A major spotlight display with possible 4th Tape arc component

45820_55a89ddab0d39973aebc6b8f6944b7ed On 23 November 2015, we were watching diamond dust halos develop under overcast skies in Rovaniemi. As we stood on a rectangular field a couple of hundred meters across, we followed halos slowly gather momentum in the spotlight beam, reaching climax when clouds were cleared away for a short while – and revealing at the same time also a lunar display. Here is an excerpt from Marko’s observation log written the next day:

„The display just adds gear. We are looking at beautiful subanthelic stuff, subanthelic arc, diffuse arc… It becomes monstrous when the cloud almost disappears. That is when we get also a moon display with full parhelic circle. No one seems to be in a rush to photograph the moon display. The beam display is sheer grotesquerity. The laser scapel sharp, 100% pure glitter of the tangent arc and uppervex Parry.“

In the photos the lamp is 5-6 degrees below the horizon. In the image above, the most noteworthy feature perhaps is the slight intensity enhancement in the 46° infralateral arc, marked with arrows in the simulation below. These could be sings of the fourth, as of yet unobserved Tape arc component (raypath 14), which is highly unlikely to be ever seen in solar displays as it appears in simulations only when light is several degrees below the horizon. Although it is probably here, we would still hope for a better example from lower light elevations that would make it easier to identify.


The poorly oriented crystal population that would make the 22 and 46° halos has been intentionally left out of the simulation. Below are parameter for the simulation. Simulation software is HaloPoint.

parametersAnother feature of interest is the „Ounasvaara arc“, which in the photos looks like an odd continuation of circumnadir arc. This Parry orientation halo is visible particularly well in the two photos below, that, unlike the photo above, were taken off-beam, from underneath. Although off-beam images are not visually as appealing for their non-uniform lighting, the halos in the brightest part of the beam are enhanced in them.

45820_91e1a2edd555970e4dbc81658c309b0845820_5ac8e0d1f35f13e2c2418cc0b61deb4dWe regret of not getting a visual of the Ounasvaara arc – judging from its intensity, it should have been easily visible to the eye. Well, something for the next time.

As usual, we also had a gasoline filled petri dish out to collect the crystals. No serious end cavities this time, which is in accordance with the observation log’s note of „100% pure glitter“.

45824_b43005ac0d8df02c06523c2195adddebAt last we switched off the lamp to give the moon display the stage. However, by the time we got to it, the lunar halos were already starting to decline. In our next post we will show nevertheless the few photos that were taken.

Jarmo Moilanen, Marko Mikkilä, Marko Riikonen


An occurrence of anomalous Hastings/Wegener

45820_e662e364876303651b6b4fbc10d66a83The image above shows what looks like a patch of Wegener or Hastings on top of the 22° halo. But instead of having the usual horizontal curvature, it is bending slightly downwards. Because of the view angle, though, the effect is not as evident as it could be. Anyway, if it were standard Hastings or Wegener, it would curve steeper up in the photo.

We have no idea how it formed, our attempts at simulating have come up empty-handed. The display was seen in Rovaniemi on 23 November, 2015, and the arc appeared at a stage when the display was still progressing to reach its peak.

Nine days later, in the beginning of December, we got another, better sighting, suggesting it is not exceedingly rare. In a similar manner, it did not occur when the display was at its best, but when the display was undergoing a momentary low. We will post about this later.

Jarmo Moilanen, Marko Mikkilä, Marko Riikonen

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

45781_ac42cf0de30dd7fde036ccaf578c1af0Crystal samples should be englightening, 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 instabilized 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.

Segments of a circular halo from Moilanen crystals observed Nov 27th, 2015, on Mt. Klínovec (CZ)

During last year’s meeting of the German halo observers, we decided to drive on top of Mt. Klínovec (Keilberg) after dinner on Nov 27th, 2015. We used the car headlights as light sources for glittering diamond dust displays from ice crystals within the first few meters above the ground, while facing temperatures in the range of –5 °C to –6 °C at wind speeds of 5 – 6 bft. Simultaneously, there appeared a non-glittering, but slowly changing moon halo display in crystals higher up, including a “traditional” Moilanen arc:

2015_11_27_2003_30s_imgp3912_usm(20:03 CET, unsharp masked, for the original image see here)

In the light of our artificial sources, we could make out several of the common halo types, but, moreover, from time to time some rather prominent glittering at about half the radius of the 22° halo. None of us could visually recognize the definitive shape of this arc. On common long-exposure pictures it did not appear very prominently due to averaging out the glitter which attracted the visual attention:

2015_11_27_2016_1s_imgp3921(20:16 CET, exposure time 1 s, another 1 s exposure from 20:11 can be found here)

Luckily I also had recorded a couple of real-time videos (at 25 fps), and this material allowed me to carry out a somewhat unusual analysis method: I extracted the individual frames and re-combined them to maximum stacks in order to preserve the “glitter enhancement” while exploiting the advantage of a longer exposure time. As a result, clear pictures of a circular arc of about 12° radius inside and concentric with the 22° halo were obtained. However, no “lower half” of this halo could be recorded:

2015_11_27_2013_imgp3919_maximum(combination of 262 frames from 20:13 CET)

2015_11_27_2017_imgp3920_imgp3920_maximum(combination of 141 frames from 20:17 CET)

2015_11_27_2031_imgp3931_maximum(combination of 450 frames from 20:31 CET)

For comparison, the source videos can be viewed here (1, 2, 3). I also calculated conventional average stacks (1, 2, 3, gamma value increased to 1.5 for better visibility) from these 6 s – 18 s long sequences. As expected from the conventional long-time exposure photos, the 12° halo segments are completely washed out in the average stacks.

