Saturday, 27 June 2020

A fine reflection subsun in northern Finland

Heikki Kainulainen uploaded this reflection subsun a couple of days ago to the Finnish observation site Taivaanvahti. It was seen in the Muonio region of northern Finland on May 29. The photo was taken through window at 04:41 when the sun was 6.9 degrees above the horizon. In the direction of the sun there is, starting at 11 km distance from the observation site, a 3 km transect of water. This is lake Pallasjärvi, and could be the source of the reflection. It is the only large water body for 140 km in the sun direction. 

However, according to Kainulainen, Pallasjärvi was still frozen. So maybe the reflection was from the lake ice, or, as Kainulainen suggest as one possiblity, from the possible flood waters in the bog areas. 

As reflection subsuns go, this is a beautiful specimen which shows also the mysterious effects of vertical striation and larger-than-sun width. Below is an usmed close up of the photo above to highlight these features.

Yet one more image is shown, in which a separate photo of the sun has been superposed with the pillar. The width of the brightest part of the pillar is about equal with the sun disk, so this part most likely was located on the solar vertical. The sun reference photo was taken with the smallest aperture of the lens and shortest exposure time, but it may still be overexposed slightly.

Wednesday, 17 June 2020

High Quality 28° Arcs in Ji'an, China

Moments before sunset on June 17 2020, a high quality odd-radius plate display with bright and vivid 28° arcs was documented by multiple observers in Ji'an, Jiangxi Province, China.

© HUANG Qian, shown with permission. Single exposure.

© ZHOU Ling, shown with permission. Single exposure.

Annotated version as follows:

The intensity of the display rivals the 2016 Chengdu display as the 28° arcs stand out even in smartphone photos above. 20°, 24° and 35° plate arcs in the photos are also quite well defined.

Unfortunately, like previous displays, no other exotic arcs are found in the photos we received from the community.

Now that we have a great and early start of the season, let's hope for more great stuff to come.

Monday, 15 June 2020

Odd radius sub-plate arcs

Odd radius plate arcs have copies on the other side of the horizon, grouped around the subsun. People have been aware of the possibility of such arcs I call them unofficially here as sub-plate arcs at least from the 1990's, but no observations have surfaced.

Here I show three odd radius sub-plate arc displays that I photographed in Rovaniemi in the winter 2016/2017. The first display was seen on the night of 12/13 December 2016. Only one odd radius sub-plate arc was visible, the 18 sub-plate arc. Below are photos and animations of its appearance.

Flashing two stacked images from a continuous photo series, the other half which had subparhelia and the other which had 18 sub-plate arcs dominating.

Versions of a stack with a slightly different set of photos than above and a simulation. Also other odd radius halos are visible: 18 plate arc and 18 halo, 35 plate arcs and faint 35 halo.

An animation of transformation from subparhelia to 18 sub-plate arc.

A stack of photos from another stage in the display. We see here three halos flanking the subsun: 18 sub-plate arc, subparhelia and a mysterious arc outside the subparhelia. The last one seems to be part of a longer arc that extend faintly to normal parhelion and reminds me of the arc in the 28/29 December 2016 display in Rovaniemi. 

An animation showing the changing display.

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Next I got odd radius plate arcs on the night 4/5 January 2017. This was the winter's coldest night in Rovaniemi. At the location which was the same as above, the Oikarainen gravel pits the temperature was around -35 C. This also was the night with the best sub odd radius stuff: in addition to 18 sub-plate arc there was an upper 23 sub-plate arc. Both were easily visible to the naked eye.

