Wednesday 25 January 2017

The Salzburg halo

On the 19th October 2013 Zbyněk Černoch was in Salzburg keeping an eye for halo events, since high cirrostratus clouds were rolling in. As they drifted towards the Sun, he first noticed bright parhelia. A few moments later, rather bright and colourful upper tangent arc with Parry arc and upper Lowitz appeared.





What stunned him later was a vivid arc similar to a parhelion, but located in the 18° region.



The arc lasted only a few minutes as the clouds were drifting away quickly. He haven't seen anything like this before.
After a few discussions the observation fell into oblivion. Until few days later when we thought we would like to figure it out once and for all.
It would seem that the arc itself is a fragment of 22° halo, but the halo itself was quite sharp and narrow in this observation.

After Zbyněk put together 3 photos to show that the 22° halo lies between the mystery arc and 22° parhelion, I decided to run some simulations.

I used pyramidal crystals with std of 15° in the c-axis and joined the simulation picture with the collage from Zbyněk.



My theory is that the mystery arc is actually an 18° plate arc. But there are some inconsistencies in the theory.
  1. The color vividness indicates that oriented crystals formed the arc. However 18° plate arc formed by oriented crystals has a typical convex shape which is absent here. 
  2. By increasing the std in the c-axis, halos from poorly oriented crystals are becoming more apparent, which is not the case here. 
  3. No other odd radius halos were observed. 
If it really is a case of an 18° plate arc, it is a really decent one.
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8 comments:

  1. Marko Riikonen26 January 2017 at 01:30

    Have you tried blue-minus-red for the 18° plate arc image? It could pick up some more details, like the 22° halo in between. The br is powerful in cases where the cloud in not uniform.

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    1. Matěj Grék26 January 2017 at 09:16

      I tried it now, the 22° halo is clearly there. No more detail to the plate arc though.
      http://imgur.com/a/MUcB0

      Delete
  2. Marko Riikonen26 January 2017 at 16:11

    Br does wonders with uneven cloud. You might get also better definition on that Lowitz arc with it. But this suppposed 18° plate arc is really an odd man out, it is too vividly colored and the display looks nothing like an odd radius display.

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    1. Matěj Grék26 January 2017 at 22:04

      Yes, i did that too and the Lowitz is really nice.
      I thought so, too.. no other odd radius halos there and it is really too colorful. So I guess it still remains a mystery.

      Delete
  3. Nicolas Lefaudeux28 January 2017 at 12:16

    i have had a handful of such cases with the halocamera, of sharp and vividly colored 18° plate arcs, and in display that do not look like odd radius but indeed are.
    although rare, such appearances are not uncommon, but were also very short lived.
    https://drive.google.com/file/d/0ByI0HyhWSn8Bd3QxYVJiVVluTE0
    https://drive.google.com/open?id=0ByI0HyhWSn8BYjJTc2ZxS3UxeWs


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    1. Matěj Grék30 January 2017 at 22:50

      Wow, okay.. so the chance of it being an 18° plate arc is still there. Thanks.

      Delete
  4. Marko Riikonen31 January 2017 at 05:51

    Such anomalously defined 18 plate arcs probably should sometimes be joined by a similar caliber 23° plate arc. Maybe not at the same time exactly, but a bit later if the cloud happens to move in the right direction.

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  5. Nicolas Lefaudeux2 February 2017 at 06:40

    On what halocamera recorded, 18deg plate arcs were the only bright plate oriented odd radius. No other bright plate arcs were visible, no bright 23deg, neither before nor after (halocam is good in this respect that we can study afterwards what has happened). I agree that this is not what one would expect. Might be a sign of triangular crystals with full bottom pyramid and almost full top pyramid?

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