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Kelvin-Helmholtz?

April 25 2014 at 3:07 PM
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Jelte  (Login JelteV)

 
Spotted these from the train, they weren't accepted in the app as Kelvin-Helmholtz, but I'm pretty sure they are. [linked image]
Anyone can confirm if it's Kelvin-Helmholtz?
Thanks!

 
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GWW/Granny Weatherwitch
(Login imaweatherwitch)

K-H

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April 25 2014, 3:36 PM 

Certainly looks like K-H oscillations to me. I'd usually expect them to be at a higher level and so they are more often white against a blue background (maybe that is why the "app" (WHAT APP?) did not confirm them. I'd say they are unusual at low level and below other cloud layers, but not impossible. BUT it's half a century since my days at Met. Office Training School and they didn't teach us about K-H's then, so I'll have to defer to modern experts if they disagree with me ! Well spotted, as such things usually don't last long. GWW/ Ex. London Heathrow Airport Met. Office in 1960's

 
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'H'
(Login beacontrigpoint2)

Re: Kelvin-Helmholtz?

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April 25 2014, 11:37 PM 

Hi, Jelte, and welcome. Authorities can differ; Hamblyn says Cirrus or Cirrostratus, whereas Pretor-Pinney says all three cloud levels and allows Stratocumulus and Altocumulus too.

Thus it is difficult for a human to say nay, but I don't know how the app works - it might be difficult for it.

GWW, check out the CAS home page where I am sure the app must be mentioned (but I don't have a smartphone).

 
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Graham Davis
(Login Cloddy)

Re: Kelvin-Helmholtz?

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April 26 2014, 10:33 PM 

Looks like K-H to me. Can occur at any level. Here's some really fine examples of them at various levels, including some on the top of a bank of fog or stratus near Birmingham, Alabama: http://webecoist.momtastic.com/2011/12/27/roll-call-kelvin-helmholtz-clouds-make-weather-waves/

 
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'H'
(Login beacontrigpoint2)

Re: Kelvin-Helmholtz?

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April 26 2014, 11:59 PM 

Nice link, Graham. It is not very obvious from the link, but the cloud literature emphasises that the boundary condition requires a warm layer over a cold layer. Some sources, including Wiki if I remember right, allow a continuous change rather than a distinct boundary - this is helpful for an explanation of K-H in contrails.

I find the link confused over the direction of the breaking crests; the first diagram suggests counter to the direction of flow (and counter to my intuition). Arrows imposed on photos suggest crests break with the direction of flow.

In The Times 25APR14 Paul Simons re-tells the Crocker Land (q.v.) story. He points out that 'In polar regions, layers of warm air often lie over dense cold air and where these meet they can bend light from over the horizon, creating mirages.' Inuits call them 'poo-jok' meaning mist. A multiple mirage is called a 'fata morgana'.

 
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Graham Davis
(Login Cloddy)

Re: Kelvin-Helmholtz?

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April 27 2014, 4:08 PM 

H, I don't see any problem in the first diagram. The upper layer of liquid is moving slower than the lower (U1 < U2) so the rotation at the interface is anticlockwise with the tip of the wave taking the lesser speed of the upper liquid. Perhaps if you regard the frames showing the developing wave as being relative to the average speed of both layers - which I suppose it is - then the arrows of the motion of each liquid are relative to this average speed. Thus, in the second frame, you could have U1 and U2 depicted by equal-length arrows, the U1 arrow pointing to the left and that for U2 pointing to the right.

 
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Jelte
(Login JelteV)

Re: Kelvin-Helmholtz?

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April 27 2014, 9:12 PM 

Thanks everyone for the replies! The app I was referring to is the CloudSpotter app for iPhone - cloudspotterapp.com. I guess the photos sent in are downgraded to a smaller file, so judging this cloud from a downgraded cell-phone-quality image can be quite challenging I guess!
These waves were formed on Stratocumulus, Nimbostratus was quite obvious present too and a few kilometers away Cumulonimbus was forming. So I guess conditions were right for a KH to form. When I'm looking up photos of the KH, I notice that almost all waves are huge. Does the size matter when defining them?
Two years ago I saw KH's in the Swiss alps, they were huge and formed high. I added quite some contrast to the photos to make them better visible. They weren't higher than the Altocumulus...
[linked image]


 
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andrew
(Login poth1)

Re: Kelvin-Helmholtz?

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April 28 2014, 12:50 AM 

Hi Jelte,

Just a word for you and the others about how the app works. There are moderators who look at every picture (I'm one of them) and give it "yes" or "no". And if they're unsure, they leave it for the next moderator. (And if that moderator is unsure, they can leave it for the next there's a number of volunteers!) Clouds being what they are, some don't fall into exact, clear-cut categories and for KH, there's a desire to be more exact and not say yes if there's a good chance that in fact it could be a cloud shape that happens to suggest KH rather than clearly is, for example. (Because the submitter gets more points for KH, for one reason!)

I didn't moderate this picture, but maybe someone was just being cautious, rightly or wrongly! So this is just to say it's a human judgment in the app, and if its not a clear-cut picture someone may have voted on the side of caution. Maybe a judgment for what they thought could be cumulus and undulatus imitating KH? I can't say, but I hope that explains the process a little and how judgments can be strict or occasionally debatable (or involve occasional human error)! As you say, zooming in on the photo doesn't necessarily give more information!

Sorry it got rejected. Please keep submitting!


 
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Jelte
(Login JelteV)

Re: Kelvin-Helmholtz?

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April 28 2014, 7:02 PM 

Hi Andrew,
Thanks for your reply! I understand it's difficult to judge the photos and cautiousness with rarer clouds is logical. I'm very grateful for all effort the volunteers put into judging the photos! After all it's still the clouds themselves that I'm looking for and gaining some points besides makes it extra fun of course wink.gif. Knowing that this cloud was probably a KH is nice, and that sometimes we're not completely sure is one aspect of why clouds are fascinating to me.
Jelte

 
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'H'
(Login beacontrigpoint2)

Re: Kelvin-Helmholtz?

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April 28 2014, 10:35 PM 

Jelte, I would say for K-H size does not matter. We have had some strong cases put forward for K-H in relatively slim contrails.

Graham, thank you for that explanation which I think I can follow. I notice the text says of the air masses 'regardless of their directions'. So is there a relationship between speed and crest direction i.e. if U1 > U2 the crests would break the other way?

 
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