9°-parryform arcs (EE66)

At the top and bottom part of the 9° ring, one can observe white light enhancements in rare cases. These are the 9° parry-like arcs, known in the scientific literature as 9° plate arcs. They almost always occur alongside other halos created by pyramidal crystals. The upper arc forms up to a sun elevation of about 55°, but above about 30° it is only faint. The lower arc is possible up to high sun elevations, but becomes increasingly faint. In the simulation above, pyramidal crystals with an almost ideally vertical main axis (tilt angle 2°) were used. However, this hardly ever occurs in nature. Therefore, the 9° parry-like arcs are generally seen only as a brightening in the 9° ring and not as an arc bending away from the sun.

The 9° parry-shaped bows form on pyramidal crystals with a vertical main axis. The main axis passes through the two base surfaces. Such crystals are essentially plate crystals with pyramid caps on the base surfaces.