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Circumzenithal Arc high over the Rockefeller Centre, New York City imaged by Ken-Shing Law ©Ken-Shing Law, shown with permission.

The 'upside down rainbow', is not a rainbow but an ice halo produced by flat plate ice crystals in high cirrus cloud. It is perhaps the most beautiful of all the halos and to call it a rainbow rather insults it because the CZA has much the purer colours.

Circumzenithal Arc     Formation from plate crystals   Rainbow colours overlap