The best odd radius stage did not last long, here are shown three and five images stacks. The display is all but straighforward case. Like in the first display, there are three halos flanking the subsun: 18 sub-plate arc, subparhelia and, well, something. The vertical feature at the horizon should, according to the shown simulation, contain both 35 plate arcs and 35 sub-plate arcs, and while there may be both or either one, that is likely not the full explanation. See again the 28/29 November 2016 display, which is not an odd radius display, but which nevertheless contains a similar kind of thing. Yet another detail to pay attention is 20 sub-plate arc, which is in the simulation, but absent from the display. It was difficult to avoid it in simulation, it should have been in the display. Concerning the lack of normal parhelia and 18 plate arcs in the display, that may be explained by the lamp not being centered exactly on the camera, but shooting slightly over it. I am not sure, this is something to pay attention to in the future (hopefully the nightly diamond dust halo chase is not dead, as it seems now).

This single photo has the 23 sub-plate arc (arrow) somewhat better than in the stacks above. I turned camera here to try catching one of the distant, unobserved odd radius arcs that I had seen in simulations. Alas, no success. After this photo the odd radius stuff deteriorated quickly, so there was no getting a stack that might have actually gleamed out something novel.

Even though the odd radius halos deteriorated, and the display started turning into a normal plate display, here 18 sub-plate arc is still visible inside subparhelia.

Another photo from another place that night with odd radius stuff. We see odd radius parhelia and circular halos at 18, 20, 24, 35 and a possible odd radius helic arc. No odd radius sub-plate arcs here.

And an animation showing the transformation from conventional display to an odd radius display.

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The third and the last odd radius sub-plate arc occurrence was in the display on the night of 9/10 February 2017. Only a weak 18 sub-plate arcs were found from photos, as shown below. Again, the location is the Oikarainen gravel pits. It is a good place, with plenty of possibility to play with lamp elevation. It is quite far from the ski center, though, 12 km as the crow flies, so the diamond dust can reach there only on the coldest nights.

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Taken that odd radius sub-plate arcs were found in three displays in one winter, they can't be regarded much of a rarity just get to the right place, switch on the spotlight, and you got them bagged. With spotlight it is in general much more easier to catch subhorizon halos (well, any halos, really) than in solar displays. In case of the odd radius sub-plate arcs, the subhorizon spotlight method has also the advantage that, because of how pyramid crystals orient to keep the larger basal face as the upper face, these halos are easier to form than in sun light, the latter way of which requires the inner reflection to take place from the smaller, and thus less effective, down facing basal face.

For the last, I add here a collection of photos from first two of these displays, and the 28/29 December display (the bottom row), to more easily compare the features in them:

All simulations in this post are made with HaloPoint2.

Friday, 12 June 2020

Surface halos from uniformly oriented crystals

I was searching for surface halos on ice plates for two years without luck because of unfavorable weather in my area (Romania and Hungary). On 18 January 2020 I finally observed my first subparhelia and 120 degree subparhelia complete with the colored nadir spot on a small icy patch near a lake in Romania. In February I continued my search for these kind of halos in northeastern Hungary. Clear weather after a rainy period with minimum temperatures between -5 and -10 degrees ºC promised me a good opportunity, so I went out to a nearby field with plenty of frozen puddles, many of which had air below the ice. Since most of them formed in shady hollows I broke off pieces of ice and placed them in the sunlight to produce the halos. To my surprise these halos appeared more vividly when I held the pieces upside-down (relative to their original position on the ground) indicating more and/or better quality ice prisms on the underside, so I used them in this way. On the plates there were large patches of uniformly oriented prisms, this feature made itself noticable as a break in the observed halos. I tried to take pictures of the crystals, but it was difficult because of their very small size (to the naked eye the surface seemed completely smooth). The only usable image I could get is shown above depicting crystals pointing away from the plate surface in the same orientation rather than being parallel with it.
Below I present the most interesting halo elements with screenshots grabbed from my videos made on 8 and 13 February.  Some of them were previously observed (in Hungary and Finland), others are new. The surface is defocused because I held the camera lens only a few centimeters from the ice plates. Since pictures are not showing the nature of these phenomena well, I recommend watching  my videos  available on Youtube (links can be found at the end of the post).
1. Subparhelia and other spotlike features on the pieces: bright subparhelia always accompanied the subsun on the ice plates. Sometimes a ,,duplication’’of the subparhelia appeared when I held the plate in an almost horizontal position and watched it in a flat angle. When holding the plates in the opposite direction of the Sun, white and colored spots could be observed.kukk
From left to right: colored spot (looking away from the Sun), white spot (probably related to the white arcs), a ,,duplication’’ of the subparhelion on its right side
2. Spots made by the underside crystals of the ice sheet left in its original place: one of the puddles had crystals only on the underside of its ice. The situation was the same on both days, despite of the melting and re-freezing of the ice in the meantime. From here I could not break off pieces for further investigation because there was water only a few centimetres below the ice that instantly destroyed the crystals if I tried to do anything.
From left to right: orange spot (looking in the direction of the Sun), orange spot (looking away from the Sun), colored spots (the Sun is at the left), white spot in the place of the 120 deg. subparhelion.

