A hot air balloon floats because the air inside it is lighter than the air outside it. When the burner heats the air trapped inside the envelope (the big fabric shell), that air expands and becomes less dense. Since the balloon now weighs less than the cooler air surrounding it, it rises, just like a bubble rising through water. The entire system relies on one elegant principle: hot air is lighter than cold air.
Buoyancy: The Force That Does the Lifting
The physics behind a hot air balloon is the same principle Archimedes described over 2,000 years ago. Any object surrounded by a fluid (and air counts as a fluid) experiences an upward push equal to the weight of the fluid it displaces. A hot air balloon displaces a huge volume of outside air. If the heated air inside the envelope weighs less than that displaced outside air, the difference creates an upward force called buoyancy.
Think of it this way: the envelope holds roughly 77,000 cubic feet of air in a typical passenger balloon. At ground level, cool air at that volume weighs quite a lot. Heat that air to around 100°C (212°F) above the surrounding temperature and its density drops significantly. The weight difference between the lighter inside air and the heavier outside air is what lifts the basket, passengers, fuel tanks, and all the equipment off the ground. If the total weight of the balloon system is less than the weight of the air it pushes aside, it floats upward.
How the Burner Creates Lift
The burner is the engine of a hot air balloon, converting liquid propane into a controlled, high-temperature flame aimed directly up into the envelope’s opening. Liquid propane travels from onboard storage tanks through reinforced hoses into heating coils, where it’s superheated and converted from liquid to vapor. That vapor is released through precision jets and ignited by a small pilot light, producing a powerful column of flame that shoots into the envelope.
A typical balloon carries about 40 gallons of propane for a single flight, burning through 18 to 24 gallons over the course of an average trip. The pilot fires the burner in short blasts rather than running it continuously. Each blast pushes more heat into the envelope, increasing the temperature difference between inside and outside air, which increases lift. To maintain a steady altitude, the pilot periodically fires the burner to replace heat that naturally escapes through the fabric.
Why Hot Air Is Lighter Than Cold Air
Air is made of molecules (mostly nitrogen and oxygen) constantly bouncing around. When you heat air, those molecules move faster and spread farther apart. The same number of molecules now occupies more space, so each cubic foot of that heated air contains fewer molecules and weighs less. The envelope doesn’t seal the air inside like a rigid container. Instead, the bottom is open, so as the air inside heats up and expands, some of it spills out the bottom. What remains is a large volume of air that’s genuinely lighter than the same volume of cool air outside.
This is why temperature matters so much. Balloon flights typically happen in the early morning or late afternoon, when outside air is cooler. Cooler ambient air means a bigger density difference for the same amount of heating, which translates to more lift with less fuel.
The Envelope: Built to Hold Heat
The balloon’s envelope needs to trap hot air while being light enough to actually fly. Most envelopes are made from nylon 6.6, the same family of material used in parachutes and high-performance textiles. Cameron Balloons, the world’s largest balloon manufacturer, uses nylon for its superior strength-to-weight ratio, heat resistance, and ability to absorb energy from sudden stress like gusts of wind.
For commercial passenger balloons that fly frequently, Cameron developed a fabric called Hyperlast. It uses individually coated silicone-elastomer yarn that’s woven and then compression-rolled to create a smooth, strong surface. The result is a fabric that handles repeated heating cycles, resists tearing, and stays lightweight. Some manufacturers use polyester as an alternative, but nylon remains the industry standard because of its combination of durability, flexibility, and heat tolerance.
Going Up, Coming Down, and Steering
Controlling altitude is straightforward. To climb, the pilot fires the burner, heating the air and increasing buoyancy. To descend, the pilot stops firing and lets the air cool naturally, or opens a vent at the top of the envelope called a parachute valve. This valve is a circular panel of fabric that can be pulled open with a cord, releasing hot air from the top of the envelope where it’s hottest. When the pilot lets go, the panel reseals under the envelope’s internal pressure.
Horizontal steering is a different story. The FAA classifies hot air balloons as free-floating aircraft with no direct means of directional control. Pilots can’t point the balloon left or right. Instead, they navigate by changing altitude to catch different wind layers. Wind direction and speed shift at different heights due to changes in atmospheric pressure, temperature, and terrain. A wind blowing northeast at 500 feet might blow east-southeast at 1,000 feet. By ascending or descending into these different layers, a technique called vertical tacking, pilots can nudge the balloon’s path in a preferred direction. It’s not precise steering. It’s a continuous process of reading conditions, adjusting altitude, and working with whatever the atmosphere offers.
What Determines How High It Can Go
A hot air balloon’s maximum altitude depends on how much you can reduce the density of the air inside relative to the air outside. As the balloon climbs, the surrounding air gets thinner and cooler. Thinner air means there’s less weight to displace, which reduces the buoyancy force. At some point, the air outside is so thin that even maximally heated air inside the envelope can’t create enough of a density difference to keep climbing.
Practical limits kick in well before that theoretical ceiling. The envelope fabric has a maximum safe temperature, and pushing the burner harder won’t help once you reach it. Passengers need breathable air, which limits most recreational flights to a few thousand feet above ground level. The total weight onboard also matters directly: more passengers or fuel means more weight the buoyancy force has to overcome, which lowers the achievable altitude for any given temperature difference.
The core idea never changes, though. Every aspect of balloon flight, from the propane burner to the nylon envelope to the way pilots find the right wind layer, comes back to one physical fact: heat the air, make it lighter, and it rises.