Not So Mundane Halo?
After the Kern arc of the previous OPOD, a ‘commonplace’ 22° halo might seen mundane. But in some ways the 22° halo is more mysterious than the Kern.
Conor McManmon captured this fine example from Cambridge, England on the morning of 1st May, ’15.
Notice the dark 'hole' inside the halo where crystals do not glint. The halo is red rimmed and outside it tapers through straws and pastel blues to white. The halo's light extends a considerable distance. A faint sundog and hint of parhelic circle complete the picture.
©Conor McManmon, shown with permission
The usual explanation for the 22° halo is that refraction of rays passing through 60° inclined side faces of tumbling hexagonal ice prisms forms it. Tumbling confers an average random orientation and thus leads to a circular halo rather than a sundog or an upper tangent arc. These latter arcs have the same ray path.
But hexagonal ice prisms do not tumble. Short prisms - plates – drift with their large hexagon faces horizontal to within a few degrees and usually are even more precisely aligned. Long prisms – columns – are well aligned with their long axes horizontal. Aerodynamic drag forces do this.
And stubby crystals of intermediate length? Wouldn’t they tumble and generate the 22° halo? Perhaps. But crystal samples from displays with a 22° halo have precious few stubby crystals!
Something else is in most case generating the halo. Perhaps cluster crystals or sometimes very large imperfect crystals more prone to tumbling?
Upper tangent arc ray