Rare Ice Halos at Porsgrunn, Southern Norway imaged by Morten Kleiva on the morning of 31 December '08 with the temperature at minus 12C. ©Morten Kleiva, shown with permission.

A beautiful photograph of a beautiful and rare display.

The obvious features are the bright sundogs generated by diamond dust plate crystals, a sun pillar and the almost semi-circular arch of a 22-degree halo. Touching its top is an upper tangent arc from column crystals. At this low sun the UTA curves sharply upwards. It opens up into a gull wing shape as the sun climbs.

Within the UTA and better seen in the second photo is a rare halo, a sunvex Parry arc.

Between the UTA and the sun is a rarer halo still. The faint V shape of a Moilanen arc is faintly visible. Check the labeled simulation below for its location.

There is a hint in the second image of another rare halo - a helic (old name heliac) arc produced by external reflection from the side faces of Parry oriented crystals.

Primed by foreknowledge and averted imagination you might also discern traces in the second image of a middle Lowitz arc curving upwards outside the 22 degree halo to become tangent with the base of the Parry arc. No claims are made for this but strong image enhancement gives indications of it plus an upper Lowitz arc. See this image for the appearance of an a middle Lowitz at a similar solar elevation.

The HaloSim ray tracing used a mix of 55% random oriented crystals, 16% plates, 25% singly oriented columns and 4% Parry columns.

That the Parry arc is roughly as bright as the UTA highlights the very high efficiency of Parry arc generation. Without that efficiency we would almost never see the arcs of William Edward Parry.

The Moilanen arc was separately simulated with ice wedges of 34° angle. We do not know what crystals actually form it in nature except that its rays somehow pass between facets inclined 34 degrees to each other.


About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed