A principal cloud type (cloud genus) in the form of a gray layer with a rather uniform base. Stratus does not usually produce precipitation, but when it does occur it is in the form of minute particles, such as drizzle, ice crystals, or snow grains. Stratus often occurs in the form of ragged patches, or cloud fragments (stratus fractus), in which case rapid transformation is a common characteristic. Stratus clouds have characteristically low vertical velocities, usually less than 1 m s⁻¹. When the sun is seen through the cloud, its outline is clearly discernible, and it may be accompanied by corona phenomena. In the immediate area of the solar disk, stratus may appear very white. Away from the sun, and at times when the cloud is sufficiently thick to obscure it, stratus gives off a weak, uniform luminance. The particulate composition of stratus is quite uniform, usually of fairly widely dispersed water droplets and, at lower temperatures, of ice crystals (although this is much less common). Halo phenomena may occur with this latter composition. Dense stratus often contains particles of precipitation. The prior existence of any other cloud in the low or middle levels is seldom required for the formation of stratus. A common mode of stratus development is the transformation of fog, the lower part of which evaporates while the upper part may rise (St nebulomutatus). As can be expected by its close relationship to fog, stratus follows a diurnal cycle with a maximum (over land) in the night and early morning. Insolation tends to dissipate this cloud rapidly, and often brings about the transformation of stratus fragments to cumulus clouds. Fog arriving from the sea frequently becomes stratus over the adjacent land. Coastal regions also provide the low-level moisture and frequently the lapse-rate stability conducive to its formation, and therefore these areas have the greatest stratus status. Stratus also develops from stratocumulus when the undersurface of the latter descends or for any reason loses its relief or apparent subdivisions (St stratocumulomutatus). Nimbostratus and cumulonimbus often produce stratus fractus, as precipitation from these clouds causes low-level condensation (St fractusnimbostratogenitus or St fractus cumulonimbogenitus). Stratus fractus in this form constitutes the accessory feature pannus of these mother-clouds. Stratocumulus and nimbostratus are the clouds most difficult to distinguish from stratus. Stratus is lower and lacks the uniform undulations or relief of stratocumulus. More difficulty is encountered when differentiating it from nimbostratus. Their modes of formation are different, nimbostratus usually having been formed from a preexisting mid- or low-level cloud; nimbostratus is more dense and has a wetter aspect, and its precipitation is of the ordinary varieties. As a final distinction, the wind accompanying nimbostratus is usually stronger than that with stratus. See cloud classification.