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SYFY WIRE Bad Astronomy

Time-lapse animation: Waves of fiery clouds undulate across the sky

By Phil Plait

Mike Olbinski is a photographer and storm chaser specializing in the amazing weather systems we get here in the American plains. I’ve featured his work many times on the blog (like when he shot monsoons in North Dakota, and his phenomenal grayscale video of storms, including a tornado).

So when he sends me a note, I stop what I’m doing and pay attention. Yesterday, he emailed me letting me know that he was chasing in North Dakota for a time-lapse video. Normally he’d take all the shots and then compile them into one video at the end of the season, but what he saw on June 2, 2017, made him put out a special edition.

What did he see? Undulatus asperatus, a relatively rare and truly bizarre cloud formation, where a normally flat layer of clouds starts to ripple, creating huge waves that, well, undulate across the sky. Olbinski caught a ridiculously amazing display of them, and even better: He caught them at sunset, when the lighting makes them go from weird to epically weird. Watch (and make it the highest resolution your monitor will handle):

Holy wow! What I would give to see that.

It’s not clear how undulates asperatus form, or why they behave the way they do. I suspect it has to do with gravity waves. These are waves in air that make it bob up and down, like sea waves (and they do look very much like standing underwater in a storm and watching the choppy sea surface undulate).

You get them when air traveling along in one direction hits an obstacle, like a mountain. This forces the air up. This air is denser than the air around it, so once it passes the mountain, gravity causes it to sink. But it has some momentum, so it sinks right past the point where it has the same density of the air around it, and falls through to the denser air below that. Now it’s less dense than the air around it, so it rises again. This can happen many times, and if the air has moisture in it, it forms clouds when it hits the peak of its travels, and when it sinks, the water evaporates again. This can create huge fields of striped clouds, which are pretty fun to see.

But that’s not precisely what’s happening here; in undulatas asperatus the entire cloud deck is waving, sometimes quite deeply. It may be due to a shear wind, when air blowing at the bottom of the deck is moving faster or slower than the air above it, and this self-reinforces into the huge amplitude waves. But how? Beats me. From what I can find, no one really knows. They’re rare enough that they’re hard to study, so in fact videos like Olbinski’s can help meteorologists figure this out.

I’ve seen a lot of freaky clouds, especially since moving to Colorado. Undulatus asperatus is definitely near the top of my list of things I really want to see. What an incredible thing it must be to witness!