I received a fine treat from Mother Nature as I finished my lunch break this afternoon. A brilliant arc from a would-be solar halo skimmed the trees outside the office windows. There was briefly a fainter, apparently complete, halo of much smaller diameter closer to the Sun. Icy cirrus clouds –natural or spawned by aircraft vapor trails– made interesting colors and textures within the arc. A few minutes between two exposures made a big difference in the appearance of the colored arc… a band of clouds passing in or through the effect gave it added dimension.
Addendum: An interesting photo was posted to the Palomar Skies blog… yes, the historic and world-famous Palomar Observatory! See: Palomar Flies its Colors! It turns out the technical name for this phenomenon is "circumhorizon arc." More on this may be found at: Atmospheric Optics.
PhotonStopper 
FANTASTIC!!! What a gorgeous photo. Very pretty how the sky is lighter blue below the rainbow and dark above. Great catch, JamesG!!
I was surprised at the contrasting areas myself, Freedom! That's not a Photoshop "burning" effect, it's for real. I think it's a band of clouds just to the south of the arc. Earlier a band of clouds was passing across the arc and gave it a three-dimensional effect looking like bunched fabric or a wave of sherbet. I'll probably post that image later but #4737 is my favorite. Thanks for your compliments! — JG
Oh, do post it. I would love to see that one as well. Unusual to see such beauty on any given day!
Your wish is my command, FS: I've posted the second photo. I'll probably rework the images once I get home to make the "levels" adjustment equal between them. Or I may not. — JG
Thanks, JG!! Wow, my wish is your command? 🙂 I will remember that. (Just kidding.)