On Thu, 18 Oct 2012 04:36:33 -0700, Edward Erbeck
wrote:

This happens when there is a thin layer of high Cirrus Clouds.

Crazy Ed

Very pretty, and one of my favorite subjects. Actually occur best in
very thin 'young' (recently condensed) altostratus clouds or cirrus
that hasn't glaciated yet, with nearly uniform-sized cloud droplets
small enough to refract spectral colors. Normally found with 10~20°
degrees of the sun.

When the droplet sizes are even more uniform, you can get more
organized circular coronae.

Last image shows a altostratus corona, with the much lager, diffuse
Bishop's ring in the background caused by refraction in the
stratospheric sulfuric acid emissions of Mount Pinatubo, erupted
nearly 18 months earlier.

Bob ^,,^