24° Column Arc, Finland

Markku Könkkölä (photography, blog) imaged this very rare arc in July 1985 at Utsjoki by the Teno River in Finland.

At right is the original film image, at left it is processed by a colour subtraction technique of Nicolas Rossetto that enhances coloured halos. The sun was 42.5° high and the rare arc nestles between the bright circumscribed halo and a the much fainter inner 22° circular halo.

This is only the second known image of the arc. The first was by Frank Nieuwenhuys at The Hague, Netherlands in 1973**. It was recently imaged by Radek Grochowski in Poland.

Lowitz arcs might seem commonplace by comparison! However, these arcs are easily mistaken for fragments of circular halos. Check 'circular' halos carefully for they could be something more exotic.

*Images ©Markku Könkkölä, shown with permission

**Riikonen M, Moilanen J, Sillanpää M & Nieuwenhuys F, Rare odd-radius column arcs, Weather, 57, 346-351 (2003)
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The halo belongs to a family of ‘Odd-radius column arcs’ produced by oriented pyramidal crystals. Common pyramidal crystals have short central hexagonal prism sections and tend to take random orientations. We then see a series of circular ‘odd-radius’ halos.

Long hexagonal columns with pyramidal ends can, in contrast, take precise orientations. The long axis is horizontal, often to better than a degree, while the crystal is free to take any rotational position about the axis. An individual crystal does not necessarily ‘spin’. Instead, millions of crystals happen to have all rotational positions to produce ‘column arcs’.

The 24° column arc of the image was produced by rays entering a pyramidal face and leaving through an ordinary prism side facet (path 15,3). The reverse path gives an equivalent arc mainly beneath the parhelic circle.

Other ray paths generate 20°and 9° column arcs. The 20° rays (13,16) pass through two pyramidal faces which, if small, would give only faint halos. The 9° column arc (13,6) is out of the camera field and may or may not have been visible.

HaloSim ray tracing
Pyramidal crystals are mostly rather rounded from an aerodynamic viewpoint. They tumble and tilt in all directions
A few take up plate-like orientations to generate pyramidal plate arcs.
Singly oriented column with pyramidal ends generating a 24° arc.