Peter O'Toole imaged this fine circumscribed halo from near Creeslough, Letterkenny, Co.Donegal, Ireland. The oval circumscribed halo surrounds a 22° halo and touches it above and below the sun.
©Peter O'Toole, shown with permission.
|The two halos are formed by the same ray path through ice. It enters a side face of an hexagonal prism and leaves through another facet inclined 60° to the first.
Crystal orientation makes the difference.
Randomly or poorly oriented crystals (of some sort, not necessarily simple columns) give the inner and circular 22° halo. Hexagonal columns with their long axes nearly horizontal form the outer circumscribed halo.
The circumscribed halo is a closed form of upper and lower tangent arcs. When the sun is above 29°, they fuse into a droopy oval.
As the sun climbs the oval shrinks and eventually it is almost indistinguishable from a circle.
Many colourful tropical high sun "22 degree halos" are in fact circumscribed halos.