- Crystal orientation:
- main axes vertikal
- Path of light:
- 13-5 (upper), 3-25 (lower)
- Occurrence:
- very rarely
EE 70
Halodisplay on 04/03/98 in Chemnitz / Germany, which was caused by pyramidal ice crystals.
There is a 9°-parry-shaped arc, 9°-halo, 9°-tangent-arc,
22°-halo, parhelia, upper tangent arc, 24°-halo and a left upper 24°-lateral-arc.
Photo: © Claudia Hinz
Description:
Subject to the sun elevation up to four white brightenings can appear in the 24°-halo. These lateral arcs are situated at the upper respectively lower right and left section of the 24°-halo. In the picture the upper left 24°-lateral arc can be distinguished as a faint brightening in the 24°-halo. Only when the main axis of the crystals is orientated horizontally in an almost perfect way, one can recognize that this halo is shaped like an arc with the lateral arcs curving away from the 24°-halo (see also the simulation). At sun elevations of 30° or more the two upper lateral arcs become very faint and cannot form at higher sun elevations. The two lower lateral arcs, however, can form up to a sun elevation of 60° whereby they move away from the 24°-halo.
Computer simulation for a sun elevation of 20°.
The 24°-lateral arcs are situated at the upper and lower right and at the upper and lower left part of the 24°-halo. Halos which are caused by vertically orientated pyramid-shaped crystals are drawn in violet colour, halos caused by randomly orientated pyramid-shaped crystals are drawn in red.
Formation:
24°-lateral arcs are caused by pyramid-shaped crystals with their main axis orientated vertically. The main axis passes the two base faces. Face number 1 in the diagram of the crystal is the upper base face, the 2nd base face is situated opposite No. 1. Observations show that pyramid-shaped crystals do not orientate as exactly as do plate- or column-shaped crystals. In most cases the tilted angle is more than 10°. Plate- and column-shaped crystals, however, often are tilted less than one degree.