Chuck O'Connell wrote:
>There is a solar refraction phenomenon called
>'circum zenithal arcs' which occur only when the sun is
>between 18 and 26 (or is it 16-28) degrees in the sky.
>It appears as a blue shifted rainbow that appears directly
>overhead as a circle centered at the zenith!
The circumzenithal arc can occur when the altitude of the sun is less
than 32.2 deg. It is best seen when the sun's altitude is around 20-25
degrees.
It is a partial circle centered on the zenith.
The arc (and other atmospheric halos) usually occur when there is thin
cirrus cloud containing ice crystals. Most circumzenithal arcs result
from refraction of sunlight through plate-like hexagonal ice crystals
falling through clouds with their larger hexagonal faces horizontal.
Light enters the top face and leaves one of the thin side faces. The
light is highly dispersed by the refraction through faces at right angles
to each other and this causes the pure and beautiful colours of the
circumzenithal arc.
Les Cowley
Atmospheric Halos on the web at:
http://dspace.dial.pipex.com/lc/halo/halosim.htm
