17 December 2011

Kelvin-Helmholtz instability


Photos posted at Reddit by alison bee, taken near Birmingham, Alabama.  Explanation by zensunnioracle:
These are indeed Kelvin-Helmholtz waves. What is happening is that the nocturnal near-surface layers (lowest 50-100m) of the atmosphere are much more stable than the layers above it in the mornings. Until the ground heats up due to daytime heating, the surface layers stay more stable than the air over it. Kelvin-Helmholtz waves occur when the wind shear between the layers destabilizes the topmost portion of that stable layer, and entrains the air into the unstable layer. What you see is stable air being lifted, cooled, and condensed so that this process becomes visible, though this commonly happens many places without being visible. 


I'd also like to note that this is different from gravity waves as stated elsewhere as these are completely shear induced while gravity waves are usually from lifting buoyant air into a stable region and gravity pulling that air back down. When that air is pulled back down, it can overshoot it's location of being stable, and a wave pattern forms.
More at Wikipedia, where I found this photo of the same process happening on Saturn!


You learn something every day.

11 comments:

  1. Rounding that post off with a Saturn photo is really cool.

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  2. These are the most interesting clouds I've ever seen. I live in Huntsville and these photos were posted on one of the Facebook pages I follow. Thanks for posting them.

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  3. Right you are! You learn something every day. I have never seen this phenomenon before. That's one of the things I love about this blog.

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  4. Wow! Totally using these when I teach about clouds. The only K-H instability cloud pics I've ever seen besides these were ones that were corkscrews in the middle of the sky (which are pretty cool too). Nothing ever like this! You should look for images of mammatus and lenticular clouds too. Those are pretty impressive as well.

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  5. These were amazing. I was driving back to work from lunch (in Birmingham) and saw these on the ridge or Red Mountain.

    There were seven running down the ridge from left to right. They appeared to be perfectly shaped funnels (like cooking funnels) that were sucking the mist from the ground and trees and ejecting the misty-cloud out the top of the upside-down funnel.

    All seven looked exactly alike and shared the same mechanics (ie moving misty-cloudlike material from ground level and shooting it out of the top.

    I even called my wife (she works roughly underneath where this was happening) and she went out and looked at it. When I drove under it I could see the material moving up through the funnel.

    Have I said 'Amazing'? Amazing.

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  6. Wow, my jaw dropped when I saw those photos!

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  7. Is it just me, or do the last 2 cloud pics (not the Saturn one) look like they were taken by a 70's-era Instamatic camera? That graininess and washed-out colour... If it weren't for the modern cars, I'd swear they were shot with my mom's old Kodak back in the day!

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  8. I've seen similar shapes used on American Indian woven baskets, neat, thank you!

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  9. little bit of a difference when talking about this particular effect in relation to saturn. the same shape and relative idea though

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  10. My family and I were in Birmingham during this phenomenon. We are sorry to have missed it! Yes........it is the most interesting cloud formation ever seen. Happy that someone sent it to us.
    Lou Dillard

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  11. Woow! I've never seen such a great scene! Thank you.

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