halo ray path through a randomly aligned
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
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
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
of 35° produce both 46° and 22° halos that look almost
the same as those of randomly oriented
In some cases there is no real distinction
between 46° halos
and supralateral arcs. Some "46° halos" are the products
of wobbly column crystals.
quoted are the standard deviation of a normal distribution
with a mean of zero.