On May 7, I was working on my programming side project. At about 17:15 local time (UTC+2h)
I was finally able to take a short break. I decided to get some fresh air on my balcony (facing south).The temperature was about 17-18°C and a substantial fraction of the visible sky was covered in rather thick clouds. The zenith area, however, was quite clear, except for a layer of nebulous cirrostratus clouds and some broken mid-level clouds. I could see a faint circumzenithal arc, so I went back inside and grabbed my camera. I took only three pictures of the CZA at 17:20, 17:21 and 17:22, before the arc disappeared completely. I noticed it was a little broader than usual, but I thought that maybe it had something to do with the solar altitude. At 18:30, I finished my work for the day and looked out of the window again. The CZA was back and I thought I could see a supralateral arc as well, so I took some more pictures. About 25 minutes later, thick clouds rolled in from the west and obscured the display. 
Since it wasn’t a particularly bright display, I didn’t think about it much until the next day when I began processing the photos I took over the last week. I was pleased to find CZA, supralateral and a suncave Parry arc (Apr 30) and another CZA + supralateral display (May 6), and then began to process the few photos that I’d taken on the previous day. The first one showed just a normal CZA, but in the second one I could see a faint rainbow smudge right under the CZA. At first, two things came to my mind: 46° halo and camera artifacts. Stellarium told me that the sun altitude was 25°. According to a chart I found on the internet, at this solar altitude the CZA should be quite narrow and the 46° halo should touch the CZA. Also, the arc in my pictures followed the shape of the CZA instead of curving downwards. I could see it in the third picture (taken in a slightly different direction) too, so I thought I could safely assume it was not an artifact or a lens flare. The rest of the photos I took later in the evening revealed a regular CZA, supralateral arc and a 46° halo – they looked completely different to the weird double arc. Then I remembered reading about an unusual type of halo related to the CZA before, but I couldn’t remember where, so I submitted my report to spaceweather.com in hope of it getting some attention there.
Meanwhile, I went on looking for the mysterious CZA phenomenon I read about earlier. I stumbled upon it a few days later, by accident. Someone had posted a link to a Halo Vault article on reddit describing a secondary CZA spotted in China. Then I found the report of A.F. Jensen. I was really excited when I realized that the second report fitted my observation quite well, so I decided to write you immediately.
Lately, halos have been keeping my quite busy. I could see another CZA / supralateral / suncave Parry arc combo in the morning of May 14 and there is a nice 22° halo visible right now. Honestly, I can’t remember seeing that many displays in a row since I’ve seen my first one in May 2005!
- Tomasz Adam
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