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.