Another occurrence of an anomalous Hastings/Wegener

By Aleksi Velhola and Marko Riikonen

In an earlier post we showed a photo of a weird downward curving patch of Wegener/Hastings. Here is another case that occurred 9 days later on December 2, 2015. This time we also got a nice comparison to normal “Wegstings” by superposing photos taken from the same camera position.

The anomalous Wegstings appeared on two occasions and was well visible to the eye. At both times the display was less brilliant than at peak stages, during which normal Wegstings was seen. Same was true of the earlier display on 23 November.

A closeup of normal and anomalous Wegstings appearances superposed to one image.

The crystal sample is interesting. Crystals range between plates and equidimensionals, which one would not expect from the strong column orientation halos in the display. It just adds to the ever increasing file of cases where the samples are not what would be expected. The dish was out long time, something like half an hour, so it represents all different stages of the display put together. However, column orientation halos were all the time present.

Did near ground turbulence cause the 22° and 46° halos in this spotlight display?

By Marko Riikonen

Crystal samples should be enlightening, but all too often they just make you confused. The observation I made on 22 November 2015 in Rovaniemi is a case on point, although an observation recently published by Alexander Haußmann may now provide a solution.

The issue is about the halos from poor crystal orientations – the strongly developed 22 and 46° halos. One explanation for such halos is that they are born from conglomerates of crystals, which, because of their irregular shape, tumble instead of falling stably.

Yet here the crystal sample did not reveal any compound crystals. Sampling bias is pretty much out of question as such crystals, due to their large size, would be expected to fall faster than single crystals and thus should not be able to avoid the collecting dish.

Another often-heard explanation is equidimensional crystals. They can’t decide which orientation they want to be in, so they tumble too. Looking at the crystals of this display, such an explanation feels tempting.

But then we have other displays where the crystals are pretty equidimensional and yet there is no strong random halo component. Moreover, evidence has been building up that equidimensional (or even slightly plate like) crystals of triangular habit can take Parry orientations. So what’s going on?

A diamond dust display photographed by Alexander Haußmann last winter at Mt. Klínovec in the Czech Republic suggests an answer. In the beam of car headlight there was visible Moilanen arc, but photographic stacking of the video frames revealed additionally a bit of a circular “Moilanen halo”. No such thing has been known before, which talks of the high stability of the Moilanen arc crystals. That it was seen nevertheless, Haußmann thinks, was due to strong wind, which near the ground became turbulent and destabilised the crystals so much that even Moilanen arc crystals were thrown off-balance.

As the wind was exceptionally strong also in Rovaniemi display, it could in a similar manner answer for the strong 22 and 46° halos seen in the photo above. Just like in the Czech display, the light beam was close to the ground, so turbulence had a chance to randomize the crystal orientations. And because the crystals were equidimensional and quite large, that would have served to kick them out of balance with ease.

All rare halos are missing in this spotlight display, but why?

By Marko Mikkilä, Jarmo Moilanen and Marko Riikonen

The mystery deepens. In two previous posts we wondered why some displays are great in their column orientation halos even though the crystals have well caved ends. Here we show a case that appeared on November 22, 2015 in Rovaniemi, where crystals seem not much different, yet rare halos requiring basal faces are completely absent. Even the 46° supralateral arc gives just a whiff. Poor crystal orientations can’t explain the absence of rare halos as the tangent arc is quite sharp. Had we known only about this display, we would be quite happy to explain with cavities, but knowing about the other displays, it is quite puzzling.

Solar diamond dust display with 87° arc

A stack of 40 photos. An average stack has been combined with maximum stack to show the crystal glitter of the 87° arc. The photos were taken during ~2 minutes. Sun movement has not been accounted for.

By Jarmo Moilanen and Marko Riikonen

The diamond dust season is soon to arrive in Finland and it is time to wipe the dust off the equipment. In a meanwhile, here is the last winter’s starter for Rovaniemi, on October 30 2015. The temperature during the display was -5° C, a guaranteed number for great stuff.

So, what do we have here? First of all, visually the upper tangent arc was a breathtaking sight. In addition to its brightness, the myriads of moving crystals made it “swarm”, as if it were alive. The subhelic arc was also intense.

Then take a look at the zenith. There is an intensity threshold very familiar from spotlight displays, which we have been calling the 87° arc and which is made by 357 raypaths in rotating columnar crystals of triangular habit. Perhaps surprisingly, this is only the first time it has been observed with traditional light source. Most likely it would have been observable in some earlier photographed solar displays, but usually the photos don’t offer the luxury of all sky views.

We spotted 87° arc first visually, which we were quite happy about, as often faint effects only turn out from photos. Sun was behind the hill and not shining where we were standing, but some way up the crystals were lit. We saw crystal glitter on the sun half of the sky, cutting off abruptly at the zenith, leaving no glitter whatsoever on the opposite side.

Simulation and two versions of the stack which has 50 photos taken during 2 m 31 s.

Another feature of interest is the dark band between the Parry arc and the 46° halo. A simulation with HaloPoint having four populations of columnar crystals reproduced it quite well. Its formation is contributed by the 87° arc, 46° halo, Parry and 22° tangent arc. Crucial was making the area between Parry and tangent arc to have plenty of light by giving the four populations a continuum of limited rotations. Also, to enhance the upper edge of the gap it was necessary to keep 87° arc from extending inside 46° halo. The triangular column population with 10 degrees rotation did the work (the uppermost active population in the parameter table – number 3). Fully spinning crystals would have extended the 87° arc all the way to the sun.

Assuming the live crystal sample of this display is representative, we see that most crystals have end cavities and yet the subhelic arc (that uses both basal ends) is striking. According to the traditional picture, hollows are bad for halos, but this is not the first display to shake that belief, and actually simulations with column oriented crystals by Nicolas Lefaudeux in 2011 using idealized hexagonal cavities boosted the subhelic and particularly the Tricker arc. But then there are column displays where cavities seem to do their expected work and all rare halos are absent (we will post an example later). So it looks like it might not be a question of whether there are hollows, but rather of what kind of hollows there are.

Or maybe in nature the cavities are always bad, but in this case there were enough optically high quality crystals to make the strong subhelic arc. After all, not all crystals in the sample have cavities or the cavities are so small that their effect is negligible. It is also possible that there was more high quality crystals in the display than the sample lets us know – if they were small, we don’t see them much in the collecting dish because larger crystals have faster falling velocities and will dominate the sample.