OPOD - Lowitz Arc over California

Lowitz Arc, California - Andrew Kirk captured this bright Lowitz and Parry arc display over California on 13th September '12. Not so long ago the very existence of Lowitz arcs was doubted and examples of this clarity still remain rarities. Images ©Andrew Kirk, shown with permission.

The traditional place to search for Lowitz arcs is, where Lowitz first recorded them in 1790, as small arcs extending sunwards from the sundogs and joining the 22° halo. That is perhaps one reason why they were so rarely observed.

The upper Lowitz arc is more frequently seen, as here, curving downwards from an upper Parry arc - crossing the upper tangent arc - and then merging with a 22° halo.

The HaloSim ray tracing at right shows Lowitz crystal arcs in red. Arcs near the sundogs are prominent because traditionally oriented Lowitz crystals were used. Those in Andrew Kirk's display might have been columnar and rocking back and forth around the Parry orientation. This strongly favours the upper arc that was observed. There is no evidence - see lower image - of arcs near the sundogs.

The HaloSim rays tracing shows extra Lowitz arcs approximately 46° from the sun. These are from rays passing between a hexagonal end face and a prism side face. One of Andrew Kirk's images - enhanced at right - shows fragments of a double arc in this region. It is dangerous to over-analyse or speculate on a single image. However, the outer arc might be an ordinary supralateral arc - or a rare Parry supralateral. The inner arc might be a 46° Lowitz arc fragment. Another possibility is that the latter is a small part of a 46° halo.

We shall likely never know. As here, always take time to observe and photograph the sky away from the showpiece halos!