Kern with subhelic arc in the UK

Berkshire (UK), 28 April 2019: Top left corner shows what's left after applying the gradient subtraction and stretching of histograms on a 38-frame average stack. Top right and bottom left, respectively, are with additional blue-minus-red subtraction and colour-channel enhancement. A HaloPoint simulation is included for reference at the bottom right corner.

While relaxing at home in the evening of a mostly cloudy day, a ray of light caught my eye and I checked the halo situation. In fragmented cirrus there was a mediocre circumzenithal arc (CZA) but not much more than that (my view towards the setting Sun is not that great). I took notice of the Sun being low in the sky so odds for Kern arc were on the rise: in the next moment I was setting up my DSLR to get some photos from my backyard.

CZA disappeared soon after I started shooting the first set of images, but it came back a few minutes later so I chose to do a re-run. Fortunately so, as the latter set turned out fruitful indeed in the post-processing.

The processed stack indicates presence of a faint Kern arc as an extension of CZA at the left. Slightly further to the left, there is a white arc that best matches the subhelic arc in the simulation. I think there is also a subtle suggestion of helic arc alongside of the subhelic, but this is less convincing. Furthermore, there are colored patches below the CZA approximately where 46° contact arcs would appear if there were Lowitz-oriented crystals in the mix. The simulation shown above is a synthesis of three distinct populations, including singly-oriented plates, singly-oriented columns, and Lowitz-oriented plate crystals.

Odd radius halos with Wegener and subhelic arcs in the UK

A short-lived display of relatively high quality occurred in Berkshire, UK, in the morning of 4th March, 2017. A routine check at 9:35 local time suggested the presence of faint 18° and 23° parhelia, in addition to the usual 22° stuff, so I collected my camera, tripod and blocker and set up the system in a nearby park area. After just 25 minutes the display was all over, but there had been enough time to capture three 30-frame series for stacking.

In addition to the 18° and 23° parhelia, both visible to the eye during the display, the processed stacks suggest parts of 20° and 35° (and possibly 24°) circular halos. Unfortunately, the region of 9° halo is over-exposed in my photos. However, as far as my visually-based observations are to be trusted, there simply were no halos 9° from the Sun.


Given that we are dealing with a cirrus display here, I find the presence of both supralateral and infralateral arcs indicative of fairly ideal column crystals. This is confirmed by the side-view stack, as faint Wegener and subhelic arcs can be identified. The latter crosses the parhelic circle near the tall tree at the bottom. Just slightly further away from the Sun, there is a 120° parhelion too.

The great 24 February 2017 halo display in Pskov region of Russia

In the evening, I looked at meteoblue's meteogram (like I do everyday), and saw on it plenty of high-level clouds, but at the same time together with middle-level clouds. At 09-30 I woke up that to check whether middle clouds would not allow observing halos.

I came out and saw that the sky indeed was pretty dirty and contained not only cirrus but also low and middle clouds. But above the sun were the central part of UTA and diffuse pillar. I could leisurely to prepare my camera before halos became brighter.

After a few minutes, UTA became seen pretty well. Viewing the sky I was very surprised to detect the supralateral arc to the left of the sun. The arc was not bright but it was clearly visible. It was weird to see such good halos in such dense clouds. Clouds from different levels merged with each other. So at first, I did not understand that it was a big display that penetrated through the gaps of the lower clouds.

After 10-15 minutes came a big gap in those clouds and I saw the full Supralateral arc which was noticeably brighter than previously. I already was very glad because the arc in this form I saw only on my screen when I processed stacks. And at the moment I saw it in the sky with my own eyes. However, this was only a prelude of what was to come.

Wegener arc, subhelic arc, and blue spot on parhelic circle

Around 11-00 the sky started to clear and above the supralateral arc and a nice circumzenithal arc appeared. In that moment I noticed to the west a bright fragment of the parhelic circle which was low over trees. Cirrus arrived from this direction. While I changed the direction of my camera's field of view, I noticed also the anthelion. High clouds arrived quickly, and they were very uniform and smooth. The brightness of halos quick started to accrue. I ran in my home in order to show the current display to my father, and during this couple of minutes the brightness of halos increased even more. I ran back to my camera, and in this moment halos became extremely bright! The upper tangent arc was very perfect and sharp. The supralateral arc was brighter than a common primary rainbow! Probably it reached such extreme brightness not only because crystals were perfect, but also because the sun elevation was almost optimal. The parhelic circle was also incredible and looked like jets of light. Wegener arc was visually visible as well. Its oblique wide lines pointed out on far and weak anthelion. Around anthelion from time to time X-shaped arc was visible.

