arc, Atacama Desert, Chile. Imaged
at the European
Southern Observatory by Sylvain Rondi (site)
on the morning of 21st March '02 when the sun was 22° high, the
ideal altitude for CZAs. Image©2002 Sylvain Rondi,
shown with permission.
The circumzenithal arc, CZA, is the most beautiful of all the halos.
The first sighting is always a surprise, an ethereal rainbow fled
from its watery origins and wrapped improbably about the zenith.
It is often described as an "upside down rainbow" by
first timers. Someone also charmingly likened it to "a grin
in the sky".
Look straight up near to the zenith when the sun if fairly
low and especially if sundogs
are visible. The centre of the bow always sunwards and red is on
The CZA is never a complete circle around
the zenith, that is the exceptionally rare and only recently photographed