The
Wegener anthelic arcs are ice-based
atmospheric effects. They are also called
Wegener arcs. They are two huge, very rare white
arcs that stretch in a
teardrop shape whose point is the
anthelion and whose bowl rests tangent to the upper
tangent arc. They are distinguished from each other simply by position as you face the sun: the left and right Wegener arcs. They are enormous when the sun is low in the sky, and grow smaller as the sun rises higher.
These arcs are named for Alfred Lothar Wegener, a German meteorologist who in 1918 developed a principle for understanding celestial mirages and astronomical refraction.
Les Cowley's HaloSim simulator reveals that if the Earth were invisible, you would be able to follow the diffuse arcs from from where the Wegener arcs touch the anthelion to below the horizon where they would create a second gigantic teardrop shape, creating an enormous infinity symbol (∞) with little old you in the center.
They form when singly-oriented hexagonal needle-shaped ice crystals, as in cirrus clouds, float in the Parry orientation with their long axis horizontal and one edge face vertical. Sunlight passes into one edge face, reflects internally off of a base face of the crystal, and diffracts out an edge face adjacent to the entrance face plus one, making a 60° bend from sun to eye. The same ray path less the internal reflection creates the circumscribed halo.
Sources
- http://www.sundog.clara.co.uk/halo/spzenith.htm
- http://www.weather-photography.com/Photos/gallery.php?cat=optics&subcat=wegener_arcs
- http://www.meteoros.de/arten/ee56e.htm