Circular halos are most
facilely explained by positing randomly oriented ice crystals.
why do aerodynamic forces, otherwise so remarkably effective, not
orient them? Equidimensional column crystals would in principle
be randomly oriented but they are not found in any significant
numbers in displays showing good circular halos.
However, total randomness is not essential, poorly aligned crystals – ones
with large tilts from their equilibrium orientations, can produce
circular halos. Very large columns can have poor orientations.
Column clusters might also sometimes be responsible.
Rays passing through column side faces with
wedge angles of 60° give
a 22° circular
halo when the crystals
are poorly oriented. Rays traversing a side face and an end face – wedge
angle 90° - produce the very rare 46° halo.
Supralateral arcs are more frequent and often mistaken for the