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).