A spectacular (and very rare) ice halo photographed (on an iPhone) by Michael Schneider this month in the Swiss Alps. According to Schneider, the phenomenon developed gradually as fog dissipated at the top of a ski resort.
The annotated overlay (using information from this site on atmospheric optics) was created by Mark McCaughrean.
Previously: Dublin Rainbow
(…all the way)
Actually, an ice halo photographed over Dublin, Ohio, in 2009. To wit:
The reason here is that ice crystals in distant cirrus clouds are acting like little floating prisms. Sometimes known as a fire rainbow for its flame-like appearance, a circumhorizon arc lies parallel to the horizon. For a circumhorizontal arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds are present. Furthermore, the numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight in a collectively similar manner. Therefore, circumhorizontal arcs are quite unusual to see.