3.  Arcs crossing at the subsun with a white spot on them: these arcs change their configuration as one rotates the ice plate. Sometimes a faint subparhelic circle is also visible. The white spot is usually located at the intersection of the subparhelic circle and a white arc, but with changing the angle of the plate it can appear elsewhere.


White arcs with a spot visible on the right one at the intersection (top) and above the intersection (bottom) with the subparhelic circle.
4. Faint parhelia-like spots near the bright spots of the white arcs, in the direction opposite to the Sun: if one follows the white arcs pointing away from the Sun, the white spots can be found there as well, accompanied by two faint spots on both sides which look as if they were parhelia of the white spot. Also there is a subtle brightening on the arcs above the white spot. Clipboard01d
Parts of the white arcs with the bright spots pointing away from the Sun, the arrows mark the faint ,,parhelia’’ on the sides.
5. The colored nadir spot: sometimes the nadir spot was present, situated on one of the white arcs.  For this feature one must look at the plate from above in a steep angle.  
The nadir spot is visible on the ,,vertical’’ arc
6. Looking through the ice plate: when looking at the Sun through an ice plate (holding it perpendicular to the Sun-observer axis), a regular six pointed star became visible ending in parhelia-like spots 25 degrees from the Sun (measured by Marko Riikonen using a starfield photo as reference). As I changed the angle and position of the plate, white and colored spots moved away from these parhelia. 
The six pointed star with parhelia on the ends

Changing the angle of the plate and looking through it to the side and below the Sun gave another set of white and colored spots
7. White star on the opposite side: holding the plate away from the Sun another six pointed star appeared with the reflection of the Sun at its center. In my opinion the arcs that make up this star are strongly related to the white arcs discussed at (3.). 
The opposite-side star with the Sun’s reflection at its center
Links to my videos (montages created using the best parts of the original material):

Wednesday, 10 June 2020

Kern and Hastings arcs make appearance in the UK

Only three days before the low-sun odd radius display ( ), I observed a brief but intense display that included well-defined suncave Parry arc as the visual highlight. Shown above are the two 50-frame stacks that I managed to extract out of the display. Both cover five minutes and they are separated by another five-minute interval during which I was too busy to collect data. Solar elevation is 26° in the first (top panels) and 25° in the second (bottom).

Applying the usual background-subtraction on the average stacks (on the left) makes helic, Tape, and Lowitz arcs all stand out reasonably well in addition to the circumzenithal, supralateral, upper tangent, and Parry arcs. As the simulation (HaloPoint 2.0) in the top right-hand-side corner demonstrates, the anthelic arc close to the far left edge is Hastings rather than Wegener. Kern arc only appears in the second stack, coming out the clearest in the middle panel. Here, blue-minus-red colour subtraction is applied on top of the background subtraction. The bottom right corner is the average 50-frame stack without further processing.

Sunday, 7 June 2020

A possible surface 28 halo

On 14 March this year in Tampere, on lake ice that was covered with thin snow a 22 halo was visible. Its glitter was not that bright, likely because of small crystals. But the glitter was relatively dense. Because the inner edge of the halo was not well defined, I took a long photo series as I have linked this impression to odd radius halos. I snapped photos for 23 minutes duration and after rejecting a few frames the stack had a total of 290 frames.

