What Causes Stripey Clouds?

Contributed by
Jun 5, 2016

Sixty Symbols is a great effort: A group of professional scientists of various stripes putting out short videos, each one tackling a different symbol in science. ∆F, -K, kg… each one is explained clearly and simply by actual scientists.

The videos proved so popular (more than 53 million views as I write this!) that they kept going. They’re all good, but one popped up recently that I like in particular. Not because of the symbol — capital N is not really all that sensational by itself — but for what it’s used for: the Brunt–Väisälä frequency.

What’s that, you ask? I’m glad you did. It has to do with clouds, and you know how much I love clouds. I’m sure you’re also wondering how that’s pronounced. I’ll let the video explain:

I live very close to the base of the foothills of the Rocky Mountains, and we get these stripey parallel clouds sometimes. The mountain range runs roughly north/south, and the clouds do too.

Sometimes, though, the clouds run perpendicular to the mountains, the parallel lines stretching to the east, and that’s due to a similar process involving gravity waves (not to be confused with gravitational waves), but when the wind is blowing from the north or south. As one layer of air is blown over another layer, it bobs up and down. Gravity pulls it down, it becomes buoyant, then it rises back up, creating ripples.

gravity wave clouds

I took this shot of gravity wave clouds in Boulder county in January 2016. I was facing south; the lines in each ripple run east/west. Credit: Phil Plait

We also get Kelvin-Helmholtz waves, and those are the best. They’re very rare, but I’ve seen them at least twice over the past few months. It happens when a layer of air blows over another layer. At the interface there’s a shear force; it’s like the two are rubbing together. This can lift up a bit of the lower layer, which then gets pushed faster by the layer above it, causing the “breaking wave” appearance.

Kelvin Helmholtz clouds

I took this shot of Kelvin-Helmholtz clouds in February 2016. The wind above the cloud was blowing southward. You can see the "breaking wave" appearance (inset) to the clouds. Credit: Phil Plait

Isn’t that cool? Finally, and related, we also get orographic clouds, which form when moist air runs into a mountain, lifts up, and forms clouds. We get steady winds over the Rockies for days, and these winds form orographic clouds.

The result is bizarre: A demarcation line just above the mountains, with clear air to the west and clouds to the east; a long, long nearly straight line running along the mountains, north/south. That line will wiggle a bit but stay remarkably steady for days. You expect clouds to move, so that line looks like it should blow east, but it never does. The clouds themselves do, but the line where the clouds start to form holds true. It’s really disconcerting.

You probably see clouds all the time. But do you really see them? It’s worth taking  another, deeper look.