The Billy Lids

According to an article on the National Geographic News website, this particular fire rainbow was caught on film on June 3 2006 in northern Idaho not far from the Washington State border. A fire rainbow is more technically called a "circumhorizontal arc" or "circumhorizon arc" by weather scientists.

A circumhorizontal arc only appears when a specific set of atmospheric conditions occur. The display is caused when light entering hexagonal ice crystals in the clouds is refracted. But, for a fire rainbow to appear, the sun must be very high in the sky, the clouds must be high altitude cirrus clouds and the ice crystals must be shaped like thick plates and aligned with their faces parallel to the ground. Thus, fire rainbows are indeed quite rare, although it is probably an exaggeration to describe them as "the rarest of all naturally occurring atmospheric phenomena".

A Scienceline article discussing another fire rainbow photograph taken on the same day as the one shown above notes that even diligent weather watches in the United States might only see one or two circumhorizontal arcs in a given year.

A list of relative frequency for rare atmospheric halos published on the Atmospheric Optics website shows that circumhorizon arcs (circumhorizontal arcs) occur very infrequently. However, others such as the Kern Arc and the Lowitz Arc may be even more infrequent.

Another spectacular shot of a circumhorizontal arc taken in Canada in 2003 is available on the Atmospheric Optics website.