I made different versions of the stack that were adjusted with Photoshop. They are shown here in non-mirrored - mirrored pairs:

pair 1: double usm + HDR Toning
pair 2: background removal (BGR) + HDR Toning
pair 3: background removal (BGR) + HDR Toning. BGR with different values than for pair 2
pair 4: BR + BGR 

It seems this was an odd radius display. A weak 24 halo appears to show up in pairs 2 and 4. Images also seem to have 35 halo. The most interesting thing, however, is the feature that looks like 28 halo. I measured it from BR image using a star field photo and the inner edge was at 27 plus some. Also, I measured the apparent 35 halo and got 35 degrees.

The night's lowest temp was at the sunrise, -7 C at the airport. Windy, clear skies overnight. Not quite the foggy, calm conditions that I have previously associated with surface odd radii. Unfortunately I forgot to take the microscope as I headed for the lake with bicycle. Realized this pretty soon after leaving my place, but didn't turn back.

This is not the first suspected surface 28 halo. On 7 April 2012 Jari Luomanen and I photographed on a small lake in Eastern Finland an odd radius display that by all looks has a 28 halo:

Friday, 5 June 2020

Halo displays from stricly regimented crystals on ice surface

In early 2019 Dávid Hérincs saw halo formations on the surfaces of ice puddles that didn't look like anything we had seen before: white arcs of unusual geometry and spots in unexpected locations. This winter I happened to see for the first time similar stuff on lake ice in Tampere, thanks to a heads up from Petri Martikainen in Juva.

Hérincs did not photograph the crystals, but simulations that I made at the time suggested a possibility that crystals are all oriented exactly in the same way. This time we got the crystals under a microscope first by Martikainen and a wonderful landscape was revealed in which crystals were packed next to each other, and indeed locked in the same exact orientation. Below is one typical photo that I took.

The displays are formed when deposited rain water freezes over the night. In Hérincs case it was ponds that froze, in Tampere it was water freezing on lake ice. Martikainen also successfully performed his own freezing experiments in his backyard. I followed suit, this video shows one of these home made ice plates with a Hérincs type display.

On that plate we see distant spots, both white and colored. Some move along roughly horizontal plane as the plate is rotated, while others are more or less stationary. Below is a freeze frame from the video with some stationary spots marked.

A frame from the video. The double arrow marks a white and colored stationary spot, which can probably be regarded as 120 subparhelion and rotated subparhelion analogues. The singular arrow marks another, much more tighter spot that also is stationary. The formation of this spot from the two others may differ in some way.

The stationary spots could be seen as formed from raypaths similar to sub-120 parhelion and rotated subparhelion (120 degrees rotated subparhelion). The moving spots, in turn, would be made by raypaths similar to subparhelic circle. Below is an animation where azimuthally locked plate oriented crystal has been rotated in 1 degree steps. Similarities with what is going on in the rotated ice plate are apparent. The rotated subparhelion actually move too, but not much, similar to the normal subparhelion.

Concerning the colored moving spots (not including the subparhelia and rotated subparhelia), in simulations many of them arise from three-hit prism face ray paths plus basal face reflection. In Parry oriented crystals such ray paths make reflected Hastings arcs when the inner prism face reflection is from horizontal basal face. Below is a stacked segment of the video demonstrating how the moving spots draw out subparhelic circle when the plate is rotated, i.e. when the crystal is allowed to rotate azimuthally.

 An average stack from a segment of video. The moving spots draw out a subparhelic circle on which the stationary spot of 120 subparhelion stands out. There is also a colored rotated subparhelion spot. 

The distant white spots and colored subparhelia were seen also on lake ice as shown by this video. Below is a photo of the same situation. Both the colored and white spots were blindingly bright, you couldn't look at them at all, a heavy underexposure was necessary for the colors to not wash out.


