At this time of year, the weather changes often. Two days back, the usual low clouds and rain were unexpectedly interrupted by clear sky. In the midst of the plain blue sky, I noticed a small thin cloud above the sun. It seemed high enough to be cirrus, though I did not know what type. Did it have ice?
Five minutes later, I step back outside on a brief errand and see that the small cloud had drifted into a revealing location, one in which the ice crystals could disperse the light into the telltale colors of the circumzenithal arc. Yes!
Few people ever notice this display even though it is actually fairly common, definitely more common than the rainbow (unless you often visit a waterfall), and also of greater spectral purity. You just have to know when and where to look.
But the display is fleeting. A minute later, the cloud had drifted out of the location and the arc was well on the way out.
You need to check the skies often.
— Jon
I saw the explanation in The Story of Ice. Nice to see this other picture of CZ arc.