxygen or pressurized capsules.
Does a flying balloon weigh less than a parked one?
No. The balloon, its hot air and its passengers weigh exactly the same in flight. It rises because the cooler air it displaces weighs more than the whole package, so the atmosphere pushes it up, precisely as water floats a ship.