Another kind of spot which was also photographed by Herincs is the colored spot on the white arcs in the nadir region. In this video several instances of this spot are seen. These spots have been assumed to be segments of circumnadir arc from plate oriented crystals. One can simulate the white arcs by azimuthally locked column oriented crystals and the circumnadir spot can be made to align with this arc as demonstrated below.

Two simulations that have between them 10 degree difference in the azimuthal rotation of the column and plate population. The circumnadir arc spot (arrow) stays on the parhelic circle. In the left side simulation the other white arc is subparhelic circle. Its analogy may not appear in real displays.

It is noteworthy that no sub-Kern spots, which would be expected to have color order reversed (in the area opposite to the cna), are not seen.  There is also another colored spot on one of the white arcs at 01:13-01:17 in the video. Below is a still frame of that double cna spot, though the other spot doesn't show very well in it.

In simulations cna spot can be off-set by tilting the crystal, as demonstrated below. The other spot is made by exactly horizontally oriented plates, the other one by plates tilted 10 degrees. So it looks like there are crystals on this particular patch at two tilting angles.

The white arcs are really parhelic circles. However, the column oriented crystals go only as far as explaining the geometry. As far as the microscopic examinations are believed, the crystals are always locked in the same exact orientation in each patch, and thus should only be able to make spots, not arcs (although it must be admitted that at least two separate orientations seems to be possible as evidenced by the doubled cna spot above). But I won’t say no more about this, as Petri Martikainen has been studying the problem, and will hopefully write his own post in the future.

In the simulations above it is seen that also a moving white spot from plate population is aligned with the white arc. Here is a video showing such white spots on white arcs in a real display. Below is a still frame.

The simulations must be taken only as a rough guide. Simulation software assume halos formed in singular crystals floating in the air, on the ice plate the situation is very different. The crystals are packed next to each other and the attachment of the crystals incapacitates some faces. Then there is the inner reflection from the bottom of the ice plate to be considered. It could be that a multiple scattering formation might better describe some features. Moreover, we have seen under the microscope some scattered crystals rising above the basic level of the packed crystals. These may explain features that look like they are consisting of separate crystals rather than being made of solid light. The circumnadir spot, for example, is in many displays visibly made of individual crystals.

I throw in also some simulations from randomly oriented crystals that show the area where any given raypath can make a halo spot in strictly regimented crystals if the orientation of the crystals only is right to light up the particular spot. I haven't covered it all, for example the rotated subparhelion type raypaths are not simulated.

 325 ray paths

  An example of distant 325 spot at the far edge of the area for this raypath

  325 in extent in plate oriented crystals is confined to only about a 20 degree horizontal segment  

123 and 321 ray paths (circumnadir arc in plate oriented crystals)

 1342 and 3568  (120 parhelion in plate oriented crystals)

12341 and 31568 (120 subparhelion in plate oriented crystals)

13 and 32 (circumzenith arc in plate oriented crystals)

Well, I have been haphazardly touching some issues here. This stuff is way over my competence. There are tons details that I did not address, tons of open questions. Hopefully more capable guys will step in and explain it all in a coherent manner.

Let's end with some practical tips for those wanting to get their share of these displays. What you are looking for is a mosaic ice surface with fleeting reflections everywhere as you walk on it. The mosaic comes about because the crystal orientations are identical only in small patches.

To see these displays, you need lean really close to the surface: in my videos, the camera lens may be just a few centimeters above the surface and is occasionally even grazing it. From standing height only subparhelia and perhaps weak sub-120 parhelia are created over the patch mosaic.

The backyard versions I made by freezing water in a kind of plastic tray and metallic oven plate. In the metallic plate I never got a good display, so this may be something to avoid. You may want to remove the ice plate from the tray for various reasons, so anything that helps to do this without breaking the ice plate is good.The plastic tray that I used was slightly pliable, bending it detached the ice plate quite handily from it.

An old ice plate may give even a better display than a fresh one, so it is good to keep the plates around. The crystals don't grow straight from the freezing water, but from atmospheric moisture, so if frost is developing in the night, then the prospects for having the Hérincs type growth on the ice plate are good.

