Why do gases float? Don't they have gravity?

 (Asked in Quora. My answer)

All gases have weight. Weight is the force of gravity acting on an object and is equal to the mass of the object multiplied by the strength of the gravitational field at the specific point in space.

If you created an imaginary open pathway (or vacuum tunnel) a gas on Earth would be pulled towards the center of the Earth and would travel as such. However such an open pathway only exists in theory. In reality there is material in the way of the gas that is usually more dense than the gas. The material exerts a normal force upwards which pushes the gas backward until it reaches a stable balanced position that we call floating.

We also call this force the buoyancy force and it counteracts the downward pull of the gases weight.

Less dense gases are pushed upward by the buoyancy force provided by the more dense gases below them. These lighter gases will eventually settle in an equilibrium position where the weight pulling down is equal to the buoyancy force pushing up.

Balloons filled with the light gas Helium demonstrate this phenomenon clearly by rising to a level in the atmosphere where the density of gas around then is more rarefied and therefore comparable with the density of the Helium gas itself.