Thursday, 13 December 2018

Elliptical halos 12-12-18

Early in the day before leaving for work saw some AC clouds moving over our area and whenever I see AC clouds or altocumulus clouds first thing that comes in mind well you guessed it elliptical halos. While driving down a back country road I slow down and park the car and right there I saw an elliptical glow manifest its self and I watched two halos form both red on inside and blue on the outside. There was no telephone pole for me to use to block the sun. I know if I used the macro lens and try to cover the sun with my hand it would be blurry so I used the wide angle.

This is my second elliptical halo observation for the 2018 halo season and this is a good way to wrap it up. 

Wednesday, 14 November 2018

Possible observation of exotic 28d upper plate arc

Upper left and upper right are image processing by N. Lefaudeux. Lower left is my own processing, and lower right the simulation. All versions of processing are based on the background subtraction method, but use different ways of enhancing contrast. Sun elevation is about 36 deg.

Some time after the publication of the 31 May 2018 display (see previous post), I returned to analyze the photos in order to take a closer look at one suspicious feature. After strong contrast enhancement, one stack showed a brightening in the area just above the upper 23d plate arc as though a halo may be present. At first, we assumed it was an artefact caused by strong enhancement of the 23d plate arc. However, after fine-tuning of the image processing, we have managed to better reveal this feature, and we can say with the enough confidence that this feature is an another exotic halo — an upper 28d plate arc.

For simulating this halo, I have used a poor plate oriented crystals with a triangular habit, which consist only of a lower exotic (2 0 2 3) pyramid. These crystals are also responsible for the 28d halo, the very faint parts of which you can try to see diagonally upwards from the sun on colored photos above. I have used a triangular habit instead a hexagonal one to remove a 13d halo, which is absent in the observation. In the table below you can find all the crystal populations used in the simulation, and also their some parameters. For ease of comparison, the simulation is placed in the photo scene. It moves into alignment with the photo, as far as field distortion (and other geometric imperfections of the resulting image) allows. As always, the simulation was made with the aid of Jukka Ruoskanen's HaloPoint 2.0. According to the photo, it seems there is faint 18d plate arcs just above 19d plate arcs. If that is the case, this feature makes the display even more similar to the Lascar one (on correspond sun elevations, of course). In order to display this feature in the simulation, regular pyramid crystals in a poor plate orientation were used. But mostly these crystals contribute to 18 and 23d halos. The crystals do not have any prismatic faces because 9 and 24d halos are not present in the display. Based on appearance of the 28d plate arc, the responsible crystals were rather small. Unfortunately, HaloPoint does not allow to change size of crystals, therefore, it is not possible to display the fuzziness of the arc.

Thursday, 18 October 2018

New case of exotic 19d plate arcs

When I first noticed the display at 08-10 local time on 31 May 2018, it was already well-developed. The high cloud layer was very thin and it was visible only in the area close to the sun. At first the display consisted of an upper 23d plate arc and an upper quarter of a 23d halo. A little later I noticed arcs as light spots in the side area of the sun, that reminded me lower 24d plate arcs.

When I processed images, I noticed that these arcs are more like 18d plate arcs than lower 24d plate arcs. But during the observation I distinctly saw that the arcs were located at an elevation lower than the sun, while 18d plate arcs are always located at the same elevation as the sun. I applied stronger processing and revealed a gap between the arcs and a 18d halo. As a result, it became clear that the arcs are exotic 19d plate arcs that were first observed during the legendary Lascar display. In addition, a trace of exotic 28d halo was also revealed.

Sun elevation is about 36 degrees

Some analysis

The halos, known as Lascar halos, are caused by exotic pyramidal crystals with pyramidal faces of (2 0 2 3) Miller index. These exotic pyramids have a 39.1 apex angle while pyramids from regular pyramidal crystals have a 56.1 angle. To simulate the display, I used four different crystal populations. Not one of them have basal crystal faces. The first population is plate oriented pyramidal crystals with upper exotic and lower regular pyramidal faces. This population makes most visible features of the display (19d and 23d plate arcs). The second population consists of crystals with regular upper and lower pyramids, and it contributes to 18d and 23d halos. The population is poorly oriented, in order to  reproduce some features of 18d and 23d halos. The third population contains plate oriented regular pyramidal crystals consisting only of lower pyramidal faces. It needs only to enhance the upper 23d plate arc. Finally, the fourth population is added to reproduce the 28d halo. Its crystals is randomly oriented and consists of upper exotic pyramidal faces in triangular habit. That is, the crystals are almost regular tetrahedrons.

