'Upside down rainbow', 'Grin in the sky'...

The circumzenithal arc is one of the most colourful of the ice halos. It presents an almost pure spectrum as near parallel rays from the sun enter the top faces of hexagonal plate crystals and leave through a side face. The refraction through the equivalent of a 90° prism produces a wide separation of colours.

Look for it overhead whenever the sun is low ( <32°) and cirrus wraps the sky.