|Circumzenithal Arc pictured by Nigel Blackwell Feb 14,2009 at Copthorne near Crawley, West Sussex, England. ©Nigel Blackwell, shown with permission.
Someone called it a 'grin in the sky'. Produced by refraction through hexagonal plate ice crystals in high cirrus, look for the CZA when the sun is low (less than 32° from the horizon. Look directly overhead. Often it makes only a fleeting appearance and changes its brightness quickly because clouds move fastest near the zenith.