Kim Huelsman (Member 53,048) was on a Father’s Day outing when she noticed this fragment of bright prismatic colour in the sky over Escondido, California, US.

It isn’t a rainbow but part of an optical phenomenon known as a circumhorizon arc. Formed by sunlight shining through a cloud’s ice crystals, it’s a halo effect that can appear as a horizontal band of colour when the Sun’s high in the sky up above. If the ice crystals cover enough of the sky, the effect looks like a wide, flat, horizontal band of colours. Usually, we see only fragments of it, visible where streaks of Cirrus or patches of Cirrostratus cover the right parts of the sky. Their ice crystals act as tiny prisms and mirrors that refract, or bend, the light, dispersing it into a band of pure spectral colours.

The optics work only when the Sun’s shining onto the cloud from high above, from at least 58 degrees elevation. So in middle latitudes like California, a circumhorizon arc only appears in the summer months. To see one, you’ll just have to be, like Kim, in the right place at the right time.