Lone Halo, Norway ~ Upper tangent arc imaged by Lars Brubæk February 25, 2010. The location is Ytre (outer) Bogge  , a small community on the eastern shore of Eresfjord. ©Lars Brubæk, shown with permission.We usually see a 'gull wing' shaped upper tangent arc atop a 22° halo. However, the two halos have separate origins. The upper tangent arc is the glints of sunlight passing through hexagonal column ice crystals oriented by aerodynamic drag forces to have their long 'c' axes nearly horizontal. The better the crystal alignments, the sharper is the halo. The crystals can take all rotational positions about the long axis although they do not necessarily individually 'spin'. The rays pass between two upper prism faces inclined 60° to each other. The plane of the ray can be skewed relative to the axis. Red light is refracted less strongly than blue giving the halo a red fringe on its sunward edge. Equivalent rays passing between lower prism faces give a 'lower tangent arc' beneath the sun. When the sun climbs above 29° the upper and lower tangent arcs join to form a circumscribed halo. |
