From Glossary of Meteorology


Like cirrus; more generally, descriptive of clouds composed of small particles, mostly ice crystals, that are fairly widely dispersed, usually resulting in relative transparency and whiteness and often producing halo phenomena not observed with other cloud forms.

Irisation may also be observed. Cirriform clouds are high clouds (
see cloud classification) existing between -25° and -85°C. As a result, when near the horizon, their reflected light traverses a sufficient thickness of air to often cause them to take on a yellow or orange tint even during the midday period. On the other hand, cirriform clouds near the zenith always appear whiter than any other clouds in that part of the sky. With the sun on the horizon, this type of cloud is whitish, while other clouds may be tinted with yellow or orange; when the sun sinks a little below the horizon, cirriform clouds become yellow, then pink or red; and when the sun is well under the horizon, they are gray. All species and varieties of cirrus, cirrocumulus, and cirrostratus clouds are cirriform in nature.
Compare cumuliform, stratiform.