- (meteorology) A principal high-level cloud type appearing as a whitish veil, usually fibrous but sometimes smooth, which may totally cover the sky and which often produces halo phenomena, either partially or completely. Sometimes a banded aspect may appear, but the intervals between the bands are filled with thinner cloud veil. The edge of the veil of cirrostratus may be straight and clean-cut, but more often it is irregular and fringed with cirrus. Some of the ice crystals that comprise 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 almost indiscernible, especially through haze or at night. At such times, the existence of a halo may be the only revealing feature, such as producing a halo around the moon. Abbreviated: Cs.