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SYFY WIRE Bad Astronomy

How Do Clouds Form?

By Phil Plait

As someone who loves looking at clouds, and may have a somewhat scientifically directed brain, I’m fascinated by the shapes and structures of clouds. Where I live, at the foot of the Rocky Mountains, provides endless examples of them.

I write about them a lot, but I don’t think I’ve ever explained just how clouds form. After all, it’s a little weird: A typical storm cloud can have a mass of millions of tons, yet they float! I know this is because they’re less dense than air, but that still seems weird.

But, if you watch this great video on how clouds form by my friends at Minute Earth, you’ll understand exactly how this all works.

There were two parts in that video that I want to emphasize. One was just why clouds are buoyant; it’s for the same reason a helium balloon is. At a given temperature, a volume of a gas will have the same number of atoms or molecules in it no matter what those atoms or molecules are. Helium atoms have way less mass than oxygen and nitrogen, so it weighs less, so up it goes. And it’s the same for humid air!

The other part was why clouds have flat bottoms. The video makes that really obvious: As you go up in altitude, the temperature of the air drops (this is called the lapse rate, which for some reason is a term I think is really cool). Water vapor (water as a gas) is transparent, so you don’t see that big bubble of rising humid air until it gets to the part of the atmosphere where it’s cool enough for the water to condense into droplets (and becomes visible). If the air over a wide area over the ground all reaches that temperature at the same altitude, it forms a plane parallel to the ground. As the humid air rises through that plane it condenses, forming a cloud that is round on top and flat on the bottom. It’s actually a bit of an illusion; the parcel of rising air is still balloon-shaped (very roughly), but you only see it where the water condenses.

I have to admit: I knew all the pieces of this but hadn’t really put it all together in my head all at once. Seeing it drawn out this way in animated form made it very clear (so to speak, har har).

At the top of this post is a picture of clouds forming with flat bottoms, and you can see their bottoms are all at about the same altitude. I have always thought this would explain why we see the sky as a flattened dome over our heads; a cloud directly overhead is close to us, but one near the horizon is much farther away, giving the sky a flattened look. The perspective effect is strong; you can see how the clouds appear to bunch together when they’re farther away. That’s not real; they’re probably scattered just as much 50 kilometers away as they are overhead, it’s just that when they’re farther away you see more of them in the same area of sky because they appear smaller with distance.

If all of this seems obvious to you, then yay! But I know that many times, when we live our lives in the natural world, there’s a lot of stuff we see, and even a lot of pieces of it we understand. But putting it all together, turning it from a lot of jigsaw puzzle pieces into a single glorious picture, well, sometimes you need someone to show you that part.

If you want more about clouds, then watch this other video by my pal Joe Hanson from It’s Okay To Be Smart. There’s a lot more there.

Tip o’ the brolly to the good folks (who are also friends of mine!) at Science Alert, too, for linking to both videos.

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