My attempt to simulate the display.
Software: HaloPoint 2.0 by Jukka Ruoskanen
The result shows quite good agreement with the observation, except for an exotic lower 3d plate arc. There are two possible reasons for it. The first is that a glow around the sun has much more intensity than halos presented here. It does not allow to reveal a 3d arc, unlike the Lascar display, whose observing place was located at an altitude more than of 4000 m above sea level. At this altitude the atmosphere has a low level of aerosols, and therefore the glow around the sun is very small, and the sky background is dark. My observation point was in Pskov Oblast, which has a flat topography with usual atmospheric conditions. The second reason is that exotic crystals may have triangular, but not hexagonal habit. The 3d arc disappears when triangular exotic crystals are applied.


 - Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 2 3) Miller index faces explain exotic arcs in the Lascar halo display"
- Nicolas A. Lefaudeux (personal communication, 2018)

Sunday, 7 October 2018

Time machine: the Chengdu display from July 20, 2016

The 1997 Lascar display ( ) opened the door to a world of exotic halos. Halo researchers and enthusiasts alike have all been eagerly waiting for a repeat event. Twenty years have passed and not a single reappearance was reported, until recently.

On July 20 2016, photographer Jin Hui captured an odd-radius halo display from Chengdu, China and later shared his photos with the Chinese sky-watcher community. The significance of the display wasn't immediately recognized and the halos involved were mistakenly identified as ordinary pyramidal plate arcs. Fortunately, the photos were brought back up on the table for better scrutiny earlier this year when members from the community performed housekeeping on past digital archives.
© Jin Hui, shown with permission. Taken from Chengdu at around 22:00UT, July 19, 2016.
 In the reprocessed images, we noticed that the two colored arcs sitting below the 35° plate arcs seem too far out to be 24° plate arcs. The observation was quickly verified by simulations - the arcs are actually positioned at an angular distance of around 28° from the sun. The overall appearance greatly resembles the 28° plate arcs in the Lascar display at low solar elevations ( ). 
Dr. Nicolas Lefaudeux, who carried out in-depth research[1] on the Lascar display, confirmed our findings with his outstanding post-processing techniques. In the stacked B-R image, the arcs exhibit excellent color separation. At this point the presence of the arcs is unmistakable - we now have the world's second known record of the 28° plate arcs.
Post-processing by Nicolas Lefaudeux
Compared to the Lascar display, what happened in Chengdu is different in several ways:
  • no other exotic arcs/circular halos
  • 9° and 24° plate arcs are present
  • 28° circular halo is weaker, if present at all
Unfortunately, the lack of other exotic arcs makes it impossible to pin point what produced the display. At least two types of crystals, pyramidal crystals with 30-32 pyramidal faces and octahedral cubic ice crystals, possess the interfacial angles suitable for 28° plate arcs ( more discussions can be found at: ).

Facing a dead end with the Chengdu case, we took a deeper dive into the archive hoping to find more sightings of the same event. The effort paid off with three photographic records recovered. Though these records contain no additional exotic halos either, they do help us paint a better overall picture of what happened geographically on July 20.

100km southwest of Chengdu, photographer Lin Yong recorded an almost identical scene from the summit of Mt. Emei, except that the 28° arcs are much weaker. Further southwest in Yuexi, crystal quality in the clouds plummeted. Founder of the Chinese sky-watcher community Ji Yun saw only a poor, traditional odd-radius plate display. These reports combined suggest that crystals responsible for the 28° arcs only appeared regionally that morning and probably require more demanding conditions to form.
© Lin Yong, shown with permission. Taken from Mt. Emei at around 22:00UT, July 19, 2016.
© Ji Yun, shown with permission. Taken from Yuexi at around 23:40UT, July 19, 2016.
According to the photographers, the halos over Chengdu and Mt. Emei quickly weakened and disappeared after sunrise. However, four hours later on Mt. Emei, Yang Jialu captured a display with 18° and 23° plate arcs with her handphone. Unfortunately the 28° area above the 23° plate arc was left out of the frame, making it impossible to know whether the 28° plate arc showed up or not. 
© Yang Jialu, shown with permission. Taken from Mt. Emei at around 2:00UT, July 20, 2016.
It's a real bummer that the display didn't last longer after sunrise in Chengdu and Emei. Studying how the 28° arcs changes with solar elevations could be another approach to closing the case. Anyways, what we have here is undoubtedly a milestone on our way to fully working out the Lascar puzzle. Till then, let's enjoy the era we're living in where there're still puzzles to be solved.

