Cirrus clouds are composed entirely of ice crystals, and when the Sun is high in the sky – at an angle of more than 58 degrees above the horizon – there’s a chance these ice crystals can refract the sunlight to create a horizontal band of colours known as a circumhorizon arc. Elena Correa spotted this circumhorizon arc in a Cirrus vertebratus cloud above Aptos, California, US. We use the term _vertebratus _to refer to clouds that resemble backbones. But this cloud looks more like an invertebrate. It’s a caterpillar that swallowed a rainbow – or, as Elena called it, ‘a cloudapillar’.