It certainly pays to investigate something you're not sure about, don't understand, or can't explain because I did a bit of research into the sun halo in combination with the horizontal rainbow mentioned in my most recent post and came across this information posted in Wikipedia:
A circum-horizontal arc is an optical
phenomenon— an ice-halo formed by plate-shaped ice crystals in high level cirrus
clouds.
Other currently accepted names for the phenomenon are circumhorizon
arc or lower symmetric 46° plate arc. The misleading term "fire
rainbow" is sometimes used to describe these phenomena, although they
are neither rainbows, nor related in any way to fire. The name comes from
its appearance as a rainbow taking the shape of flames in the sky.
 |
| Fire Rainbow beneath Sun Halo |
The complete halo is a huge, multi-colored band running
parallel to the horizon with its center beneath the sun. The distance below the
sun is twice as far as the common 22-degree halo. Red is the uppermost color.
Often, when the halo-forming cloud is small or patchy, only fragments of the
arc are seen.
How often a circumhorizontal arc is seen, depends on the
location and the latitude of the observer. In the United States it is a
relatively common halo seen several times each summer in any one place. In contrast,
it is rare phenomenon in northern Europe for several reasons.
Formation of the halo requires that the sun be very high in
the sky, at an elevation of 58° or greater, and that a cirrus cloud or haze be
present and contain plate-shaped ice crystals. The sun's altitude determines
the visibility of the halo; it is impossible to see at locations north of 55°N
or south of 55°S (although a lunar circumhorizon arc might be visible at other
latitudes).
At other latitudes the phenomenon is visible, for a greater
or lesser time, around the summer solstice. For example, in London, England the
sun is only high enough for 140 hours between mid-May and late July. Contrast
that with Los Angeles, with the sun higher than 58 degrees for 670 hours
between late March and late September.
The halo is formed by sunlight entering
horizontally-oriented, flat, hexagon ice crystals through a vertical
side face and leaving through the near horizontal bottom face (plate thickness
does not affect the formation of the halo). In principle, Parry oriented column
crystals may also produce the arc, although this is rare. The 90° inclination
between the ray entrance and exit faces produce the well-separated spectral
colors.
The arc has a considerable angular extent and thus, rarely
is complete. When only fragments of a cirrus cloud are in the appropriate sky
and sun position they may appear to shine with spectral colors.
A circumhorizontal arc may be difficult to distinguish from
an infra-lateral arc when the sun is high in the sky. The former is
always parallel to the horizon, whereas the latter curves upward at its ends.