I was looking in the pond when I saw rainbow colours reflected there, and so I looked upwards and saw a circumzenithal arc. I wouldn’t have noticed it if I hadn’t been looking in the pond.
As with the sun dogs, this is caused by hexagonal ice crystals, but these are lying horizontally. The colours are purer than in a rainbow, because there is less overlap, but mine look milky because the high cirrus where the crystals are is fairly thick. In fact the clouds were beautiful.