A principal cloud type (cloud genus), appearing as a whitish veil, usually fibrous but sometimes smooth, that may totally cover the sky, and that often produces halo phenomena, either partial or complete. Sometimes a banded aspect may appear, but the intervals between the bands are filled with thinner cloud veil. The edge of a veil of cirrostratus may be straight and clear-cut, but more often it is irregular and fringed with cirrus. Some of the ice crystals that compose the cloud are large enough to fall and thereby produce a fibrous aspect. Cirrostratus occasionally may be so thin and transparent as to render it nearly indiscernible, especially through haze or at night. At such times, the existence of a halo may be the only revealing feature. The angle of incidence of illumination upon a cirrostratus layer is an important consideration in evaluating the identifying characteristics. When the sun is high (generally above 50° elevation), cirrostratus never prevents the casting of shadows by terrestrial objects; and a halo might be completely circular. At progressively lower angles of the sun, halos become fragmentary and light intensity noticeably decreases. Cirrostratus may be produced by the merging of elements of cirrus (Cs cirromutatus); from cirrocumulus (Cs cirrocumulogenitus); from the thinning of altostratus (Cs altostratomutatus); or from the anvil of cumulonimbus (Cs cumulonimbogenitus). Since cirrostratus and altostratus form from each other, it is frequently difficult to delineate between the two. In general, altostratus does not cause halo phenomena, is thicker than cirrostratus, appears to move more rapidly, and has a more even optical thickness. When near the horizon, cirrostratus may be impossible to distinguish from cirrus. See cloud classification, cirriform.