|Rare Diamond Dust Halos, Switzerland
Ken Jay took the images and made a montage. The display was at Tuftern, close to Zermatt. The Matterhorn juts upwards between the sun and the right hand sundog. December 18, '08 with a sun ~7° high. Images ©Ken Jay, shown with permission.
|The outer halo is a rare 46° halo, truly circular and at this solar altitude far from the uppermost circumzenithal arc. The more common supralateral arc, oft mistaken for a 46° is weak as is the upper tangent arc from the same horizontal column crystals.
Parry oriented crystals produced a ‘V’ shaped arc convex to the sun and above it a faint suncave Parry arc. The shape and position of these two rarities change quickly as the sun’s altitude alters.
Parry crystals produced two even more rare halos. One is just visible in the montage as brighter smudges on the 46° halo. These are Parry supralaterals or Tape arcs named after halo expert Walter Tape. More images in a forthcoming OPOD feature.
The other Parry generated rarity are helic arcs – glints of sunlight externally reflected from crystal side face.
"Snow cannons were operating all day less than 1 kilometre from the site of photograph."
That's probably the key to the visibility of these rare halos. Before snow guns were regularly used they were seen only in Antarctica. Ice crystals nucleated by snow machines and growing downwind of them produce some wonderful halos.