I know it's probably the most stupid question there is, but why do they fly are the clouds lighter than air? What's keeping those tiny ice structures floating miles about the ground? I've been looking all over the internet and I can't find acceptable answer to this basic physical question. Can you please help me?

This question was marked as answered - but I don't believe it has been explained satisfactory. Please bare with me on this one.

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    $\begingroup$ Similar question here physics.stackexchange.com/q/9898 also see this ppt. google.gr/… $\endgroup$
    – user288447
    Commented Apr 20, 2014 at 22:07
  • $\begingroup$ Clouds are not 'floating block of ice'. Clouds are made of many gases and when that clouds get cold, vapor condenses to water (if its cold enough to snow or ice (so clouds have ice particles but they are so small that they float)). Because of Water vapor is less dense than air at sea level (because the more temperature object has the more volume it has) it goes up. Force of that is described using this equation: $F=\rho_{air}gV_{watervapor}$ $\endgroup$
    – user21420
    Commented Apr 20, 2014 at 22:12
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    $\begingroup$ This link to a scientific american post seems to give a convincing answer. $\endgroup$
    – BMS
    Commented Apr 21, 2014 at 1:56
  • $\begingroup$ Thank you for your answer. So the clouds are generally less dense that air bellow them [is this right?]. One additional questions pops into mind: What force causes these small ice particles to float for prolonged periods of time?? I am left little confused with this response - are water vapour particles microscopic ice particles??? :O $\endgroup$ Commented May 28, 2014 at 7:39

2 Answers 2


Have you ever walked through fog? What is it made of?

Lots of very small water droplets. They are so small that their weight is negligible compared to the motion of air molecules, so they fall too slowly to ever notice.

Next time you boil water, and you see "steam", what is it? Same thing.

The only difference between fog and a cloud is that fog is a cloud that happens to be on the ground.

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    $\begingroup$ The "steam" in quotes is good. Rule of thumb is if you can see it, it isn't steam. I used to do a demonstration in which I started paper on fire with superheated steam. Steam from a flask through a copper tubing coil over a Fisher burner. Melt a candle too. $\endgroup$ Commented Apr 22, 2014 at 1:44
  • $\begingroup$ So we have a water droplet - why does it starts to rise and why makes it fly so long? I can imagine boiling kettles, fogs, particles suspended by motion of molecules, but why exactly causes them to behave in this way - a huge pile of something that looks as cotton [most likely made out of water mostly] flying for prolonged periods above our heads? Is it result of pressure or temperature ratio..? Water is relatively good thermal conductor. And one more peculiar thing is: why do they move towards the low pressure areas all the time? IS there any simple explanation to this, please? $\endgroup$ Commented May 28, 2014 at 7:35
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    $\begingroup$ @user2820052: If you were your present size, but you only weighed 1 gram, and if air molecules kept bumping into you like bouncing bowling balls on all sides, you'd fly too. And if there was a region of fewer bowling balls up ahead, it would attract many of them, and you'd be carried along. $\endgroup$ Commented May 28, 2014 at 15:42
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    $\begingroup$ @user2820052: forget the density and temperature. What matters is the ratio of volume to area. If a droplet has a diameter of 10^-4 cm, then its ratio of volume to surface area is 10^-4 that of a cc of water. In other words, its weight becomes insignificant compared to its area, so gravity has no more effect on it than a hair on your head. $\endgroup$ Commented May 29, 2014 at 0:01
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    $\begingroup$ @user2820052: NP. The air has a temperature, which is just the air molecules moving around excitedly, bumping into themselves and everything else. That bumping is what pressure is. If you're teeny-tiny and weigh nothing (or practically nothing) you will just bump around in the general chaos. Now, take a cubic liter of water - it has 100 square cm area on a side, and weighs 1kg. Now take a cubic cc of water, it has 1 square cm area on a side, and weighs 1 gram. 100 times less area, 1000 times less weight. Keep going. That's why dust motes fly and mountains don't. $\endgroup$ Commented May 30, 2014 at 13:02

This is easier to picture with a particular example. Have you seen clouds with flat bottoms, like a lot of small clouds that all have flat bottoms at the same level? These clouds are blobs of air with more moisture than the surrounding air. The flat bottom type are from rising air from heat convection. This is what pilots call "thermals". The air cools as it rises both from the surrounding air being cooler and because the air is expanding.

The flat bottom you see is the altitude at which the temperature equals the dew point for the humid air rising up and there is condensation. If you watch you can often see that the cloud grows as more humid air rises. It only becomes visible as it passes the dew point. The types of birds that soar will be under these clouds getting a lift.

  • $\begingroup$ You're on the right track. The flat bottom is where the temperature falls below the dew point. However, air is not always rising in thermals - that's called unstable air, as opposed to stable air. When the environmental lapse rate exceeds the adiabatic lapse rate, the air is unstable. See Convective instability. $\endgroup$ Commented Apr 21, 2014 at 12:19
  • $\begingroup$ Thanks Mike. I never really THOUGHT about this until a friend who para-sails described it this way. And I TA'd Atmospheric Science as an undergrad! $\endgroup$ Commented Apr 22, 2014 at 1:40
  • $\begingroup$ All I know is what I learned in pilot training (plus undergrad thermo). $\endgroup$ Commented Apr 22, 2014 at 12:23
  • $\begingroup$ Thank you for this interesting example. The thing is I am interested in learning what is keeping these particles afloat... What precisely is causing clouds [H2O and other gases] say at 5000m altitude to stay aloft??? Are they lighter than air at [750g/m^3]and why to they move towards low pressure areas ??? $\endgroup$ Commented May 28, 2014 at 7:16

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