Near sun there are 9° halo together 9° column arcs

All sky view. Near anthelion there is Tricker arc

Anthelion and some of diffuse anthelic arcs

Upper tangent arc with diffuse upper 23° plate arc and sharp 24° column arcs. Thanks to M.Riikonen for the confirmation

Halos lasted in this bright form around 45 minutes. After the brightness of complex a few decreased, but all details continued were visually visible. I remember that I saw well crossing of Supra/Infra-lateral arcs on the parhelic circle. Also from time to time appeared 120° parhelia.

On the left there is 120° parhelion

At 13-10 was the end of this great display, only halos near the sun remained. Parhelia were most bright in this moment and seemed to me that near them were Lowitz arcs. But stacks showed that is not true.

Thin plates in rotating Parry orientation as an explanation for thedisplay on the night of 8/9 November 2016, in Rovaniemi

In an earlier post I told simulation attempts were not successful for this display. Well, I really did not put that much effort into it. Now I have given it a fresh look and managed to get some success.

The problem was the subhelic arc and anthelic arcs that could not be get rid of. In new simulations made with HaloPoint the subhelic arc issue is pretty much resolved and the anthelic arcs also play it low key.

As usual, the simulations do not stand scrutiny of details, but that does not matter regarding main message: that it was necessary to use thin plates in rotating Parry orientation to keep the subhelic arc and anthelic arc stuff in check.

Two simulations are shown above together with the photo. They are identical except that in the other the thin Parry crystals are rotating 15 degrees and in the other 5 degrees. The subhelic arc is actually in there, but it is masked by background noise from the random population. Well, maybe its curve can be detected in the 5 degree simulation, but it is shadowy. It becomes clearer with more burn and finer dot.

The simulation with 5 degree rotation replicates also the diffuse area of light seen above the Wegener arc in the image. It is a spread out helic arc. So maybe we could regard the Wegener rather as an intermediate form between Wegener and Hastings, the “Wegstings”.

If thin plates indeed are the culprit, how can they fall with their basal faces vertical? If we maintain that singular plates can not knife through the air in such orientation, then perhaps there were copious 90 degree crosssed plates in the air, built so that one was glued to the center of the other one’s basal face to provide the right balance for the required orientation. I did not take crystal samples, with one camera it is too much a hassle, particularly as we have again and again come to learn that samples rarely give answers.

Parameters for the other simulation in HaloPoint software. Shown is the thin plate in rotating Parry orientation.

 

 

 

 Finally, a technical factor could have influenced the subhelic arc visibility in the photo. I had placed the camera slightly outside the center of the beam sideways to enhance the sides of the display and this may have had a dampening effect on the visiblity of the higher located subhelic arc. The effect is probably not significant, the camera offset was just a little, but in the future this practise must be dropped in order to get best comparability with simulations (of course, as the spikes at the anthelic region attest, my beam is not regular anyway, so to some extent this issue is there aways). Off the record, I don’t really think that this could have made subhelic arc disappear, rather the opposite, because when the camera is right in the center of the beam, you have more masking bright glitter than when the camera is offset and any threshold intensity halos should be then lost easier. But it is good to let this issue have known.

A pure breed uppervex Hastings

In snow gun diamond dust displays Parry orientation is often strongly emphasized in relation to column orientation. There may be no signs of column orientation at all, except for perhaps a slight tanget arc brightening on top of 22° halo.

Such displays have made observers to ask themselves whether the uppervex Hastings arc – the Hastings arc component that is touching the uppervex Parry arc – could be sometimes be obseved in addition to the usual Wegener, or even without it. So far displays where light source elevation allows separation of uppervex Hastings and Wegener have not resulted in any candidates.

