Thursday, 21 November 2013

Subparhelic circle on snow surface

[gallery columns="4" ids="1543,1542,1538,1555"]

Subparhelic circle is traditionally seen from an airplane in ice clouds below. But it is exceedingly rare. A more productive way of observing subparhelic circle is in diamond dust using bright spotlight as a light source. A patch of subparhelic can be seen also on snow surface. Here I present such a case.

On snow surface subparhelic circle appears as crystal glitter on the sides of observer's head shadow. This is shown in the first two photos, which are different versions of the same peak hold stacked image of 46 frames. A single photo (third image) is not as convincing - snow surface halos have tendency to weaken markedly in single photos which is why stacking is good idea to bring them to the level of visual impression or even exceed it. In the first image the effect is somewhat on par with the visual impression.

The formation of subparhelic circle on snow surface requires two things: ample of sectored plates and strong wind. A dense snowfall of sectored plates in still air is not enough, because crystals falling the snow surface will orient themselves according to the irregularities of the snow surface - that is, in more or less random fashion. However, if the sectored plate fall is associated with strong wind, then the crystals are oriented on the snow surface with their large basal faces in more or less horizontal orientation to minimize the air resistance. This is the crystal orientation required for the formation of subparhelic circle.

Meeting these two requirements seems not easy. I saw subparhelic circle on snow surface first time in Canada's Resolute Bay on 15th March 1999. A couple of weeks later there was another appearance, indicating that the effect may not be that rare in the High Arctic. But I never went back and was from then on trying to see the effect here in Finland. On 15th March 2013 - exactly 14 years after the first observation - there was finally a success. The photos above are from that day when subparhelic circle appeared here in Joensuu.

I first saw a wide sun pillar on the snow surface, as shown by the fourth image. Because in Resolute the subparhelic circle was associated with similar pillar, I knew that in the opposite direction there must be subparhelic circle. The subparhelic circle did not seem completely symmetrical, the right side seemed to curve down more than the left side. The stacked images seem to confirm this slight asymmetry. This is probably due to crystals that had a slight tendency to point their tilted basal face to certain azimuthal direction. One can imagine this happening with a wind blowing from one direction.

In computer halo simulations that use simple hexagons subparhelic circle appears from raypaths that involve reflections inside the crystal. While in airborne crystals subparhelic circle is most likely formed by such raypaths, this probably can not happen in the sectored plates that form the subparhelic circle on snow surface. My suspicion is that in the latter case the subparhelic circle is formed from reflections on the external surface of these crystals. Sectored plates have ridges and depressions that form faces in 90 degree angles on their surface and a reflection from these faces produces subparhelic circle opposite to the sun in crystals having plate orientation. According to calculations by Lauri Kangas, external reflections produce three times fainter effect than what would appear from internal reflections in simple hexagons.

Probably the best places to see subparhelic circle on snow surface are in the High Arctic and Antarctic where strong winds are commonplace and presumably abundant snowfalls of sectored plates are not rare either. In more moderate climates treeless mountain tops could be most productive. In Resolute Bay strong wind had actually collected the sectored plates in large horizontal sheets of crystals. In the Joensuu case shown here, the crystals seemed to be in isolation.

So, next time you are surrounded by sunlit snow, look around. If there is crystal glitter, usually it is uniformly distributed, perhaps showing some circular halos. But if there is pronounced glitter in the form a wide pillar in the sun direction and not much at all glitter elsewhere, turn around and look at your shadow. You will most likely see the glitter of subparhelic circle emanating from the sides of you head shadow.

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