Cirrostratus is a subtle, understated cloud that can easily go unnoticed – except, that is, by CloudSpotters, keen to complete all ten of the main cloud types in their collection.
A delicate layer of ice crystals, often spread over vast areas of the sky, Cirrostratus can appear as no more than a light, milky whitening of the blue. It can sometimes look stripy or fibrous (the species known as fibratus) but more commonly lacks any variation in tone.
It also distinguishes itself as the best of the high clouds at producing the coloured arcs, rings and points of light known as halo phenomena. These can appear as the sunlight shining through a layer of Cirrostratus is refracted and reflected by the ice crystals, each of which can behave like a tiny prism. Halo phenomena certainly don’t always appear but, when they do, they can exhibit beautiful rainbow colours, and are a sure way to distinguish Cirrostratus from Altostratus, which, being lower and consisting (at least partially) of droplets, doesn’t produce them.
Image: Spotted over Harburn, Ontario, Ontario, Canada by Jdapb.