Explained: Flying Carpet Rhombus-shaped Cloud in Exeter

[Mick West]

via: http://www.westernmorningnews.co.uk...carpet-cloud/story-22011903-detail/story.html

The straight-edged rectangular formation was seen in the sky in Exeter, Devon - and images were quickly posted on social media.

One picture of the rhombus-shaped cloud was snapped by local Philip Mussel - and another put online by Laura Gilchrist, an employee at the Met Office.

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The Met office gives the correct explanation:

"In this case an aircraft has moved through a pocket of air with the right conditions for the contrail to form, meaning there is just a short section.

"The wind is blowing at right angles to (ie across) the contrail, and due to the right atmospheric conditions this is causing the ice crystals in that trail to spread outwards - creating this rectangular shaped cloud.

"It's unusual to see the cloud edges quite so well-defined as in this example but the processes involved are very common."

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However this is a little hard to visualize from the text alone. Perhaps a better way of looking at it would be to see what it would look like if the contrail were not broken:


Now that looks a bit more like a regular contrail that's been around a few minutes, and has spread out a bit. But how does it end up like the "magic carpet"? Why does it get cut up so neatly.

The answer is that it does not get cut off along a long straight line, but rather gets cut off when the contrail is narrow, and then the longer "edge" forms as the contrail spread vertically, and the wind shear spreads it horizontally. So what might the original contrail look like?



Now that looks more reasonable. Contrails are generally not continuous. Most contrails start and stop. A contrail is basically the jet engine revealing regions of the atmosphere that have a lot of water in them - kind of like invisible clouds. In this case there was just a single "invisible cloud" in that particular region of the sky, and the plane flew through it and made a contrail.

So why did it spread out? Well, a contrail is a type of cloud. More specifically it's a cirrus cloud, a high altitude cloud made of ice crystals. It spreads when there is enough moisture in the air for the ice crystals to grow. They grow at different rates, and the larger ice crystals will sink through the air faster than the smaller ones. With a cirrus cloud, this can lead to the "cirrus uncinus" (curly hook) clouds because the wind speed is generally lower as you descend, so the falling crystals will trail behind.


While this gives a "tail" for an individual cloud, when the cloud is a long contrail, you actually get a whole sheet. Have a look at the progression of an individual contrail from the initial narrow trail (starting around 400 feet wide), to a much thicker contrail, often over a mile wide.



Now imagine what would have happened if the trail was only a short segment, passing through a small region of ice-supersaturated air


So we can see how a short segment of a normal contrail can end up like a "rhombus" shape.

 

[Joe]

Are there any actual pictures of the same thing ?

"It's unusual to see the cloud edges quite so well-defined as in this example but the processes involved are very common."

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[Mick West]

Are there any actual pictures of the same thing ?

"It's unusual to see the cloud edges quite so well-defined as in this example but the processes involved are very common."

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As they say, it's quite unusual to get one quite like this. You it's ice supersaturation in a fairly limited region, so the odds are against it. Not something you will see often.

 

[Mick West]

Like here's a pic I took years ago of a broken contrail. However it's much longer, and less dense.

 

[deirdre]

this is totally not the same thing, but for laymen who cant picture such square clouds i'll stick this in, to show it is possible for clouds ( contrails are cirrus clouds) to have very straight edges.

 

[Pete Tar]

The picture illustrating the wind moving the main body of the cloud was interesting because I had thought the wind would be in the opposite direction, blowing the lighter bits to spread out, but it makes sense that the heavier portion gets blown leaving the thinner smear behind.
This may be why the edges are so straight, if it were blowing and spreading out the other way perhaps it would not be so well defined.

 

[Trailspotter]

I've looked for other images of this cloud on the web, almost all of them are copies or crops from a few images taken from essentially the same spot at approximately the same time.

This one taken at a wide angle shows that the rhomboid cloud was rather small compared to other clouds in the sky and could readily be missed if it was not lit by the setting down Sun:

from http://www.dailymail.co.uk/news/art...Encounters-theres-one-shape-magic-carpet.html

The only notably different image I found so far is this:

from http://www.stunning-news.com/a-flying-carpet-spotted-in-england/

The cloud's reddish hue and different proportions indicate that it has been taken later and probably from a different location. The low light highlights the underlying structure of the cloud. The light and dark stripes in the upper corner are more consistent with the cloud spreading along the long side of the rhomboid, rather than along the short one, as proposed in OP.

 

[Pete Tar]

The light and dark stripes in the upper corner are more consistent with the cloud spreading along the long side of the rhomboid, rather than along the short one, as proposed in OP.

I don't follow? There's 4 sides made of two parallel short and two parallel long sides.
The OP proposed it was pushed from the 'long' left to the 'long' right, creating the 'short' sides in the process.
(unless I've pictured it all wrong)

 

[Trailspotter]

I don't follow? There's 4 sides made of two parallel short and two parallel long sides.
The OP proposed it was pushed from the 'long' left to the 'long' right, creating the 'short' sides in the process.
(unless I've pictured it all wrong)

There are two roughly perpendicular directions: in one the plane flew, in the other its short contrail spread. In OP, for illustration, the plane direction was selected as from top-left to bottom-right (or vice versa), whereas the last picture suggests that the plane probably flew in the perpendicular direction, that is, from bottom-left to top-right.

 

[Mick West]

There are two roughly perpendicular directions: in one the plane flew, in the other its short contrail spread. In OP, for illustration, the plane direction was selected as from top-left to bottom-right (or vice versa), whereas the last picture suggests that the plane probably flew in the perpendicular direction, that is, from bottom-left to top-right.

I was thinking there seemed to be the reman of an original contrail along the right edge



This is not that apparent in the second photo, but I still think it's consistent that the trail being there. Here's a color corrected version I made, with what I think is the direction of the plane.

 

[Trailspotter]

I still think it's consistent that the trail being there. Here's a color corrected version I made, with what I think is the direction of the plane.

The rib-like structures, like those in oval, usually occur across the spread contrails, not along them. Also, in the wide angle picture above (#7), wisps of cirrus clouds show the likely direction of the wind, that is, the direction in which the contrail has spread.

 

[Mick West]

The rib-like structures, like those in oval, usually occur across the spread contrails, not along them. Also, in the wide angle picture above (#7), wisps of cirrus clouds show the likely direction of the wind, that is, the direction in which the contrail has spread.

Those look more like ripples to me. There's some more distinct linear features in the bottom half.

 

[solrey]

Those look more like ripples to me. There's some more distinct linear features in the bottom half.

Right, like an undulatus cloud produced by wind shear where the ripples are perpendicular to the wind, same as wind driven waves on a pond. The spreading of the contrail is indicative of wind shear. I see some streaks of the kind that are formed parallel with the wind as well, like short 'cloud streets' which I would imagine the turbulence from the wing tip vortices/pendules might contribute to the cloud street pattern.

http://namesofclouds.com/types-of-clouds/undulatus-clouds.html

http://www.wunderground.com/blog/JeffMasters/cloud-streets

 

[Trailspotter]

Those look more like ripples to me. There's some more distinct linear features in the bottom half.

Here is a picture from my iPhoto archive featuring a spread contrail illuminated from below (the top one):

The ratio of spacing between 'ribs' across contrail to the contrail width looks more like that of 'my' ripples than 'your' linear features.

 

[Mick West]

Consider the feathering though. The bottom left edge is feathered. The other edges are not. Feathering is apparent in the trailing side of your contrail.

 

[Mick West]

Here's another possible square cloud from contrail exampl. Less well define dedges

source: http://sylphsandufocloudships.wordpress.com/2014/05/

And here's one that I thought was not from a contrail, but the accompanying explanation says

The "square" cloud is in fact a mid to high level cloud that possibly may have started as a contrail (a thick one) and that cloud moisture has drifted perpendicular to the direction of the original aircraft.. making for a "straight" edged cloud.

All the credit goes to CBC Meteorologist Claire Martin for cracking the case on this one : )

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source: http://www.cbc.ca/nl/features/nlweather/2008/08/the_weekend.html

Possible contrail:

Source: http://starsimplified.wordpress.com/2010/12/page/2/

 

[Trailspotter]

Consider the feathering though. The bottom left edge is feathered. The other edges are not. Feathering is apparent in the trailing side of your contrail.

The discussion of such fine details would make sense if we had an original photograph in original resolution, not the one that was reduced in size and probably enhanced in some way. My point was that the additional details seen only in the last picture are normally observed oriented across the contrail rather than along it. Before continuing our argument about which direction is which, we should find the cloud dimensions first.

 

[BdeB]

Hello Mick
First of all I love your site.
I'm from the Netherlands and probably my english won't be perfect, so you know
I've read about this phenomena at a dutch alternative media site. The most ridiculous explanations appeared in the comments section. It bugged me that all the pictures were up close, so I searched the internet for other pictures. This is the only one I found from more of a distance.



The upper right looked like a piece of a contrail to me, maybe the same contrail even.
Maybe you can shed a light on this. Contrails are your expertise.

 

[Mick West]

Interesting, here I've cropped and enhanced it:


The cloud in the upper right does seem similar in density to the "square cloud". Hard to say though.

 

[Pete Tar]

Upper left maybe?

 

[Trailspotter]

A quick pseudo stereo image of the rhomboid cloud with the left part cropped from the new image and the right part from the previous one:

It shows that the cloud is not flat, it is slightly bent in the middle along the darker line parallel the short edge and bulges down on the left side.

EDIT: rescaled and adjusted the right part; in stereo the long sides now look even longer. The long right edge is fairly straight, the bulging down increases to the left of it. It is the most prominent at the left side of the cloud, but the right side bulges down a bit as well. This is consistent with Mick's suggestion of the long right edge being the remain of an original contrail.

 

[solrey]

Took these shots just minutes ago of a spreading contrail above the full moon rise. I was out finishing up in the garden and actually saw this contrail and a few others as they were formed overhead. Saw them spreading out and heading toward the east so I grabbed the camera when I finished up. This contrail was somewhere around 30 minutes old +/- 5 min. when I snapped the shots. Direction of travel was from L (N)-R(S), upper level winds were blowing "top" (W) to "bottom" (E). The second pic was taken just a few minutes after the first one.

 

[Bob Solomon]

Here's another candidate if anyone's interested. Not so well defined along 2 of the edges, but there seems to be more than one section.

 

[Trailspotter]

Here is a similar formation that looks more like a flag:

https://www.flickr.com/photos/37423935@N00/846756309/

There are two earlier photos of the same formation in the jeremyhughes' photostream showing its evolution.