Buoyancy is the upward force exerted by a fluid that opposes the weight of an object immersed in it. This principle, described by Archimedes, states that an object will float if the buoyant force acting on it is equal to or greater than its weight.

In high altitude ballooning, buoyancy is the fundamental force that enables a balloon to ascend through the atmosphere. The balloon is filled with a gas that is less dense than the surrounding air, such as helium or hydrogen. As a result, the balloon experiences a net upward force, allowing it to rise. As it ascends and atmospheric pressure decreases, the gas inside expands, causing the balloon to grow in volume and continue to rise until it reaches an equilibrium point where the buoyant force balances the weight of the balloon and its payload.