Detached clouds in the form of white, delicate filaments or white or mostly white patches or narrow bands. These clouds have a fibrous appearance, or a silky sheen, or both.
MAIN DIFFERENCES BETWEEN CIRRUS AND SIMILAR CLOUDS OF OTHER GENERA
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. Cirrus near the horizon may be difficult to distinguish from Cirrostratus, owing to the effect of perspective. Cirrus clouds are distinguished from Altocumulus by their mainly fibrous or silky appearance and by the absence of cloud elements in the form of laminae, rolls, etc. Thick Cirrus clouds are distinguished from Altostratus patches by their smaller horizontal extent and their mostly white appearance.
CLOUDS FROM WHICH CIRRUS MAY FORM
Cirrus clouds often evolve from virga of Cirrocumulus or Altocumulus (Ci cirrocumulogenitus or Ci altocumulogenitus), or from the upper part of a Cumulo nimbus (Ci cumulonimbogenitus). Cirrus clouds may also form as a result of the transformation of non-uniform Cirrostratus by evaporation of its thinner parts (Ci cirrostratomutatus).
Cirrus is composed almost exclusively of ice crystals. These crystals are in general very a fact which, together with their sparseness, accounts for the transparency of most Cirrus Dense Cirrus patches or Cirrus in tufts may nevertheless contain ice crystals large enough to an appreciable terminal velocity, so that trails of considerable vertical extent may form. though not very frequently, the ice crystals in the trails melt into small water droplets; tnills are then greyish, in contrast with their usual white appearance, and may give rise to the of a rainbow. The trails curve irregularly or slant as a result of wind shear and of the variation in size of the particles; consequently, Cirrus filaments near the horizon do not appear parallel to it. Halo phenomena may occur; circular haloes almost never show a complete ring, owing to of the Cirrus clouds.
Cirrus tufts with rounded tops often form in clear air. Fibrous trails may appear under the tufts; the tops then gradually lose their roundness. Subsequently, the tufts may disappear completely; the clouds are then in the form of filaments. Cirrus in the form of filaments may develop also from dense Cirrus patches, from Altocumulus castellanus and floccus and, occasionally at very low temperatures, from Cumulus congestus. The following special remarks can be made with regard to the colours of Cirrus. At all times of day, Cirrus not too close to the horizon is white, in fact whiter than any other cloud in the same part of the sky. With the sun on the horizon it is whitish, while lower clouds may be tinted yellow or orange. When the sun sinks below the horizon, Cirrus high in the sky is yellow, then pink, red and finally grey. The colour sequence is reversed at dawn. Cirrus near the horizon often takes a yellowish or orange tint owing to the great thickness of air traversed by the light in passing from the cloud to the observer. These tints are less conspicuous in lower cloud genera.
SUPPLEMENTARY FEATURES AND ACCESSORY CLOUDS
Cirrus sometimes shows mamma.