|Welsh Pyramidal Halo - Imaged this morning (July 13) near Bangor, North Wales by Graham A Stephen. ©Graham A Stephen, shown with permission.|
|The outer circular ring is probably** a common or garden 22° halo from poorly or randomly oriented ice crystals in the cirrus cloud.
The inner halo is a rarer 9° radius halo produced by pyramidal ice crystals.
Most cloud crystals are varieties of hexagonal prisms with flat ends perpendicular to the prism side faces. But sometimes the crystals are capped instead by hexagonal pyramids. The additional faces (up to 20) offer extra routes for light rays and can produce a number of extra ‘odd radius’ halos.
The 9° halo is formed by rays passing between one of the ‘original’ prism side faces and an opposite pyramidal face. The two faces are inclined equivalent to a prism of angle 28°. This compares with the 60° inclination that yields the outer 22° halo.
**Probably – When pyramidal crystals are about, outer halos can be one or combinations of 20,22, 23 and 24 degree radius halos!