A principal cloud type (cloud genus), predominantly stratiform, in the form of a gray and/or whitish layer or patch, which nearly always has dark parts and is nonfibrous (except for virga). Its elements are tesselated, rounded, roll-shaped, etc. ; they may or may not be merged, and usually are arranged in orderly groups, lines, or undulations, giving the appearance of a simple (or occasionally a cross-pattern) wave system. These elements are generally flat-topped, smooth, and large; observed at an angle of more than 30° above the horizon, the individual stratocumulus element subtends an angle of greater than 5°. When a layer is continuous, the elemental structure is revealed in true relief on its under surface. Stratocumulus is composed of small water droplets, sometimes accompanied by larger droplets, soft hail, and (rarely) by snowflakes. When the cloud is not very thick, the diffraction phenomena corona and irisation appear. Under ordinary conditions, ice crystals are too sparse even to give the cloud a fibrous aspect; however, in extremely cold weather, ice crystals may be numerous enough to produce abundant virga, and sometimes even halo phenomena. Mamma may be a supplementary feature of stratocumulus, in which case the mammiform protuberances may develop to the point where they seem about to detach themselves from the main cloud. Virga may form under the cloud, particularly at very low temperatures. Precipitation rarely occurs with stratocumulus. Stratocumulus frequently forms in clear air. It may also form from the rising of stratus, and by the convective or undulatory transformation of stratus, or nimbostratus, with or without change of height (Sc stratomutatus or Sc nimbostratomutatus). Stratocumulus is analogous to altocumulus and forms directly from the latter when the elements grow to a sufficient size (Sc altocumulomutatus). The further humidification, accompanied by turbulence and/or convection, of an already humid layer of air near the base of nimbostratus or even altostratus can form stratocumulus (Sc nimbostratogenitus or Sc altostratogenitus). If the ascending currents that produce cumulus or cumulonimbus approach an upper layer of stable air, they slow down, and all or a portion of the mother-cloud tends to diverge gradually and spread horizontally, often producing stratocumulus (Sc cumulogenitus or Sc cumulonimbogenitus). A particular form of Sc cumulogenitus, previously called stratocumulus vesperalis, often occurs in the evening when convection decreases, resulting in the gradual dissipation of both bases and tops of cumuliform clouds. Since stratocumulus may be transformed directly from or into altocumulus, stratus, and nimbostratus, all transitional stages may be observed. By convention, altocumulus is composed of apparently smaller elements (often simply because of its higher altitude); stratus and nimbostratus do not show regular subdivisions or wave form, and they have a more fibrous aspect. When the base of stratocumulus is rendered diffuse by precipitation, the cloud becomes nimbostratus. See cloud classification.