23° Parry type arcs (EE69)

The upper 23° parry arc is a concave arc that sits like a hat on the 23° ring . It can only form at a sun elevation of about 15°. Between a sun elevation of 30° and 55°, the upper 23° parry arc is most intense. As the sun elevation increases, the arc dissolves. The lower 23° parry arc is visible only from a sun elevation of about 47°. In general, the 23° parry arcs do not touch the 23° ring. The upper arc touches it at a sun elevation of about 47.6°. In the scientific literature these arcs are called 23° plate arcs.

The 23° parry arcs form in pyramidal crystals with a vertically oriented main axis. The main axis passes through the two base surfaces. Observations show that pyramidal crystals do not align as precisely as plate or column crystals. The tilt angles are usually more than 10°. Plate and column crystals, on the other hand, often have tilt angles of less than one degree.