If a cloud is a kilometer long and a kilometer tall and weighs about as much as 100 elephants, why it does not fall to the ground under the influence of gravity?

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    $\begingroup$ There is one thing called density? And Archimedes principle? And because of this a ship weighing more than 10 tons doesn't sink. Read Archimedes principle for more explanation. $\endgroup$ – user5954246 Nov 3 '16 at 4:58
  • $\begingroup$ In short the buoyant force is more than weight (or) force applied by warm wind up is more than force applied by gravity and this is because surface area of cloud is quite large. $\endgroup$ – user5954246 Nov 3 '16 at 5:07
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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/9898/2451 and links therein. $\endgroup$ – Qmechanic Nov 3 '16 at 7:56

A cloud is not one single aggregated mass, but a vast collection of tiny droplets (less than 100 micron). Each droplet falls individually under the influence of gravity, but their terminal velocity is much smaller than turbulent fluctuation velocities prevalent in clouds. To overcome this and reach the ground as rain, those droplets have to grow to a size much larger than ~ 200 micron.

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  • $\begingroup$ "their terminal velocity is much smaller than turbulent fluctuation velocities prevalent in clouds" - and since as a whole, clouds don't appear to fall, that means that the net turbulence in clouds is always upwards? Why don't clouds ever exist in places where net turbulence is downwards? $\endgroup$ – Cory Klein Jun 16 '17 at 20:22
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    $\begingroup$ @CoryKlein When one speaks of turbulence, one subtracts the mean motion and considers only fluctuations. So your question is to be interpreted as why is mean motion in clouds usually upwards? Clouds experience updraft (which can be up to 6 m/s) because of buoyancy generation due to latent heat release which is due to condensation of water vapor into liquid droplets. There are however regions where there could be downdrafts, especially at the top of the cloud due to radiative cooling. But even if a few droplets were to be brought to earth by these downdrafts it wouldn't be considered rain. $\endgroup$ – Deep Jun 17 '17 at 4:37

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