>> So is there any agreement on what would be "the rarest of all naturally >> occuring atmospheric phenomena" Probably no single event/phenomenon. There are, for instanace, various classes of halo-type things that ought to occure from rare crystals (such as ones having pointed ends rather than flat ones) but simply haven't been observed. A lot of crazy stuff has been observed at the South Pole where the requisite crystals are floating right in front of your nose. The Cowley Web site mentioned previously presumably has very rare/not-yet-seen phenomena listed somewhere. See his images/simulations under 'infrequent halos' and 'complex displays' starting at: http://www.atoptics.co.uk/halosim.htm \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.