Twin Halos ~ An unusual view of circumscribed and 22° halos generated by glinting diamond dust crystals.   Captured by Ryan Skorecki while at the Don Sheldon Mountain Hut on the Ruth Glacier, Denali National Park, Alaska.   ©Ryan Skorecki, shown with permission.

The inner 22° halo is circular while the outer circumscribed halo is a drooping oval.

Both are made by rays refracted through the side faces of hexagonal ice crystal prisms. Rays enter a prism face and leave through another one inclined at 60° to the first – equivalent to passing through a triangular 60° prism.

There the similarity ends. The 22° halo forming entities are probably crystal clusters that tumble in the air and almost certainly not the ‘randomly aligned pencils’ or column crystals usually accused of this halo’s formation.

The circumscribed halo – the higher sun cousin of the upper and lower tangent arcs – is the product of column crystals. These crystals are always oriented by aerodynamic drag forces as they drift slowly downwards relative to local air currents. The drag aligns the crystals so that their long axes are to within a degree or so – and often much closer – to horizontal.

At left a HaloSim ray tracing calculation shows the two halos and a by-product of the oriented column crystals, the parhelic circle.


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