However, in spotlight displays where lamp is at or below the horizon, we have managed to photograph during the last and this winter a several of cases that are suggestive of an uppervex Hastings. And not even suggestive, but outright assertive.

The uppervex Hastings intensifies with lowering light source elevation, only complication is that it then starts to overlap with Wegener and identification must be done on the basis of other halos in the display. If Parry orientation halos are prominent and column orientation halos such as 46° lateral arcs, diffuse arcs and subhelic and Tricker arc are very weak or absent, then we might say that the “Hastgener” in the display is indeed Hastings arc.

As an example of a display that leaves little doubt of its Hastings nature, here is shown a one that Marko Mikkilä observed this year, on the 2nd January in Sievi. The lamp was resting on the ground on a rather level field and is according to Mikkilä about 3 degrees below the camera.

We see no evidence of column orientation – no 46° lateral arcs, no Tricker, no subhelic arc. Instead, in the image there is an overwhelming helic arc and Tape arcs, which both are solely Parry orientation born. Thus the faint “Hastgener” must be an uppervex Hastings arc, rather than Wegener.

Above is a attempt at simulating with HaloPoint. It supports the Parry scenario untarnished by columns, even though the details may not be quite right. Perhaps the most striking mismatch is with the subanthelic arc, which is bright in simulation but seems to be missing from the photo. However, there is a darker area which looks as if shaped by subanthelic arc. This seems to be a real effect, we have seen similar looking dark voids inside anthelic arcs before and in an upcoming post I will be showing a good example of such darkness associated with this halo. Perhaps the more intense parts of subanthelic arc are outside left outside of Mikkilä’s photo, although in that case it should have been possible to make a matching simulation. I actually did find an option which was better in that respect, but other parts got too wrong to take it seriously.

An earlier display in the same location by Mikkilä seems to also contain a pure uppervex Hastings.

A distinct Wegener but other reflection halos from column orientation lacking

By Marko Riikonen

Spotlight displays are great in that almost every time you photograph them, you realize you understand halos less and less. This time the puzzle is: Why Wegener in the image above is so strong in comparison to other reflection halos? No subhelic arc is visible and neither there seems to be diffuse arc – I think the spikes at the subanthelic point are lamp artefacts. Of course I can’t not say that for sure, but around the subanthelic point even weak stuff shows up easily to the eye, so had there been diffuse arcs, I should have noticed it. If we accept this, then, in addition to the Wegener, the only suggestion of column reflection halos is what looks like a short patch of Tricker arc cutting across the sub-Kern arc (see the simulation below for comparison).

As for the posed question, I don’t have an answer. In simulations, using plate like column oriented crystals weakens subhelic and diffuse arcs in relation to Wegener, but they are still well visible, as shown by the simulation in the image below, which was done, I think, with h/d 0.3 plates.

Of the other stuff worth a mention in the image – which is a total of 27 min 20 sec exposure – is the sub-120° parhelion. It is quite common in spotlight displays, but to see it one typically has to run alonside the beam. This time it was clearly visible while standing still. There is also a blue spot. At 9 degree elevation it shares exactly the same location with 120° parhelion, which may be there too.

Another photo taken a bit later and at another location, shows a dark circular void at the zenith (or nadir, if you look from the point of view of halos), as shown below. A much more striking instance of this effect was captured by Jari Luomanen in 2013.

This night of halo hunt started on 8 November at 10 pm and lasted until 5 am. The development of the diamond dust quality was a slow and hopeless decline, which happens quite often when temperatures drop towards -15 °C. That is considered by halo hunters the worst temperature, giving crappy displays. Unfortunately the temperature notes were lost as my halo hunting diary got corrupted and shows now page after page ####s instead of letters. But I think it was between -11 and -14 °C through the night.

Subhelic arc crossing at subsun (and some other stuff)

By Marko Riikonen

The four kaleidoscopic arcs carry in their name the location on the celestial sphere where their loops’ cross. For three of these halos – helic arc, Tricker anthelic arc and subanthelic arc – there exists photos showing the crossing.

