Alpine Halos ~ Diamond dust crystal halo display at Bettmeralp, Valais, Switzerland imaged by Bent Iversen. ©Bent Iversen, shown with permission.

Atmospheric
Optics

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The dominant halo is the circular 22 produced by some kind of non-oriented crystals. At each side is a weak parhelion (sundog) from horizontal plates. A thin white parhelic circle crosses the sun.

At top and bottom of the 22 are tangent arcs. Hexagonal column crystals drifting in the air with their long axes nearly horizontal produce them. Their shape changes dramatically as the sun climbs and eventually the two tangents join to form a circumscribed halo. At high sun the latter is frequently mistaken for the more diffuse and less coloured 22 degree halo.