Monday, 29 January 2018

Surface Halo Arcs on Icy Puddle

Surface halos were observed on frozen puddles by Hungarian observer Dávid Hérincs. He photographed the halos on the morning of 25 January at about 10° Sun elevation. As he reported, with naked eyes he could only see the glittering of ice, the halos came out only afterwards in the photographs. Besides the subsun and subparhelia, sharp halo arcs were also photographed which look like lower tangent or Parry arcs. Can anyone provide some explanation for what exactly these are? More images.

Photo of a similar phenomenon by Dávid Hérincs from 30 December 2016.


  1. Ágnes told me the sun elevation was about 10 degrees. So here is a simulation made with azimuthally locked crystals for the first photo:

    The horizontal plate crystals are tilting in an azimuthally preferential direction (120 and 240 degrees). For comparison a simulation for Parry scenario is given.

    We have before had evidence of azimuthally oriented crystals on ice surface in the form on one sided subparhelia. This one, though, needs much more precise azimuthal locking and is in the same class with those curving subsun tails that are sometimes photographed from an airplane. I can sort of understand how they form, but how in crystals grown on a surface?

  2. Thanks for the comment and the simulation. It is indeed a very interesting phenomena, and maybe not too rare also because I saw similar surface halos two times since then.

    Firstly on 15th February with 24-25 degrees sun elevation.
    Some members from the Hungarian atmospheric optics (AO) group said that these are probaly a subhelic arc (the X) and a sun pillar (in the middle).

    The second case occurred on 21 February, and this was similar with the January event (the left-side subparahelia was also visible but this picture do not show it).
    I saw a colorful subCZA-like spot in the region under the subsun too, which seemed connected to the arcs that come from the subsun.

    The forming of the ice crystals is questionable to us (Hungarian AO group) too. These halos are visible only on ice tables which do not contact the water under it directly, so there are air between the ice and the water (or the ground). This trapped cool air may help the forming and sustainment of the crystals. For example, I often see much intense rime growth around small holes on these types of ice:
    Someone in the AO group mentioned that there are crystals on the bottom side of the ice table too. She looked a table late in the morning and did not see surface halos on its top page (the crystals possibly melted by that time), but after she put up a piece from the table and turned it, she saw the subsun and the arcs which crossing it. Additionally, it seems that the outdoor temperature (on the meteorological standard 2 meters) do not play important role in the formation of the crystals because when I saw the halos, the temperature varied between -8 and -1 °C.

  3. Now I'll be... Wonderful stuff. But I am out of ideas. Probably not azimuthally locked crystals after all. Next step would be to take good close ups of the crystals as they lay on the surface. Also, have you looked all over, possibly something is seen in the opposite direction, around your head shadow? In any case, I must look for this when more springly weather arrives to Finland.