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)



  1. This is getting to be very interesting

  2. For us in the west, and by west I mean primarily northern Europe, China and the far east is something of an unknown quantity, an enigma, when it comes to the study of halos. You are obviously well versed and well organised but would you care to say something briefly about the present halo community in China, its extent and the level of serious interest amongst its members? Thank you.

    1. I hope to see some 28d stuff one day

    2. Hi Alec,

      A few years back the community started as a small hobby chat group hosted by Ji Yun covering all sorts of atmospheric phenomena. It has now scaled to a group with hundreds of members and around 20 active contributors country wide. Instead of solely relying on contributors, we proactively scout for sighting reports on various social platforms in China, primarily Weibo (the Chinese equivalent of Twitter). Thanks to the massive user base of these platforms, seldom does a major event go off our radar. Ji Yun also hosts a public Weibo account (currently with 200k followers) publishing sighting reports regularly and compilations annually to boost public awareness.

      In terms of the study of halos, I'd have to say none of the community members qualify as serious researchers and sadly, we haven't heard of any real researchers working on this topic in the Chinese academia. There're, however, at least 5 members including myself who have serious interests in halos and are experienced enough to independently identify majority of known halos. We also have a bunch of experienced programmers developing a halo simulation web app in the hope of making halo simulations more accessible and user friendly to newcomers.

      Now that the growth of the community has awarded us with two world class sightings, I'm sure there'll be more to come : )

      Jia Hao

    3. Thank you, that's most informative. I'm simply amazed that you have 200k general enthusiasts on your Weibo group. I just cannot envisage that level of interest here in the west. If only a small proportion of those followers develop a serious interest in the study of halos, I'm sure it will be a huge benefit for halo science. I for one am sincerely looking forward to any more observations you may care to share with us!

    4. Alec asked the same question that I wanted to ask you. For years I was wondering whether there were any serious halo communities outside of the realms I knew. And there you came, sharing some of the the most amazing observations in recent years! I am also looking forward to the upcoming posts from China, and I feel really happy to hear about your group.

  3. Hope more sightings of 28d halo and its arcs begin to pop up worldwide. Just another reason I carry my camera everywhere I go when driving

    1. Hi Michael, I'm quite sure more examples will be observed over time as increasing numbers of people begin to actively look for them. Having a good stacking and processing workflow will maximise ones chances. With three confirmed observations, I wonder whether the 28 arc should now be downgraded from ultra exotic to extremely rare!

  4. Replies
    1. Hi Michael, I have written to you privately.

  5. The more years passed the more I started thinking Lascar was a one-off. And now we have suddenly a bunch of Lascar displays. So, right, we can start thinking of cracking fully the Lascar mystery some day. Nicolas came up with an excellent model, but I am not sure the 13 and 3 degree arcs it predicts actually exist (they can be, though, removed by assuming triangular crystals). Measurements of the former (a fuzzy light blotch in the scans made from slides) seem to place it some 11-12 degrees above the sun (Jarmo and I just redid the measurements indepently) and I have a recollection it was V-shaped visually. We most likely did not manage to photograph its best manifestation, as the arc was flashing on and off in seconds. I have this recollection of us trying to catch it on film at its peak, but after the photo was taken, I felt like we didn't quite succeed. It was changing so fast. Other arcs in the display were of stable intensity. That sort of tells me this "Moilanen arc" -- as I seem to have referred it in some past writings -- was a separate feature, possibly even born in a cloud layer at lower level than the other arcs.

    So yeah, this is one of Lascar mysteries that I hope the upcoming Chinese observations will shed light on. And then we have actually even a realistic chance to photograph the crystals. In 2010 (or around that time) Jari Luomanen and I photographed a 28 halo in a snow surface display. We just didn't take the crystal sample. The challenge is still up.

  6. Hey Marko can you send me a copy of halo point?