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How do large clouds stay in the air if they can weigh tonnes?

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Stop to think about the massive clouds we see floating in the sky most days and it seems about as counterintuitive as a lead balloon.

Asked by: Terry Rockett, via email

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At a certain height, air cools enough for any water vapour to condense into droplets and form visible clouds. The droplets are liquid water, and therefore denser than air, but they are tiny, so they have a low terminal velocity and fall very slowly. A typical cloud only has about 0.5g of water per cubic metre in it, and if the droplets are small enough, they will be kept aloft by the thermals in the cloud as warm air rises from below. Once the droplets have fused together and grown large enough, gravity dominates over buoyancy and they fall as rain.

As for why water that evaporates rises up into the sky in the first place, air is mostly nitrogen (N2) and oxygen (O2), with an average density of 1.225kg/m3. A water vapour molecule is much lighter with just one oxygen atom and two hydrogen atoms (H), so its density (at standard temperature and pressure) is only 0.804kg/m3.


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Authors

luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.

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