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addentum 11 June: forgot to mention that simulations are made with HaloPoint2.

Monday, 1 June 2020

Odd radius display at low sun in Berkshire, UK

11-frame stack at sun elevation 2°: unsharp mask and colour enhancement (top), and background subtraction + blue-minus-red colour subtraction (bottom; the latter by Nicolas Lefaudeux).

In the evening of 24th May 2020, a notable display of odd-radius halos and their associated plate arcs occurred in Berkshire, UK. I first noticed faint traces of circular halos - 20° and 23° as I could later confirm from photos - at around 19:20 BST, when the sun was at 13° elevation. Less than 30 minutes later the sun had come down to 9° and first signs of the upper 20° plate arc appeared, making obvious the need to find a view down to the horizon. The display got weaker after some time but regained some intensity less than 30 minutes before the sunset. Unfortunately there were some lower-level cloud interfering with my view for most of the observing time.

Stacked and further processed images from the first stages of the display (below) reveal 18°, 20°, 23°, and 35° circular halos in addition to the aforementioned 20° plate arc. At the end of the display (above), the circular halos are less clear, but plate arcs at 20° and 35° show up better. Most interestingly, perhaps, there are suggestions of 28° circular halo and the associated plate arc at the upper left-hand side at solar elevations 7° and 2°. Previously 28° arcs have been reported in the Lascar display of 1997 and in a few more recent occasions in China, but possibly never before in Europe.

50-frame stacks at sun elevation 9° (top) and 7° (bottom): Background subtraction (left) and background subtraction + blue-minus-red colour subtraction (right).

Thursday, 14 May 2020

Map in the Sky - High Cloud Light Pillars in Xiamen, China

In the evening of May 10 2020, residents in Xiamen saw a strange, patterned cluster of light spots hovering over the city. Photographer YUAN Quan captured the phenomenon at its peak with his handphone.

© YUAN Quan, shown with permission. Equivalent focal length 39mm (from EXIF, unclear whether it's accurate).

Shortly after YUAN’s photo went public, LI Yahong and HUANG Tengyu from the China Sky Enthusiasts community found out that the light pattern, when flipped and rotated, perfectly matches downtown Xiamen’s map.

YUAN Quan's photo flipped and rotated 180°. Map from Apple Maps.

With the help of real night time images of Xiamen captured by Wuhan University’s Luojia-1A satellite ( and Chang Guang Satellite Technology's Jilin-1 satellite (, LI and HUANG’s finding was verified - we’re looking at a reflection of Xiamen’s city lights off the clouds.

*Special thanks to the Luojia-1A and Jilin-1 satellite teams for authorizing data usage.

Satellite images copyrighted to Jilin-1 and Luojia-1A satellite teams, shown with permission.

This phenomenon is most likely a high cloud light pillar event, similar to a previous case in Finland ( but on a much larger scale. The reflection captured in YUAN’s photo corresponds to an area of 200 square kilometers on the map. What’s even more interesting, the ground temperature in Xiamen during the event was 23°C -  a fairly warm night.

Due to the lack of background stars in YUAN and other people’s photos, it’s hard to do accurate cloud height calculations. A rough estimate by ZHANG Jiajie places the clouds between 6 and 7 kilometers above sea level. Sounding data from the night indicates the existence of a moist and wind-free layer in the 6 to 7 kilometer range so the height estimates may not be too far off.

Such warm night high cloud light pillar events are becoming more frequent in China - another two weaker ones were observed in the past few months in different cities. We believe the ubiquitous usage of decorative LED strips (which are very bright) on tall buildings, as well as the vast improvements in low-light photography on newer smartphones are two of the key factors leading to the surge in new observations.

Saturday, 2 May 2020

Hidden Treasures in the Jutland Display

The Jutland display ( introduces us to the Jensen arcs, a brand new halo form and so far unexplained. While the new halo attracts all the plaudits, there are some hidden treasures in the display which are equally interesting and worthy of more attention.

