Atmospheric
Optics
Delft Halos
The bright arc is part of the parhelic circle. The fainter halo forming a cross near the tip of the spire of the Nieuwe Kerk is the rare Wegener Arc.
Three images by Frank Nieuwenhuys on 16th July in the historic Dutch city of Delft.
All images ©Frank Nieuwenhuys, shown with permission
The Wegener arc circles the sky - twice. Above the sun it is tangent to the 22° halo and upper tangent arc/circumscribed halo. Its two arms meet at the anthelic point** 180° in azimuth from the sun. They then continue beneath the parhelic circle to meet again below the sun. There they are tangent to the lower extremity of the 22° halo and lower tangent arc.
The arc is formed by rays passing between to side faces of singly oriented columns inclined 60°. The rays do not pass straight through the ice but are internally reflected from the near vertical hexagonal end face.
**The anthelic point can appear (as here) as a bright spot on the parhelic circle. It is not a halo but is bright because halos cross there.
