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   46� Halo Formation


46° halo ray path through a randomly aligned crystal.
This is the traditional explanation for the halo.

Rays enter and leave a side face and end face of a randomly aligned crystal (paths 13 or 31).

The faces are inclined 90º to each other and the resulting minimum deviation angle is ~46°. However, a great many rays are deflected through larger angles and give the halo a diffuse outer edge.

The 90° wedge disperses the colours more widely than those of the 60° wedge forming the 22° halo.

46° halo or supra-/infralateral arc? Column oriented crystals form supra- and infralateral arcs when rays pass between a side face and an end face. These are the same paths as those that produce a 46° halo from randomly oriented crystals .

But what happens when oriented columns have large wobbles, as can large or equidimensional crystals? Mouse slowly over the slider from bottom to top to see the effect of increasing wobble.

Tilts** up to 3° produce clear infra- and supralateral arcs with a characteristic cusp at the parhelic circle. At larger tilts the arcs' inner edges become more circular. Halos from tilts of 5 - 8° would be difficult to distinguish in the sky from a 'true' 46° circular halo. Tilts of 35° produce both 46° and 22° halos that look almost the same as those of randomly oriented crystals.

In some cases there is no real distinction between 46° halos and supralateral arcs.   Some "46° halos" are the products of wobbly column crystals.
**  Tilts quoted are the standard deviation of a normal distribution with a mean of zero.