Thanks for the link. I assumed it was a recent photo, necessitating the circumzenithal arc (low Sun), whereas the caption says it was taken in June 2006, which is the season when you'd expect the circumhorizontal arc (high Sun, arc fairly low in the sky). The photo does show the irridescent character of the light very well. Yes, they are uncommon, but I wouldn't call them "rarest" by a long shot. We get them pretty regularly in Arizona particularly in late spring when there is still some cirrus around of the right sort but with the Sun above 58 deg elevation. (It doesn't have to be exactly that angle, just _above_ that elevation.) As I recall, the crystals in the clouds have to be dominated by ones hanging nearly vertical (imagine a six-sided pencil with a umbrella-like snowflake cap on end end---the whole thing about 1mm long or less). More details (and photos) can be found at Les Cowley's excellent atmospheric optics pages: http://www.atoptics.co.uk/halo/cha.htm \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.