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  1. Standard member uzless
    The So Fist
    22 Jun '10 21:48
    A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.

    How come it doesn't come crashing to earth?
  2. Standard member joneschr
    Some guy
    23 Jun '10 00:16 / 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.
  3. Standard member sonhouse
    Fast and Curious
    23 Jun '10 02:47
    Originally posted by uzless
    A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.

    How come it doesn't come crashing to earth?
    It'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.
  4. 23 Jun '10 06:31
    Originally posted by uzless
    A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.

    How come it doesn't come crashing to earth?
    Its not mass but density that counts. If you burnt an elephant, all its mass could go into the air and float away.
    Large volumes of air also weigh as much as thousands of elephants.
  5. 23 Jun '10 10:20
    Originally posted by uzless
    A thundercloud (cumulonimbus) can weigh as much as thousands of elephants.

    How come it doesn't come crashing to earth?
    Think of how much Hindenburg, the Zeppeline, weighted. It didn't crach (because it was heavy).

    Think of the difference between mass and weight...
  6. 23 Jun '10 10:44
    The keyword to this puzzle has been mentioned implicitly but not explicitly so I will do that: pressure.
  7. Standard member joneschr
    Some guy
    23 Jun '10 14:50
    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.

    --------------------

    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/(9&#951.

    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.
  8. 23 Jun '10 14:54
    Originally posted by joneschr
    No, that's really not it, at all. The air below the cloud is still lower density. Water is still higher density.
    Hindenburg 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.
  9. Standard member sonhouse
    Fast and Curious
    23 Jun '10 16:24
    Originally posted by FabianFnas
    Hindenburg 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.
    That about sums it up. Why is there such a debate over such a mundane issue?
  10. Standard member joneschr
    Some guy
    23 Jun '10 18:05 / 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).
  11. Standard member uzless
    The So Fist
    23 Jun '10 18:49
    Originally posted by joneschr
    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).
    now, we're gettin somewhere
  12. 23 Jun '10 20:02
    Originally posted by joneschr
    Because clouds still float when the mean density of the cloud is higher than the mean density of the air underneath the cloud.
    But 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).
  13. Standard member forkedknight
    Defend the Universe
    23 Jun '10 20:46
    Originally posted by twhitehead
    But 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).
    Clouds do "fall" when the droplets get too big. It's called rain.
  14. Standard member joneschr
    Some guy
    23 Jun '10 21:19
    Originally posted by twhitehead
    (the cloud would fall).
    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.
  15. 24 Jun '10 06:28
    Originally posted by joneschr
    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.
    If it was just drag, then air would fall to the surface, eventually. It doesn't, because of pressure.