This image taken with a 'phone camera by Gemma Worswick near Henley-on-Thames, England shows how very bright and colourful the circumzenithal arc can be as it rides overhead. ©Gemma Worswick.
Near parallel sunlight rays enter the top faces of horizontally aligned plate crystals where they are refracted and dispersed into spectral colours. The rays are further dispersed as they leave a near vertical ice face to be directed downwards.
The arc curves because cloud crystals have all rotational positions about their vertical axes. But the arc can never be a complete circle - that is the exceedingly rare Kern arc.