But I knew of no such images of the remaining member of the quartet, the subhelic arc. So on the night of 5-6 November 2015 it was good to give it a try as diamond dust happened to form in an area where it was possible to place the lamp low.

It worked, although I must admit that there is really no actual cross to talk of as the subhelic arc sort of vanishes near the subsun. The display was better earlier, but then I was busy photographing at another spot nearby with less relief. By the time I moved the gear to the ski jump on the slope and took the photos for the stack above, the swarm had already lost its edge.

So there is room for improvement coming winter. Or maybe someone will photograph subhelic arc crossing from an airplane next time – just like the sub-120° parhelion and the missing segment of the subparhelic circle were photographed soon after their spotlight discoveries.

Subhorizon view for 33 degree light source elevation with regular hexagons in column orientation (h/d 1.5 dev 0.5). Simulation software: HaloPoint

At the ski jump I also switched the places of camera and lamp to see how the display looks on a positive elevation. Two photos are shown here, the upper one seems to be for a little less than 30 degree elevation and the lower one for a little less than 20 degrees.

Finally, the photo below shows the display when the diamond dust had just formed. Just like here, this is often the best stage, so it pays to come early and wait for things to start. Further below are the crystals for this stage. The marked cavities are worth noting. Maybe that is why I was not able to get a satisfying simulation. The sample also contained frozen droplets straight from the snow guns – the machines were just a couple of hundred meters away.

The temperature dropped from -2 to -6° C during the time I was out (from 10 pm to 7 am). I did not take note of the temperature when the action started, but it was lower than -2° C. If there is anything at all seen before it drops down to -5° C, things tend to be very unstable, switching quickly between ice and water fog.

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.

Wegener or Hastings?

This spotlight display that was photographed in Rovaniemi on the night of 5/6 January rises the question of whether it is Wegener or Hastings arc or both together that is seen here. From the outset, as there is helic arc but no subhelic arc one could argue that it is rather the uppervex Hastings arc.

When comparing with simulations (below) it does not look that obvious anymore. The curvature and extent of the arc looks more akin to Wegener than Hastings (the side view image). And it is possible to simulate Wegener without subhelic arc by using thin plates in column orientation. Indeed, to have best agreement with the images, in all simulations thin plates were used both in Parry and column orientation.

In the image on the left the crystal h/d is 0.1 for all simulations. In the one on the right it is h/d 0.03 for rotations of 0.5 and 5 degrees, for full rotation simulation h/d 0.1 was used. Crystals are triangular, oriented in Parry orientation so that the tip is pointing up.

Well, none of these simulations come out satisfying. The light source elevation for simulations is -5 degrees.

Marko Riikonen / Nicolas Lefaudeux

– added one more image with simulation containing both Wegener and Hastings

Diamond dust halos on the night of 12/13 January, part II

More photos from the 12/13 January night. The image above is a view opposite to the spotlight. Seen is blue circle, diffuse, Wegener and subhelic arcs. Below are two more images, the blue-minus-red image shows the “column 351/361”, which is the Kern arc equivalent of 46° supralateral arc. The lamp is about 5 degrees below the camera.

Marko Riikonen

More photos from three weeks ago in Rovaniemi

It seems putting the photos of the recent displays to my web page will delay. So, in a meanwhile here are two more photos of the display three weeks back.

When light source is at 0° elevation, some difficulties arises in identifying certain halos. Tricker arc and subanthelic arc merge together, as do helic and subhelic arcs. However, the subanthelic arc is formed by Parry-oriented crystals, while Tricker arc is from singly oriented columns. Because in this display the 46° supralateral arc is rather strong, and there are only weak indications of 46° Parry or Tape arcs, probably singly oriented columns were in command. The even brightness of the Tricker/subanthelic loop is also typical for Tricker arc. Subanthelic arc has strong brightenings on the sides - at least in the simulations - and that is actually what seems to be present in the the display of last weekend.

The reason why halos are seen so strongly in the halogen light is probably the relatively dark background sky as compared to sun or moon situation. Yet all the halogen lamp displays that I have photographed so far have been hampered by city lights. Once the crystal cloud drifts to the dark outskirts of the city, more intense appearances should be expected.

By Marko Riikonen