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SYFY WIRE Bad Astronomy

Skyfall: Sunrise Fallstreak Cloud

By Phil Plait

Here at BA HQ we’re all about weird clouds. Well, maybe not all about them, but certainly a lot about them. You’d think that clouds wouldn’t be terribly surprising (unless you’re trying to just get some Sun on a beach in Brazil) since, after all, they’re clouds. But you’d be wrong.

For example, get an eyeful of this.

Whoa. What is that?

It’s called a fallstreak or hole punch cloud. That’s not really an official name, since this isn’t actually a separate type of cloud; it’s actually something that happens to a cloud.

Here’s how this works: If you get water cold enough, it’ll freeze (duh). But, water needs something to freeze around, a particle of dust or an existing ice crystal or some other such object that can trigger the formation of ice crystals.  If you have very pure water in a very smooth container, you can slowly lower its temperature to below freezing, and it’ll stay a liquid unless there’s something that disturbs it. Water like this is said to be supercooled.

These conditions can occur in a cloud, with water floating around below the freezing point but unable to make ice. If something happens to trigger ice formation—maybe an airplane going through the cloud creating ice crystals, or dust getting into the cloud—the water will rapidly freeze around these “condensation nuclei.” The resulting crystals will then fall, leaving behind a hole in the cloud.

The fallstreak cloud above was photographed by Zora Fernandez in British Columbia at sunrise in late February. The colors are from the reddish Sun lighting up what looks to me like a nice, stable layer of altocumulus perlucidus clouds (a term I just learned and which, c'mon, is really cool). Something must have happened to trigger the fallstreak; the water then crystallized, began to fall, and voilà. Awesomeness.

Note the dark shadow across the red clouds; that’s the shadow of the fallstreak cloud on them. As the crystals fell, the low Sun cast the shadow upwards, on to the cloud layer. And the greenish color you see is most likely actually just the sky seen through the hole in the cloud layer! The odd color is probably a combination of the sky color at sunrise and the red clouds, giving it a greenish or turquoise cast.

Here’s a second shot of it:

The shadow is more obvious in this one, and the sky is paler. I’ve seen fallstreak clouds a few times. The first time I remember well; I was driving home from school in Michigan and a big one appeared out my side window. I stopped and took a picture (which I’m sure I still have, if I could dig through the bazillion prints I keep in shoeboxes in my attic; this was in the 1980s). It was so odd; the feathery icy cloud that fell left a hole in the upper layer the same shape, like a cartoon character does when they run through a wall. Freaky.

Anyway, this goes to show you: You might take the sky for granted, but it’s actually an endless show put on for free. Wanna see it? Look up.

Tip o’ the cumulus to Jenn Laycock and Nathan Santo Domingo.

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