|Tropospheric Bishop's Ring
Imaged at Tampere, Finland on the morning of 31st March by Marko Riikonen (more ring images, site).
Images ©Marko Riikonen.
The upper image is not out of focus! It is a stack of several images centered on the sun to accentuate the soft details and contrasts of the ring. The lower image is a single exposure.
"I went out to look for halos. But upon looking at the mirror [to reveal faint and subtle sky phenomena the Bishop's ring caught my attention. It occurred in some cloud that seemed like it could develop to altocumulus (Ac). Went inside to get camera and took about 20 photos to stack. It was not as good [as previously] but still OK, the cloud was thinning. As the cloud thinned even more the red zone disappeared leaving only blue, which stayed."
A Bishop’s ring is essentially a huge corona, a diffraction pattern in the sky produced from light scattering by very small particles. This one is larger than a 22° halo.
The cloud water droplets that produce an ordinary corona are typically 10 to 30 micron across. The larger Bishop’s ring comes from much smaller objects and most often they are stratospheric sulfate and dust particles, the sometimes globally distributed aftermath of large explosive volcanic eruptions. The same particles give glorious sunsets and twilights.
This ring is different. Marko is sure that it was formed in the troposphere, in nascent altocumulus. The reddish ring is ~30° in radius and an IRIS calculation indicates that the supercooled water droplets were only ~0.5 micron or smaller diameter, at the very lowest end of the sizes in clouds.