23 Jun 10
2 edits
The drag force of air exceeds gravitational force for small particles (and of course, clouds are made of small particles). So, while you add up the mass of the water in the clouds, it sounds large, it's dwarfed by the force exerted by air flow from convection.
Same principle as why some bubbles in your newly poured beer can be seen moving downward in the currents of the beer.
Of course, if you could aggregate all those bubbles into one big bubble, the drag force would be insignificant, and it would overcome the current flows.
23 Jun 10
Originally posted by uzlessIt's pretty simple, cumulonimbus clouds are less dense than the air underneath, like oil floating to the top of a pond. If you captured exactly say, a cubic meter of the thundercloud and a cubic meter of air closer to ground level, the ground level air would have more mass so it would naturally be at the bottom of a gravity well.
A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.
How come it doesn't come crashing to earth?
23 Jun 10
Originally posted by uzlessIts not mass but density that counts. If you burnt an elephant, all its mass could go into the air and float away.
A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.
How come it doesn't come crashing to earth?
Large volumes of air also weigh as much as thousands of elephants.
23 Jun 10
Originally posted by uzlessThink of how much Hindenburg, the Zeppeline, weighted. It didn't crach (because it was heavy).
A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.
How come it doesn't come crashing to earth?
Think of the difference between mass and weight...
23 Jun 10
Originally posted by FabianFnasNo, that's really not it, at all. The air below the cloud is still lower density. Water is still higher density.
Think of how much Hindenburg, the Zeppeline, weighted. It didn't crach (because it was heavy).
Think of the difference between mass and weight...
--------------------
http://lamp.tu-graz.ac.at/~hadley/whydontcloudsfall.html
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.
23 Jun 10
Originally posted by joneschrHindenburg in equibrilium is the same as the watercloud. It finds its place where the mean density under it is higher and above it is lower compare to the mean density of the ship.
No, that's really not it, at all. The air below the cloud is still lower density. Water is still higher density.
23 Jun 10
2 edits
Because clouds still float when the mean density of the cloud is higher than the mean density of the air underneath the cloud. And that's the principle that addresses the OP's question.
The issues is why water droplets (which are more dense than air) don't crash to the ground. To address that point, you need to understand drag - (not density and not pressure).
23 Jun 10
Originally posted by joneschrnow, we're gettin somewhere
Because clouds still float when the mean density of the cloud is higher than the mean density of the air underneath the cloud. And that's the principle that addresses the OP's question.
The issues is why water droplets (which are more dense than air) don't crash to the ground. To address that point, you need to understand drag - (not density and not pressure).
23 Jun 10
Originally posted by joneschrBut is it? (the mean density of the cloud higher). If so, it would only be marginal or there would be a downdraft (the cloud would fall).
Because clouds still float when the mean density of the cloud is higher than the mean density of the air underneath the cloud.
24 Jun 10
Originally posted by joneschrIf it was just drag, then air would fall to the surface, eventually. It doesn't, because of pressure.
Yes, the cloud would fall. But at an extremely slow rate - as I posted earlier, a cloud will naturally fall from gravity. But again, because of drag, it would take a couple of years to reach the ground.