Kern & Lowitz Arcs
An exceptionally rare sighting by Reima Eresmaa over Berkshire, Southern England late afternoon on April 20, ’15.
“At first glance there was nothing extraordinary about the display except the intermittently visible upper suncave Parry arc. However, by stacking sequences of images on top of each other and applying some quite heavy processing several rare halos became apparent.”
The grey-scale image is Reima’s heavily processed version. Mouse over it for a version closer to the visual appearance.
The pair nicely demonstrate that the very rarest of gems can hide in even mundane halo displays.
More of Reima's images here & ones enhanced by Nico Lefaudeux here.
Images ©Reima Eresmaa, shown with permission
Kern arc rays enter a top hexagonal face, reflect internally from a side face and then exit through a second adjacent side face.
The Kern Arc is a full circular continuation of the circumzenithal arc.
The Kern is much rarer than Lowitz arcs. After one or two reports of visual sightings, notably one during the great Saskatoon display of 1970, it was finally captured on camera by Marko Mikkilä in 2007.
Like the circumzenithal arc, it is formed by horizontal plate crystals but the Kern ray path needs improbably thick crystals or – more physically likely – ones of quasi-triangular aspect where alternate side faces are long and short.
This HaloSim ray tracing shows other rare arcs in Reima's processed images. The projection is an all-sky view centred on the zenith.
The upper Lowitz arc is strong in the images and likely originated from Parry-Lowitz oriented crystals.
Besides the bright upper suncave Parry arc, there were hints of rare Parry supralaterals (Tape) arcs. Whilst these occur relatively frequently from snow blower crystals above ski slopes they are hardly ever seen in high cirrus.