Dark arc associated with cirrus ice cloud

Ian Jacobs

New Member
The halo (photographed in Bangkok) is probably a circumscribed halo (at 22 degrees) that is due to a predominance of longer hexagonal crystals that are falling horizontally.

The feature that I do not understand is the dark concentric circular arc at four-five o'clock at about 31 degrees. It persisted for about five minutes. Light is returned to the viewer from randomly oriented crystals outside the halo but why is this light interrupted at 31 degrees?

Has anyone seen or photographed similar?

Can anyone suggest a crystal shape and/or orientation that would produce this effect?


Three images taken over the course of a minute follow:




 
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Mick West

Administrator
Staff member
Here's a detail enhanced version:


Halos are formed by refracting light so in order to darken a region, the light is being redirected elsewhere. There's a dark region just inside the normal halo, and that light is being redirected to the brighter colored ring. But as we can see here there's no evidence of this redirected light in the "arc". This suggests some options.

1 - Total internal reflection in ice crystals sending the light back towards the sun. This does not seem possible.
2 - A shadow of something on the cloud layer, either:
2a - A shadow of a contrail
2b - A shadow of some random clouds
3 - A arc-shaped gap in the clouds, not dark, but simply showing the sky behind, either
3a - A gap cause by a plane (a distrail)
3b - Some random gaps that happen to line up
4 - Some random combination of 2 and 3
5 - Something else.

Looking zoomed in in more detail initially suggests to me it's option 3, and most likely 3a, a distrail.

You say on Flickr that the movement is independent of the cloud layer (which would then suggest a shadow), and this does seem to be the case with the lower cumulus fractus, but it also seem to me to be moving with the more indistinct, partially rippled layer.


The size of the dark areas is not consistent with a fixed shadow. The cloud region where the arc is seem to have three "fingers" of cloud pointing towards the sun, with darker anti-rays in-between. To my eye, the structure of the dark line moves with the cloud region. Although it's quite striking that it seems to remain centered on the sun over all three images.

Here I've added four tracking dots to suggest the movement of the arc with the cloud region:

Note the red dot, immediately to the left of it there's a small dark region, this seems like it's part of the arc, but then moves with the cloud.

So I would tentatively side with it being a coincidental distrail of a plane making a turn.
 
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Ian Jacobs

New Member
Thank you for your thought and effort on this one.

I agree that there does not appear to be any way that light can be diverted or obscured by ice crystals to form a dark halo. There are to my knowledge no images showing that ... among the thousands upon thousands of halo images made in the last two decades that have confirmed the existence of rare arcs and filled in the blanks with photographs of halos that could be simulated but had never been seen.

Distrails are not uncommon here ... I have photographed about 20 and seen three times that number over ten years. They occur in cloud at all levels, but at low levels in stratocumulus and altostratus they often fill in with condensation over a few minutes so are fleeting and not seen as distrails unless they are regularly looked for. I have not seen any examples of curved distrails (other than possibly this one) but I have seen a few (less than ten) curved contrails.

I agree that there is a different quality about the obvious stability of the halo in the clip and the weaker apparent stability of the arc in the three images. Now given that the most likely explanation is a distrail in a wave-cloud layer intermediate between the upper ice cirrus layer and the lower wisps of cumulus (the apparent rays in the layer do not actually point towards the sun) what are the chances of that coincidence? And the quite different question: what are the chances of seeing that again?

To answer the second question first. Vanishingly small. Halos like this occur here for an hour or so at most twice a year and I have not seen another curved distrail, let alone one that appears to be circular (or very nearly so) and centered on the overhead sun.

The first question is deeper. Unspecified coincidences happen quite often. If it were not so, poker would not be such a dangerous game because two or more very high hands would almost never occur at the same table at the same time. The triple coincidence here with the presence of the halo, the curved distrail, and the apparent centering on the sun is a stretch of the imagination, but not actually impossible.

I expect these images to remain unique.

Best regards ... and thank you again for your thought and effort.

Ian
 
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Mick West

Administrator
Staff member
The first question is deeper. Unspecified coincidences happen quite often. If it were not so, poker would not be such a dangerous game because two or more very high hands would almost never occur at the same table at the same time. The triple coincidence here with the presence of the halo, the curved distrail, and the apparent centering on the sun is a stretch of the imagination, but not actually impossible.

Indeed. Extraordinary unlikely coincidences are often though to signify something extraordinary or meaningful, when really all they signify is that this happens to be the time the numbers came up.

While I would not entirely discount the possibility of some anti-halo, an unlikely coincidence seems more probable.

Unlikely things happen all the time. Things entirely new to science are observed less so.
 

Pete Tar

Senior Member.
It might just be an illusion due to less reference points, but it doesn't seem to track completely parallel all the way, it looks like it'll eventually go wider than the inner arc, meaning it's not part of any halo system. It might need a grid or measurement overlay to check that properly though.
 

Ian Jacobs

New Member
It might just be an illusion due to less reference points, but it doesn't seem to track completely parallel all the way, it looks like it'll eventually go wider than the inner arc, meaning it's not part of any halo system. It might need a grid or measurement overlay to check that properly though.

Thank you.

I thought I was done with this one ... but on reflection .... I followed the instruction to put up original images and neglected to correct for rotation of the hand-held camera. For which I apologise. If you look at the clip of three images above you will see the edge of the building overhang rotating from image to image. Note also that the cloud field in the clip rotates rather than being in simple translation. I have uploaded three images corrected as nearly as I can for rotation and the effect is a little different. The upper cirrus layer now seems to me to move as a whole through the halo and the arc? The curvature of the arc seems to be maintained from frame to frame but perhaps the separation of the two may vary just a little? ... which would mean that it is not part of the halo system.

Corrected for camera rotation (as far as I am able).




Thoughtfully

Ian
 

Trailspotter

Senior Member.
I thought I was done with this one ... but on reflection .... I followed the instruction to put up original images and neglected to correct for rotation of the hand-held camera. For which I apologise. If you look at the clip of three images above you will see the edge of the building overhang rotating from image to image. Note also that the cloud field in the clip rotates rather than being in simple translation. I have uploaded three images corrected as nearly as I can for rotation and the effect is a little different.

The most of people here, who live in moderate latitudes, are not accustomed to the Sun being right overhead. At the date and time of the photos the Sun's elevation in your place was near 80°, so it is not surprising that you have rotated camera a bit between the shots. I often unintentionally do the same when follow an object above my head.
 
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