Atmospheric
Optics
Nova Scotia Halos
~ Sunset approaches in this scene by Jeff Dalton. Soft halos paint the sky with their geometry.
Image ©Jeff Dalton, shown with permission
The picture tells us that the cirrus covering the sky had many randomly oriented hexagonal ice prisms of some sort. They generated the bright inner 22° halo. Plate oriented crystals were about too, but not too many. They made the weak sundogs and perhaps added brightness to the parhelic circle. Wobbly or optically imperfect horizontal column crystals made the fuzzy upper tangent arc.
The outer halo's source is less obvious. Is it a circular 46° halo from randomly tilted crystals. Or a supralateral arc from horizontal columns.
Two HaloSim ray tracings are at right. Crystals in the left-hand half were tweaked to roughly match the image with an outer 46° halo. The randomly tilted crystals were kept short. In that way a bright 46° halo would result from the higher proportion of rays passing between prism side and end faces. A supralateral arc from horizontal columns was suppressed by making them very long. In that way most rays passed between side faces and went into the fuzzy upper tangent arc.
On
the right-hand half the crystal tweaking was reversed. It suppressed 46° halo formation and enhanced supralateral and infralateral arcs.
Add crystal imperfections, variations in cirrus thickness across the sky and lens distortions. It is just not possible to decide on the outer halo identity.
And does it matter? The scene’s beauty and tranquillity are enough.