Full circle 'pillar CZA'

During the peak of the display before the Jensen arcs appeared, another elusive, sub-visual halo lurked near the CZA and Kern arc. Anders’ Kern arc photo (likely the first high cloud Kern arc visible in a single frame), when enhanced, reveals a faint, full-circle arc outside the CZA + Kern combo.

Photo by Anders. Sun elevation 19.8°

First of all, it’s not a weaker version of the Jensen arcs. When compared to the actual Jensen arcs which emerged 10 minutes later, this one dips lower to around 57° elevation.

Left: ‘Pillar CZA’ (arrowed) , sun elevation 19.8°; Right: Jensen arcs, sun elevation 21.5°. Photos by Anders.

The 57° elevation reminds us of the multi-scattered 'pillar CZA' in the Siziwang Qi display ( The overall appearance of the arc in Anders’ photo, albeit dimmer, greatly resembles the Siziwang Qi ‘pillar CZA’.  Chances of the two being the same halo are quite high.

Siziwang Qi photo by LI Tingfang. Jutland photo by Anders.

However, unlike Siziwang Qi where multi-scattering was strong, the absence of other multi-scattered halos (such as secondary UTA and PHC from the bright UTA) in Anders' photos clearly doesn't favor the conventional multi-scattered ‘pillar CZA’ solution for this display.

Nicolas Lefaudeux proposed an interesting theory which sounds highly viable in this case. He suggests that the sunlight scattered inside the cloud layer may have acted as an 'integrated glow', feeding light to each crystal from all azimuthal angles. In other words, the crystals are seeing from their viewpoint an infinite number of 'pillars' around the horizon, and each 'pillar' creates its own 'pillar CZA' via the crystals. The combination of these infinite numbers of 'pillar CZAs' will of course be a full circle with even intensity distribution.

This theory, when extended, may potentially explain the mismatch in the Siziwang Qi case too. In the Siziwang Qi display, available materials suggest the arc likely went full-ring as well (couldn’t be confirmed due to lack of DSLR photos). The sun pillar alone can't create a full-circle multi-scattered CZA without invoking unrealistically thick and triangular plates.

Simulations with ZHANG Jiajie’s program ( Sun elevation 20°.

Ji Yun from the Chinese halo watching community came up with the idea of facilitating the 360° glow from the snow covered terrain to increase the ‘pillar CZA’ azimuthal extent. His idea is in nature very similar to Nicolas’. If this ‘integrated glow’ theory holds, the name ‘pillar CZA’ should probably be changed to ‘integrated CZA’ instead. But for now, it’s still too early to make that move. More observations are needed to see if the arc always goes full-circle.

Skywatchers world-wide should start keeping an eye out for this elusive feature in future displays. Whenever there’s a bright CZA in diamond dust or high clouds, it’s highly recommended to always photograph the entire zenith area in RAW for in-depth analysis. It may also be worthwhile to go through past images and see if this arc shows up via heavy post processing.

Mismatching Lowitz arcs

Now let’s turn our attention to those arcs above the parhelia, which are seen in many photos of the Jutland display. Below is Anders’ photo for solar elevation of 14.8 degrees, and shown are four simulations of arcs, two from Lowitz oriented crystals (simulations 1 and 2) and two from alternate Lowitz oriented crystals (simulations 3 and 4). The alternate Lowitz orientation was included because recently it was realized that alternate Lowitz arcs actually exist. Ji Yun identified them in a display that Lasse Nurminen photographed in Raisio, Finland on 28 June 2019.

None of these scenarios really work. Only simulation 2 gives a match with the photo, and that’s just the outer arc – the inner arc is not right. And even the outer arc is on a rather short side for comparison. More length would have been preferred to see whether the apparent similarity in geometry really holds.

