Sunday, 2 April 2017

Subvisual odd radius halos in the UK

In the afternoon of 26 March, 2017, I was spending time outdoors with my family and wasn't too well equipped for serious observing. Of course, I had paid some attention to cirrostratus clouds drifting in the sky, but most of the time I struggled to see any significant halos. There were just occasional patches of 22° halo: or at least that's what I thought they were. But then the upper suncave Parry arc appeared so I had no choice but to start photographing immediately. After all, you don't get Parry every week, and in fact this was my first such encounter in 2017. During the next 20 minutes or so, I took a decent set of photos, but then gave up as it appeared that the display had become insignificant again.

My hope was to find a few more halos in the post-processing, and stacking did a good job indeed. The stack shown above is from the first 50 frames and covers a total of 196 seconds. On the left the unsharp mask is applied on individual frames before stacking. The version in the middle is a gradient-subtracted average stack, which is further processed by using the blue-minus-red subtraction technique on the right. In addition to the usual stuff and the Parry arc, we can identify upper and middle Lowitz arcs as well as a short piece of helic arc to the left from the circumzenith arc. But that's not all - there are also odd radius circular halos.

For comparison, the stack below is from the last set of 30 frames that I took more than 30 minutes after the last signs of the Parry arc had disappeared. This stack covers a total of 174 seconds. I didn't really expect to find anything special at this point, but took the photos anyway out of curiosity. Apparently the odd radius stuff is still in play. My feeling is that it had been there all the time, possibly long before I noticed the Parry for the first time and also long after I had got indoors to celebrate the Mother's day dinner.

What is shown below is an attempt to make the scene as clear as possible by combining photos from two different series into one stack. 100 frames are included, covering 13 minutes in time but missing about six minutes in the middle. My interpretation is that the relatively typical pattern of 9°, 18°, 24°, and 35° halos is complemented by the exotic 13° halo. But I'm not sure and it would be great to hear what readers might think of the case. Whether the 20° halo is missing altogether or masked behind the suspiciously wide 18° halo, I am not sure of that either.


  1. "My feeling is that it had been there all the time, possibly long before I noticed the Parry for the first time and also long after..."

    I think this statement has quite important implications for all of us. A sub-visual halo is just that, below the threshold of visibility. Interesting halos may be present on a far greater number of occasions than we have hitherto thought. It's a case of out of sight, out of mind. Perhaps we need to radically alter our approach by routinely taking a test stack or two when conditions seem promising, regardless of whether or not there is anything obvious to photograph. For example, only yesterday, we had some decent cirrus in the morning but there wasn't a trace of a 22 visually, even in the mirror. Just before I went to work I took a quick stack on the off chance which upon processing later revealed 9°,22° and 23° halos. How many of you have seen a basic 22° and thought it's not even worth getting the camera out? I think we need a mental shift to help us overcome this prejudice.

    To conclude, I think the very presence and possibility of sub-visual halos is a vindication of the use of the Lefaudeux Halocam, a camera working 24/7 which will capture ALL halos, regardless of whether they are visible or not. If more of us could develop such a system, we could be secure in the knowledge that nothing would ever be missed and at the same time we would be contributing to halo science by capturing the fleeting visitation of the rarest halos.

  2. I'm really amazed at how after-processing techniques have contributed to halo-science in the past decade. It's incredible how much full-frame cameras can capture and then how much we can bring out of the raw images afterwards. First there was the advent of scientific writing with Newton and the others. Then the heroic age of arctic explorers. And now the revolutionary digital era which adds so much to halo science. Halo-history is being made in the 21st century.

  3. this is very interesting case, extremely similar to what halocamera has caught on several occasions.

    the satellite images also show very similar cloud history, with the responsible clouds sublimating over germany at the beginning of the day, then drifting as very thin over netherland and regrowing over UK. this is exactly what i have seen also on my 13d cases. sublimating clouds that start regrowing, then 13d appears.

    see the video here, can you confirm that this is indeed those clouds that produce the 13d halo?


  4. I think you are absolutely right Nicolas. Just as a clarification, if you stop the movie at 15:15 UTC, there is a thick cirrus cloud over UK Midlands. My location is right at the edge of that cloud on the Southwestern side (and that's exactly the time I took my photos).

  5. has the 13d ring been seen visually?