Upper Tangent and Parhelia
December 28, 2006
The photo above showing an upper tangent arc and a sundog (perihelion) was taken on October 31, 2006 in the Nagy-Kevély Mountains of Hungary. On this bright autumn day, various forms of halos and arcs were visible all afternoon and even into the night (lunar halo). Arcs tangent to the 22 degree halo (at top) are formed by columnar ice crystals, typically found in cirrus clouds, having their 'c' axis horizontal in respect to the surface -- think of a pencil on a desk. However, these crystals aren't all positioned similarly -- they have random orientations. Sunlight enters one of the side faces and exits through an alternate side face. Oriented hexagonal plate shaped crystals are responsible for the formation of sundogs -- think of a dinner plate on a table. Here also, sunlight enters a side face and exits an alternate side face.