Mountain Halos, Switzerland imaged by Alistair Adams 21st December '04. Images ©Alistair Adams, shown with permission.

In this very exceptional and rare diamond dust* display the parhelic circle crosses the air as a solid white bar. The low sun is to the left and the coloured halos are infralateral and supralateral arcs from singly oriented column crystals. Often mistaken for a 46 halo, the cusp as they intersect at the parhelic circle distinguishes them clearly from the more rare circular halo.

The scene is repeated in another image at right which this time shows halos of rather greater rarity. The accompanying HaloSim** ray tracing simulation identifies them. Sloping upwards from the sun to eventually cross the parhelic circle is a faint subhelic arc, the product of a tortuous ray path with multiple reflections passage inside column crystals.

Projected against the blue sky and touching the supralateral arc is a Parry supralateral arc. At first seen only in Antarctica, this is one of the earliest images showing it elsewhere. Another arc from the improbably oriented Parry crystals is the helic arc visible faintly across the sky. Helic arcs are the glints from sun reflections off the side faces of the Parry columns.

Possibly but not with certainty formed downwind of ski-slope snow machines.
  
**  The simulation uses a mixture of plate, column, Parry and randomly oriented crystals. Populations were adjusted to give an approximate match to several images.


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