A principal cloud type (cloud genus) composed of detached cirriform elements in the form of white, delicate filaments, of white (or mostly white) patches, or of narrow bands. These clouds have a fibrous aspect and/or a silky sheen. Many of the ice crystal particles of cirrus are sufficiently large to acquire an appreciable speed of fall; therefore, the cloud elements have a considerable vertical extent. Wind shear and variations in particle size usually cause these fibrous trails to be slanted or irregularly curved. For this reason, cirrus does not usually tend, as do other clouds, to appear horizontal when near the horizon. Because cirrus elements are too narrow, they do not produce a complete circular halo. Cirrus often evolves from virga of cirrocumulus or altocumulus (Ci cirrocumulogenitus or Ci altocumulogenitus), or from the upper part of cumulonimbus (Ci cumulonimbogenitus). Cirrus may also result from the transformation of cirrostratus of uneven optical thickness, the thinner parts of which dissipate (Ci cirrostratomutatus). It may be difficult at times to distinguish cirrus from cirrostratus (often impossible when near the horizon); cirrostratus has a much more continuous structure, and if subdivided, its bands are wider. Thick cirrus (usually cirrus spissatus) is differentiated from patches of altostratus by its lesser extension and white color. The term “cirrus” is frequently used for all types of cirriform clouds. See cloud classification, cirriform.