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   Randomly Oriented Crystals


Circular halos are most facilely explained by positing randomly oriented ice crystals.

But 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 former.

   22° halo  46 and 22° and other