Circular halos from poorly oriented pyramidal
crystals swamp the everyday 22° halo
in this 90 million ray simulation. Outwards from the sun are 9,18,20,23,24
and 35° radius halos. Sometimes called "Odd radius halos".
Sometimes circular halos appear that cannot be explained by invoking
'ordinary' hexagonal prism crystals.
These are the 'Odd radius halos' produced by
pyramidal ice crystals. Pyramidal crystals have one or
both end faces of ordinary ice crystals replaced by six sided pyramids.
The pyramids are sometimes truncated and the central prism section
is sometimes missing, creating 18 different variations with up to
20 sides. Very likely, all hexagonal ice crystals are actually pyramidal
but with insignificantly small inclined facets.
In contrast to ordinary prism crystals where the angles between faces
are either 60 or 90°, the angles between pyramidal crystal faces can
also be 28, 52.4, 56, 62, 63.8 and 80.2°. Each inclination or dihedral
angle forms a separate circular halo.
The odd radius halos are all minimum deviation phenomena like the
more familiar 22 and 46° halos. Their relative intensities - or whether
some appear at all - depend on the presence and relative sizes of
particular sets of faces.
Odd radius halos are perhaps not so 'odd' or rare as usually thought.
Make a point of routinely searching for them.