Crystal type:
- Pyramidal crystals
- random
- Refraction on 2 pyramidal surfaces
Occurrence:
- rare (about once a year)
Description
The 18° ring is a faintly colored ring of light with a reddish inner edge and a radius of 18° around the sun. It usually appears with other halos such as the 9° ring, which are produced by pyramidal crystals. The 18° ring is the second inner ring after the 9° ring. It is also called Rankin's halo. Pyramidal crystals with a vertical main axis also produce the 18° lateral arcs.
Origin
The 18°-ring is formed by randomly oriented pyramidal crystals. The light passes through two pyramidal faces at opposite ends of the crystal, which enclose an angle of 52.4°. The graphic above shows the light path.
Photos
18° Ring taken in Frankfurt (Main) on 06.06.2011 (Photo: Andreas Zeiske)
Halo phenomenon with 18° ring on 06.02.2001 in Schlägl (Photo: Karl Kaiser)