How do hot air balloons fly?

Hot air balloons are able to rise above the ground because they contain a large volume of air that is less dense than the air around it. As air that is less dense contains fewer air particles, this would normally mean that it was of lower pressure than the air surrounding the balloon, and one might expect that the higher pressure of the surrounding air would press on the balloon’s envelope and squash it. This does not happen to hot air balloons because the air inside their envelopes is heated using a propane burner; the particles in hot air move around more quickly than those in cool air, and faster moving particles cause higher pressure inside the balloon, equalling the pressure of the colder air outside which contains more, yet slower moving air particles. The force that tends to push lighter, less dense objects up into the air is known as buoyancy.

Hot air balloons are gigantic for a reason

The buoyancy of warmer, less dense air can be harnessed to lift a weight but it takes around 28 litres of air to raise only seven grammes. For this reason, the envelopes of hot air balloons are extremely large; they need to be big enough to hold the thousands of litres of hot air required to lift a basket full of human passengers into the sky. To ensure that the air inside the envelope can be heated directly, so that the balloon continues to rise, the propane burner is situated directly beneath an opening at the bottom of the envelope. Cylinders full of liquid propane are carried in the basket to supply fuel for the burner, which ignites it via a pilot light. The heat of the flame eventually raises the temperature of the supply tube enough to change the propane flowing through it from liquid to gas, which then burns more efficiently.

A hot air balloon typically gains altitude until it reaches a level at which the surrounding air is so thin that the air inside the balloon is only a little less dense; buoyancy decreases and no more lifting takes place. Until that point is reached, as long as the pilot fires up the burner the balloon will keep gaining height. To lose height, the pilot can open a valve at the top of the envelope to release hot air. This first slows the rate of ascent and then, eventually, causes the balloon to descend.

Balloon flying is more of an art than a science

In order to make a hot air balloon fly in a particular direction, the pilot relies on the different wind currents that are found at varying altitudes. By rising or falling, the balloon catches a current of wind that is flowing in the desired direction. Velocity also depends on these currents, as the winds found in the higher levels of the atmosphere tend to move more rapidly compared with those lower down. By making use of these natural forces, the balloon pilot flies as accurately as possible, but some improvisation is necessary and the crew on the ground are always ready to travel to pick up the craft and its passengers on landing.

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