Weak Circumzenithal Arcs

The circumzenithal arc isn't always strong so can easily be missed. It is usually worth keeping an eye on that part of the sky when there is cirrus about and the sun isn't too high. The arc is very sensitive to the shape and orientation of the ice crystals.

This one formed in an old jet contrail.

In this example the arc is only formed by one of the types of cirrus in that part of the sky.

This thin veil of cirrus showed a weak but colourful arc which lasted several minutes.

Five minutes later a different type of cirrus moved through. Although the colours were very weak the arc seemed to be associated with the thicker cirrus rather than the thinner background.

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