Circumzenithal Arc Rainbow Silhouette
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Circumzenithal Arc seen in Laguna Beach California at a solar height of 23.9º.
The red side of the arc is closest to the sun.
The circumzenithal arc is a unique phenomenon,
and much less known than the rainbow, though it is even more colourful.
The reason is that it appears high up in the sky.
If there are cirrus clouds and the sun is low,
one should look upwards from time to time.
The circumhorizontal arc appears only if the sun is higher than 58º
and therefore is very rare at our latitudes.
CZA and CHA are the most colourful arcs. Both are due to refraction by 90º
ice prisms which split the white light into the spectral colours.
If the ice crystals are flat hexagonal prisms, they tend to orient
themselves such that bases are horizontal when they float in the air.
Light rays entering the crystal through the top plane
and leaving through one of the sides produce the CZA;
light entering from the side and leaving through the bottom plane make the CHA.
Centre of both bows is the zenith, but the CHA is close to the horizon.
However, even these arcs do not exhibit spectral colours for several reasons.
First of all, they appear before a background of blue sky with thin clouds,
on the other hand, the finite angle of the sun's disc,
the not exactly horizontal orientation,
double refraction of the rays in the ice crystals,
and diffraction effects lead to some mixing of the spectral colours.
Surprisingly, this makes the arc even more colourful.