Cirrus are detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous (hair-like) appearance, or a silky sheen, or both.

Cirrus clouds are composed almost exclusively of ice crystals. These crystals are in general very small, a fact, which together with their sparseness, accounts for the semi-transparency of most cirrus clouds. Dense cirrus patches or cirrus in tufts may nevertheless contain ice crystals large enough to acquire an appreciable terminal velocity, so that trails of considerable vertical extent may form. Cirrus clouds are distinguished from cirrocumulus by their mainly fibrous or silky appearance and by the absence of small cloud elements in the form of grains, ripples, etc. Cirrus clouds are distinguished from Cirrostratus by their discontinuous structure or, if they are in patches or bands, by their small horizontal extent or the narrowness of their continuous parts.

Cirrocumulus are thin, white patches, sheets or a layer of clouds without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged; most of the elements have an apparent width of less than one degree.

Cirrocumulus differs from Cirrus and Cirrostratus in that it is rippled or subdivided into very small cloudlets; it may include fibrous, silky or smooth portions which, however, do not collectively constitute its greater part. Cirrocumulus differs from Altocumulus in that most of its elements are very small (by definition, of an apparent width less than one degree when observed at an angle of more than 30 degrees above the horizon) and without shading. Cirrocumulus are composed almost exclusively of ice crystals.

Cirrostratus presents a semi-transparent, whitish cloud veil of fibrous (hair-like) or smooth appearance, totally or partly covering the sky, and generally producing a halo appearance.

Cirrostratus clouds are composed mainly of ice crystals. The smallness of these crystals, their sparseness and the fact that cirrostratus has at most only a moderate depth, account for the semi-transparency of this cloud through which the outline of the sun is visible. Cirrostratus differs from altostratus by its thinness and by the fact that it may show a halo phenomena. Cirrostratus is never thick enough to prevent objects on the ground from casting shadows, at least when the sun is high (greater than 30 degrees) above the horizon.