Parhelic Circle and Halo
October 09, 2002
The above photograph shows a portion of both a 22 degree circumscribed halo and a faint parhelic circle (the white line to the left of the colorful halo). The parahelic circle is a colorless horizontal arc that on some occasions encircles the entire sky - it forms at the same altitude as the Sun. Whereas the 22 degree halo is caused by light refraction through the 60 degree prism faces of randomly oriented hexagonal ice crystals, the parhelic circle is caused by reflections off the vertical sides (with respect to an observer on the ground) of hexagonal crystals. Side faces of plate-shaped crystals and end faces of pencil-shaped crystals can both have vertical orientations. Reflection rather than refraction results in colorless arcs.