Jia Hao

[1] Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 -2 3) Miller index faces explain exotic arcs in the Lascar halo display," Appl. Opt. 50, F121-F128 (2011)


Tuesday, 18 September 2018

28° plate arc captured in Haikou, China

On the evening of Sep 5, 2018, an odd-radius plate display of great significance was captured in Haikou, China, by photographer Zhan Guorong. The photos, when enhanced, reveal an elusive coloured arc between 24° and 35° plate arcs, which doesn't fit into any ordinary odd-radius halo families.

© Zhan Guorong, shown with permission
The arc was later confirmed by Dr. Nicolas Lefaudeux to be the exceedingly rare 28° plate arcs, which previously had only two known records world-wide. They were first observed in the 1997 Lascar display in Chile (, and spotted for the second time in Chengdu, China by photographer Jin Hui on July 20, 2016. We've got permission from Jin Hui to share his great capture to the world.

© Jin Hui, shown with permission

Unlike the Lascar display which lasted for almost a full day with many new arcs/halos discovered, displays in Chengdu and Haikou were short-lived with no other new arcs/halos apart from the 28° plate arcs. The lack of associated arcs and restricted solar elevation make it difficult to fully understand what really happened up in the clouds. Isolated 28° plate arcs can be reproduced in simulations by either triangular pyramidal crystals with 30-32 pyramidal faces [1] or octahedral cubic ice crystals with an octahedral face horizontal [2]. Both models require rather restricted shape/orientation conditions.

photo enhancements by Nicolas Lefaudeux, simulations with home-made program by Zhang Jiajie

Dr. Lefaudeux brought up another interesting point. The 9° and 24° plate arcs were totally missing in Lascar, implicating the absence of middle column sections in the pyramidal crystals. In Haikou and Chengdu though, they were present and quite strong.

Are these displays simply variants of the Lascar display with different crystal combinations? Or are we looking at a totally new breed? We'll need more photos at different solar elevations to unravel the mystery. Good news is that now we know such displays can probably occur anywhere. Before the Haikou case, we thought that the responsible crystal clouds are high mountain related since Lascar and Chengdu sit beside the Andes and the Himalayas respectively. The clouds responsible for what happened in Haikou, however, had their origin in the middle of South China sea.

We encourage skywatchers world-wide to keep an eye out for these elusive arcs. They might just pop up in the next odd-radius display over your backyard.

Jia Hao

[1] Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 -2 3) Miller index faces explain exotic arcs in the Lascar halo display," Appl. Opt. 50, F121-F128 (2011)

[2] M. Riikonen, M. Sillanpää, L. Virta, D. Sullivan, J. Moilanen, and I. Luukkonen, “Halo observations provide evidence of airborne cubic ice in the Earth’s atmosphere,” Appl. Opt. 39, 6080–6085 (2000)

Thursday, 31 May 2018

Odd radii halos outbreak in the Czech Republic

There was an odd radii halos outbreak in the Czech Republic and Hungary in the past few days.
Multiple rare halo forms were captured.
Let me show you some of the photos:

These were taken by Jiří Kaňovský from Černotín, Czech Republic:

He managed to capture 9° contact arcs, 9° parhelion and 24° parhelion among others. Uppercave Parry was captured, too.

These were taken by me:

I am very confused about the halo forms captured here. First I thought I captured 18° parhelions, but now it turns out those are probabaly 20° and also 35° column arcs. There is also a faint lower 23° parhelion. 9° contact arcs are possibly captured, too.
The sun was 62° high at the point of the photoshoot.
I managed to stack multiple photos and edit those in Photoshop to bring out the halo forms.
Can anyone confirm that those are indeed column arcs?

Here are some .NEFs (Nikon's RAWs) if you're interested:!ArJV0E31n7WMg0jl3Lh02eZjZy0p

Saturday, 14 April 2018

Odd Radius Display, Bolton, United Kingdom

On the 5th April 2018, there was a short lived but rather nice odd radius display in Bolton, a town situated in the north west of the United Kingdom. As I was preparing to go to work, I noticed the top of what I initially took to be a 22d halo. However, it rapidly developed and became more complex and very soon an 18d became easily visible to the eye. The display lasted about thirty minutes in total and I was only able to take a few single shots and a couple of stacks before it began to fade and I had to leave. Upon processing the images, 9, 18, 23, 24 and 35d halos with attendant plate arcs were identified as being present.

Processing courtesy Nicolas Lefaudeux.