[gallery link="file" columns="5"]
Some odd radius displays from last summer in Tampere. In most of them there was only 23° plate arc visible. Although the displays were not that impressive, there were plenty of them. In July-August a record breaking heat wave produced many 23° plate arcs, I saw them on 14 days. Stacked images usually revealed more pyramid halos, mostly the 9° halo or 18° plate arcs. 23° plate arc is very easy to identify mistakenly as 22° upper tangent arc.
As a rule, when it is hot weather period, 23° plate arc is more common than 22° upper tangent arc. I am pretty sure they are not limited to Finland and Estonia only (in the latter Marko Krusel has a lot of observations). Hopefully we start getting observations of them coming summer from elsewhere.
The gallery above does not include every odd radius display from last summer. I could not find all them from the hides of my disorganized computer. I'll add them as they come around. Many displays were also not photographed. The dates are given in the file names. In some cases stacking has really made wonders in squeezing out the halos invisible in single frames. In the 15 August display, where there was only solitary 23° plate arc visible to the eye (and nothing more even through a convex "blind spot" mirror), stacking photos taken during about 2 hours unveiled a complex display with 35° halo as shown by the 8 and 9 images in the gallery. The two upper and lower images are single frames, the two in the middle are the same stacked image with different level of unsharp masking. Also the display of 19 August improved markedly by stacking photos taken during only 3 and 13 minutes, the last two images in the gallery. Again comparison with single frames is given.