Photo by Anders. Sun elevation 14.8°. Simulations with HaloPoint 2

Below is another Anders’ photo, the sun elevation is seven degrees higher. Here no outer arcs are visible but we think the inner arcs are the same as in the earlier stage. This time comparison is made only with two Lowitz orientation scenarios. The circular Lowitz arc seems a better match here, but the arc in the photo doesn’t look quite as curved as in the simulation.

Photo by Anders. Sun elevation 20.4°. Simulation with HaloPoint 2.

Possibly these arcs in the Jutland display are like the subparhelic arcs (Schulthess arcs) of scenario 2, that is, arcs characterized by an added basal face reflection to the Lowitz arc raypath. The shape is not exactly what the simulations predict, but this is a known issue: several people have pointed out the disagreement of the observed subparhelic arc geometry with the theory for various displays. One such illuminating display was seen on 7 March 2017 in Rovaniemi, which happens to have solar elevation comparable to the first photo above.

And that’s just the geometry. Another contradiction, a glaring one, is that subparhelic arcs are in simulations accompanied by much stronger Lowitz arcs, yet usually Lowitz arcs are nowhere to be seen. Actually, the subparhelic arc and alternate subparhelic arc simulations above were tweaked for clarity: Lowitz arcs were removed in order to not let subparhelic arcs overwhelm them. Moreover, only the above parhelia parts of the subparhelic arc were plotted.

Until recently the subparhelic arcs have been exclusively seen in diamond dust, in which they are a common feature. The first high cloud case to show them appears to be the display that occurred on 7 February 2020 display in Finland.

2 and 10 o’clock spots

Many photos that Anders and others took of the 14 April display show an intensification in the subparhelic arcs at 2 or 10 o’clock positions, or both. Also one video has captured this feature nicely.

In some photos an outright spot is visible. Just to be sure, we tested with simulations for one of Anders' photos whether this could be a 24 plate arc. After all, the cloud contained pyramid crystals, as betrayed by the odd radius column arcs in some photos.

The position indeed corresponded to the 24 plate arc position, but the orientation isn’t quite right, as shown below. So, considering also the lack of other odd radius plate arcs in the display, these spots are probably better understood as parts of the subparhelic arcs.

Could they be just chalked up to irregularities in the high cloud? The fact that we haven’t seen such brightenings on subparhelic arcs in diamond dust displays – which tend to be more uniform – would seem to give appeal for such an explanation. However, considering that several photos from various people show more or less well defined spots in the Jutland display, it might be the case that there is something about the subparhelic arc crystals themselves that makes the spots.

The spot (on the right) in a photo by Richard Østerballe

The simulation has a 24 plate arc to compare with the spot on the left in Anders' photo. Simulation with HaloPoint2.

Another photo from Anders showing the spot. The 24 column arc is visible here, as well as in the photo above.

Odd radius column arcs

Anders' father, Ole Jensen also photographed the display and he took note of 9 column arcs on the sides of the sun. His photo is shown below. A selection of Anders’ photos, too, have 9 column arcs and additionally a very faint 24 column arc, which is visible in the two photos above.

9 column arcs. Photo by Ole Jensen. Sun elevation 13°

Seeing red in the blue spot?

There are some reports of people claiming to have seen red in the blue spot visually. But red has never been caught on photos to substantiate these claims and the theory is not forthcoming either.

Interestingly, some heavily enhanced Anders’ photos would seem to suggest that there is indeed red in the blue spot. However, Nicolas Lefaudeux demonstrated that it is an illusion that disappears when the blue color and its cyan end is covered. 

From the optical point of view, the colors in the blue spot are supposed to add to each other starting from the violet. It should be:

violet only = violet
violet + blue = blue
violet + blue + green = cyan
violet + blue + green + yellow = light cyan
violet + blue + green + red = white

So from this, any red should be an illusion (as there is never more red than any other color), and indeed, it seems to be the case when we look at the blinking grey patch on the blue and cyan of the blue spot (shown below).

Photo by Anders

The red on the blue spot more or less vanishes when blue-cyan is covered. Photo by Anders.

- Co-authored by Marko Riikonen, Jia Hao and Nicolas Lefaudeux