Why don't clouds fall out of the sky?

d ~ 102'912'000 km: day 40 of Earth's orbit

Your average cute fluffy cumulus cloud consists of about 400 metric tonnes of liquid water. And it's up there, a couple of kilometers above your head, looking innocent while apparently defying the laws of gravity. Surely, clouds should fall down. Why don't they?

The first thing to know about these clouds is that they're not static. They're like a fountain inside, as warm air from below pushes up through the middle of the cloud. When each individual updraft reaches the top, it pushes out a bump of cloud that we can see. That's why cumulus clouds are lumpy on top - each bump is the result of one of the warm air plumes that helped build the cloud. Here's a great time lapse video of this internal fountain process.

You can see the continual building of clouds from successive thermals. The important point to take from this clip is that a lot of the air inside a cloud is moving upwards.

How about the water?

The liquid water in a cloud is in very tiny water droplets, and each droplet is around 0.01mm in diameter. That means that if you took a lump of water with the same volume as a sugar cube and you split it into a billion pieces, each fragment would be the same size as a cloud droplet. The droplets are very spaced out, so the amount of liquid water in a litre of cloud is pretty small. That's why you can't drink fog.

If there was no air in the way, gravity would make a cloud droplet and a lump of lead fall at the same speed. But air pushes back on things that are falling through it, and that push becomes more important as the object gets smaller. I like the way that J.B.S. Haldane said this, although in a different context: "You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, and a horse splashes." Yuck. The tiny cloud droplets have a very large surface compared with their mass, so the air slows them down even more than it slows the mouse and the final droplet speed is only about 3mm per second.

The end result? Individual cloud droplets are falling down, but very very slowly. And the air that carries them is moving upwards faster than they're falling. Imagine a slinky coming down an escalator moving upwards, and you'll get the idea. So those tiny droplets are stuck, up there in the cloud. And that suits me just fine. I don't have an umbrella that would withstand 400 tonnes of water falling on it all at once!

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