It seems quite likely that the observed segments belong to a “Moilanen circular halo”, i.e. the result of poorly oriented crystals with 34° wedge angles. The moon display proved that such crystals were around, and all halos were caused by a rather thin diamond dust cap wrapping closely the top of Mt. Klínovec, as illustrated by pictures taken from the nearby Mt. Fichtelberg. Near-ground turbulences generated by the rather strong wind will have caused the orientation randomization in the headlight display. If this reasoning turns out to be correct, the correspondence table between the Moilanen and 22° halo families can be completed in the following way (the first two lines were suggested by Nicolas Lefaudeux, as reported in Marko Riikonen’s “Halot”):

Moilanen arc ↔ Parry arc

Mikkilä arc ↔ upper tangent arc

this halo ↔ 22° circular halo

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

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 tanget 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.

Marko Mikkilä, Jarmo Moilanen, Marko Riikonen



Subhelic arc crossing at subsun (and some other stuff)

45512_591a70a2f38bf2de24bc1b28ceaa0b8cThe four caleidoscopic 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

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 snowguns – 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, swicthing quickly between ice and water fog.

Marko Riikonen

The lamp is 9 to 10 degrees below the horizon.


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.

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. The temperature during the display was -5° C, a quaranteed 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.

20151030_rovaniemiAssuming 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.

dsc_0874Or 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.

Jarmo Moilanen, Marko Riikonen


AKM’s Circumhorizon Arc Seasons 2015 & 2016

Starting in 2015, inscribed members of AKM’s forum were invited to post photographically documented circumhorizon arcs (CHAs). The reach of this mostly German language website and the geographical limit for CHAs at ca. 55° N, restricted the incoming reports to Germany and its surroundings to latitudes between about 47° N and 52° N, in practice. The possible maximum solar elevation angle for these places varies between 61° and 67°, thus not quite attaining the brightness optimum for CHAs, which comes at a little less than 68°. [1]

On the other hand, it has been found that some of the aesthetically more pleasing CHA apparitions occur at 65° solar elevation or less. This is because the CHA gets wider then, therefore increasing the dispersion of spectral colors. Such CHAs are also situated less high in the sky, where they are more easily seen unintentionally. Also then, an interesting landscape in the foreground may add to the scene’s beauty.

The following table contains a summary for the 2015 and 2016 CHA observations. Observations on the same day have been included, if from different places and reported in the said forum or – in just two cases – directly communicated to this author. Chance findings of related CHAs in other public networks and oral reports solicited by an AKM posts are carried separately. For consistency, three CHAs seen in far-away places by a travelling AKM member were not included in the count. The last column contains the count of AKM’s systematic visual observers.


It is clear, that the CHA campaigns for 2015 and 2016 were comparably successful, definitely surpassing the counts for any earlier year. The competitive character of a call for CHAs stipulated an increased time log from skilled observers and made them aware of quite inconspicuous CHAs. Although the meteoros.de forum boasts many hundreds of members, the number of those contributing CHAs, was quite low. The CHA reports for 2015 and 2016, respectively, were delivered by 15 and 19 observers, respectively, who lump together to just 26 different persons for both years. So every CHA reporter on the average had 2.3 specimens. Besides different personal noon-day observing options, the more southerly observers were favored, of course. The most successful six of them recorded 34 CHAs (58% of the grand total).

In concluding, some circumhorizon arc images from 2016 are duplicated from our forum. First place goes to a very nice spectrally pure example, seen June 15th at a solar elevation of 64.1o by Daniel Eggert near Neuburg (Danube).

The photo by Isabelle Klein (a) shows a CHA underneath a 22o circumscribed halo, as photographed from the Autobahn A1 near Hermeskeil nine days earlier, on June 6th, at a solar elevation of 63.4o. Unsharp masking of this shot shows an infralateral arc curving upwards from the CHA. Not shown here is the also present complete parhelic circle.

By contrast, Michael Großmann’s photo (b) from Aulendorf, taken June 18th at a solar elevation of 65.3o does not show any hints of a 22o halo.

On July 6th, Elmar Schmidt experienced a long-lasting CHA display in Heidelberg (c). The sky was uniformly covered with cirrus clouds, which washed out the contrast for the halo, which extends for more than 60o of azimuth. To show this better, two portrait oriented wide-angle photos, taken at solar elevation of 63.4o, were stitched together, therefore artificially broadening the circumscribed 22o halo disproportionately w.r. to the CHA.

