The circumzenithal arc or Bravais' arc is an optical phenomenon similar in appearance to a rainbow. However, it arises from refraction of sunlight through horizontally oriented ice crystals within cirrus clouds. The ice crystals in the cirrus clouds are generally hexagonal. A fair amount of cirrus clouds from an associated thunderstorm complex located out across Missouri were streaming southward through Kentucky Friday afternoon. Typically these arcs form no more than one quarter of a circle centered on the zenith and on the same side as the sun. The colors are inverted from your typical rainbow with blue on the inside to red on the outside of the arc.
This photo was taken Friday afternoon near Rineyville, KY.