I woke up this morning with a picture in my mind of a once in a lifetime image from a few years ago. I knew it had a name and so I consulted t.o.b. hoping he would remember. His mind went blank but he knew he had it in his well organized filing system somewhere . . .
A circumhorizontal arc is an optical phenomenon - an ice halo formed by plate-shaped ice crystals in high level cirrus clouds.
I found this photo on Wikipedia describing it as "near Coeur d'Alene, ID". It may be the same one we saw while at a Farmers Market in Liberty Lake, WA (a few miles from Cd'A) a few summers ago.
How often a circumhorizontal arc is seen, depends on the location and the latitude of the observer.
The arc has a considerable angular extent and thus, rarely is complete as opposed to the halo that is sometimes seen around the sun. When only fragments of a cirrus cloud are in the appropriate sky and sun position, they may appear to shine with spectral colours.
A circumhorizontal arc may be difficult to distinguish from an infralateral arc when the sun is high in the sky. The former is always parallel to the horizon, whereas the latter curves upward at its ends.