Karl Kaiser in Schlägl, Austria, took his photo (d) of a delicately colored CHA on July 8th at a solar elevation angle of 63.4o.

[1]        E. Schmidt, A. Haußmann, C. Hinz, P. Zenko: Der Zirkumhorizontalbogen – Teil I: Auftreten und Häufigkeit, VdS-Journal, Nr. 53 (2015), S. 70

Author: Dr. Elmar Schmidt, Bad Schönborn, Germany

Rare Subhorizontal Halos

I was flying from London back home to Berlin on September the 11th 2016 . The flight was operated by British Airways. The airplane was located above Hankensbüttel / Wittingen (DE, Niedersachsen) when I observed a bright subsun and some bringt subparhelia. The suns altitude was about 40° at this time. For this fact the halos appeared very steep below the horizon, which made it difficult to observe them.

In addition to the subsun and the left subparhelion, the sub parhelic circle was visible for some moments. It appeared in the shape of a bright tail of the subparhelion. But for some moments I could see the sub parhelic circle between them.

The following image was taken with a Samsung smartphone.


According to the theory, the sub parhelic circle can be produced by horizontal oriented ice crystals. But the sub parhelic circle grows left and right from the subparhelia, so that there is actually a gap between the both subparhelia. During my observation from the airplane, the sub parhelic circle was visible between the both subparhelia. How is it possible?

2016-09-11_simulation2016-09-11_unter-halo_usmThe simulation software called „HaloSim“ (L. Cowley & M. Schrieder) leads to a first clue. The sub parhelic circle between the subparhelia could only be simulated with the help of pretty flat Lowitz-oriented crystals. In addition, the simulation shows an „X“ which is crossing each of the subparhelia. These are the reflected lowitz arcs or also knows as „Schulthess bows“. The bows are also visible in my picture from September the 11th 2016.

The 1th picture shows a simulation by HaloSim (L. Cowley & M. Schroeder). The 2nd picture was modified with the help of an unsharp mask to highlight some details.

The simulation shows also the 46° tangent arcs (EE52), also knows as 46° lowitz arcs. Especially the lower middle 46° tangent arc (EE52B) is standing out. However, I neither saw these bows nor I found them in my pictures. The reason for that is the fact that my attention was directed to the area near the subsun. Another reason may be the fact, that there were no halo active ice crystals in the higher air layers.

The simulation and the observation do match pretty well and are convincable, so that one can say, Lowitz-oriented crystals are responsible for the presented halo display.


This picture was stacked from a video file. The frames were aligned and modified with photoshop.

I want to thank Michael Großmann and Alexander Haußmann for helping me with the analysis.

Author: Andreas Möller, Berlin, Germany

Experimental Demonstrations of Column Halos

Experimental Equipment | Photo: Michael Großmann

Did you ever think about showing in a practical way, how ice-crystal halos come to be? An individual halo like, e.g., a sundog is easy to demonstrate, but producing halo phenomena in such a way is more difficult, as a multitude of crystal shapes and orientations are involved.

Raypath sundog | Photo: Michael Großmann

In 2011, I committed myself to the task of generating halos in a darkened room by optomechanical devices, for which the term „halomators“ was coined. Any one of several models can hold an artificial crystal and make it rotate in certain ways. When light impinges on this crystal, different kinds of halos are produced by the possible reflective and refractive raypaths. By means of the so-called sky transform it can be shown that a single crystal produces the same kind of display in a laboratory environment as a multitude of such crystals in nature.

For the experiment shown here, a horizontally rotating column crystal was at first doubly oriented, then only singly.
In the beginning (with a rectangular face of the crystal staying in the horizontal plane of the rotation too) the Parry orientation tape halos very nicely showed, including the corresponding reflection halo like heliac arc, subhelic arc and the parhelic circle. When the crystal was made to rotate about its symmetry axis, the upper tangent arc also appeared.

Natural photographs, however, can show still more halos from oriented columns. This led me to further improve my halomators, for which my profession as an industry mechanic proved helpful. By 2016 I could document numerous other kinds of halos.

„Halomator IV“| Photo: Michael Großmann

The following images are stacks of photographs of halos obtained from singly and doubly oriented columns. They show the following types:

– upper tangent arc
– concave Parry arc
– convex Parry arc
– heliac arc
– subhelic arc
– subanthelic arc
– Tricker’s anthelic arc
– Wegener‘s anthelic arc
– Hasting’s arc
– parhelic circle

Remark: I had to use acrylic (PMMA) as a workable material for my crystals, which does not match the refractive index of ice. Therefore, most deflection angles change. Also, those refraction halos produced in ice by the 90° prism angle do not show at all, because of total internal reflection. (Ways out of this limitation are being investigated by other experimenters and myself, and will then possibly be reported.) Despite of these shortcomings, I found out in many demonstrations, that most people grasp the optics behind a certain halo much better than from a sketch or computer simulation.

Author: Michael Großmann, Bilfingen, Germany