Atmospheric
Optics
Parhelic Circle & Wegener Arc, The Netherlands
~ Imaged 16th July '15 by Peter Visser. A white parhelic circle curves across the frame. At top left are the 22° halo and circumscribed halos. Above and below (bottom right) the parhelic circle is the rare Wegener arc so named after the meteorologist, polar researcher and originator of continental drift ideas.
A HaloSim ray tracing using an all sky stereographic projection centred on Peter Visser's camera pointing direction and a 52° high sun. The approximate extent of his image is the blue rectangle.
Horizontal columns and some randomly oriented crystals are sufficient to reproduce the display (although plate crystals might have been present).
The complete Wegener arc circles the sky from the top of the circumscribed halo to its base. It crosses the parhelic circle at the anthelic point.
Wegener rays enter and leave column crystals through side faces inclined at 60° but differ from those generating the circumscribed halo in that they are internally reflected from an end face.
The end face reflection accounts for their rarity because end faces are often indented and optically imperfect.
Whenever there is a strong parhelic circle that might also be from the end faces of column crystals, look for the Wegener arc.
