How Do Clouds Stay Up in the Sky?

[gentle music] Okay, first we need to figure out what is actually inside a cloud.

Have you ever wondered that? If you could somehow reach up and grab a handful of cloud, what would you get? It's clearly got something to do with water, right? Well, yes, clouds are made of tiny drops of liquid water, and depending on how cold it is, maybe even tiny pieces of ice.

But here's the thing.

Here's what's different.

Those little drops of water and the pieces of ice, they are so small that they behave a little differently than what we might be used to.

See, a typical cloud droplet might be only about .02 millimeters across, which means you could line up a whole bunch of them across the width of just one human hair.

Yeah, it's smaller than a human hair.

And actually, human hair is a really good example because, think about it, what happens if you have a little piece of hair in your hand and you let it go? Does it fall straight to the floor? No, it, it kind of floats on its way down, [laughs] doesn't it? In fact, if the air or the wind is just right, it might not hit the floor at all, or at least not for a while.

It behaves a lot like a feather.

It definitely is not like a rock, where you let it go and it just thuds right down, right? You would need a lot of air pressure to stop a rock from slamming straight into the ground.

But these cloud droplets are even smaller than feathers or human hairs, so they float around even more.

As gravity pulls each tiny droplet downward, the droplet also has to push its way through the air.

The air pushes back against it, and this is called air resistance.

Because cloud droplets are incredibly small, they fall very slowly, like they're falling in slow motion.

See, in completely still air with no air resistance, a cloud droplet may fall only about half an inch in one second.

Here, I w- I want to show you something.

Will you count to 10 with me? Ready? One, two, three, four, five, six, seven, eight, nine, 10.

Okay, so in 10 seconds, a cloud droplet with no air resistance could only fall about five inches.

Yeah, five inches in 10 seconds.

[laughs] Most iPhones are taller than that, and that's in completely still air, okay? If there's air resistance, it might go even slower.

At that speed, a cloud droplet falling from about 1,650 feet in the sky could take more than 10 hours to reach the ground.

And the thing about it is that air isn't usually completely still at all.

[laughs] Clouds often form when air is moving up.

Water is constantly evaporating from places like oceans and lakes and wet ground, and even just puddles.

That puts invisible water vapor into the air, and when moist air rises higher into the atmosphere, it cools.