Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a hair-like (fibrous) appearance, or a silky sheen, or both. Cirrus is composed almost exclusively of ice crystals.
The species Cirrus fibratus is comprised of nearly straight or irregularly curved fine, white filaments, that are, for the most part, distinct from one another.
Cirrus uncinus appears as filaments shaped like a comma, terminating in a hook or tuft.
Cirrus in patches, sufficiently dense to appear greyish (unlike other cirrus species) when viewed towards the sun is the species Cirrus spissatus. It can sometimes be thick enough to obscure the sun’s outline, or even hide it. Cirrus spissatus that has originated from the remains of the icy anvil of a Cumulonimbus cloud is known as Cirrus spissatus cumulonimbogenitus.
Cirrus floccus appears in the form of more or less isolated, small, rounded tufts, often with trails.
Distinguishing Ci from other genera
Cirrus is distinguished from Cirrostratus by its discontinuous structure or, when in patches or bands, by its small horizontal extent or the narrowness of its continuous parts. Owing to perspective, Cirrus near the horizon may be difficult to distinguish from Cirrostratus.
Cirrus clouds are distinguished from Cirrocumulus by their mainly fibrous or silky appearance and by the absence of small cloud elements.
Thick Cirrus clouds are distinguished from Altostratus patches by their smaller horizontal extent and their mostly white appearance.
Cirrus is not associated with any precipitation.