Upside down rainbow - The circumzenithal arc or cicumzenith arc (CZA), also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow and arising from refraction of sunlight through non-terminated, horizontally-oriented ice crystals in certain clouds. It takes the shape of one-quarter of a circle centered at the zenith and parallel to the horizon, on the same side as the sun. Its colors run from blue near the zenith to red towards the horizon; it is one of the brightest and most colorful halos"
http://en.wikipedia.org/wiki/Circumzenithal_arc