Originally posted by FabianFnas
Think of how much Hindenburg, the Zeppeline, weighted. It didn't crach (because it was heavy).
Think of the difference between mass and weight...
No, that's really not it, at all. The air below the cloud is still lower density. Water is still higher density.
However, the drag force of the air dominates over the gravitational force for small particles. The drag force increases as the size of an object decreases. The force needed to move a sphere through a viscous medium is given by Stokes's law,
F = 6πηRv.
Here, R is the radius of the sphere, v is the velocity, and η is the viscosity. The viscosity of air is about 0.018×10-3 Pa·s and the viscosity of water is about 1.8×10-3 Pa·s. Stokes's law is valid if the Reynolds number NReynolds = 2Rρv/η is less than about 2000. Here ρ is the mass density.
A spherical particle falling under the force of gravity will reach terminal velocity when the gravitational force matches the drag force,
mg = 6πηRv.
Solving this for the terminal velocity yields,
vterminal = 2gρR²/(9η
A water droplet with a 10 nm radius falls at 12 nm/s in air. It would take 2.6 years for this droplet to fall one meter. It is only when the small droplets begin to coalesce into larger droplets that they fall with significant speed.
So ultimately, the real movement of clouds is dominated more by air current than by gravitation.