Davos, Switzerland Rare Halos
Top image by Barbara Landl, lower image Christian Rixen.
Images ©The photographers, shown with permission.
Atmospheric
Optics

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A diamond dust ice crystal display dominated by a dazzling parhelion (sundog), white parhelic circle and 22-degree halo.

It is, however, the fainter halos that demand attention.

Horizontal column crystals generated supralateral and infralateral arcs with their characteristically pure and widely separated colours. They cross the parhelic circle to form a cusp shape – a sure indication, if the sharp colours did not already tell us, that the arcs are not a 46-degree halo.

The top picture hints at two rarer arcs. The brightening at 2 o’clock on the supralateral arc ‘might’ be a trace of a Parry supralateral (Tape arc) formed by Parry oriented columns. Curving upwards from the sundog is another rare arc possibility – a middle Lowitz arc.

The lower image and its unsharp mask enhancement show helic arcs formed by external sunlight reflections off the side faces of Parry crystals.

There is also a definite Moilanen arc, a halo form yet to be explained. The hint of a halo above the Moilanen arc might be produced by the same crystals that generated the latter.