Definition
The upward lifting force produced when a lighter-than-air gas (such as helium or hot air) displaces a volume of denser surrounding air, causing the gas-filled envelope to rise. It is the principle that allows balloons and airships to fly.
Plain English
Lighter gas inside a balloon pushes the balloon upward because the air outside it is heavier. The heavier outside air sinks and shoves the lighter gas up.
Context Anchor
Seen when describing lighter-than-air aircraft, such as balloons and airships.
Derivation
Buoyancy comes from the Spanish boyar, meaning 'to float.' Originally used for objects floating on water, it was extended to gases because the same physics applies: a lighter substance surrounded by a denser one gets pushed upward.
Why Pilots Care
Understanding gas buoyancy is essential for safe operation and altitude control of lighter-than-air aircraft.
Analogy
Like a beach ball held underwater -- release it and it shoots to the surface because the water around it is heavier. A helium balloon does the same thing in air.
Grounding Statement
In a hot-air balloon, heating the air inside makes the balloon lighter than the air around it, creating lift.
Intuition Check
Gas here does not mean gasoline or fuel. It means a gas such as heated air, helium, or hydrogen that can help support an aircraft in the air.
Example Sentence 1
Hot air balloons rely on gas buoyancy -- heating the air inside the envelope makes it less dense than the surrounding air, and the balloon rises.
Example Sentence 2
A leak in the envelope reduced gas buoyancy and caused the airship to descend.