Grey or whitish, or both grey and whitish, patch, sheet or layer of cloud which almost always has dark parts, composed of tessellations, rounded masses, rolls, etc., which are non-fibrous (except for virga) and which or may not be merged; most of the regularly arranged small elements have an apparent width of more than five degrees.
MAIN DIFFERENCES BETWEEN STRA TOCUMULUS AND SIMILAR CWUDS OF OTIIER GENERA
Stratocumulus may, in extremely cold weather, produce abundant ice crystal virga, sometimes accompanied by a halo; it is then nevertheless distinguishable from Cirrostratus by the fact that it still shows some evidence of the presence of rounded masses, rolls, etc. Furthermore, the opacity of Stratocumulus IS greater than that of Cirrostratus.
Stratocumulus may sometimes be confused with Altocumnulus having dark parts. If most of the regularly arranged elements, when observed at an angle of more than 30 degrees above the horizon, have an apparent width of more than five degrees, the cloud is Stratocumulus.
The differentiation of Stratocumulus from Altostratus, Nimbostratus and Stratus is based on the fact that Stratocumulus shows evidence of the presence of elements, merged or separate. Furthermore, in contrast with Altostratus which often has a fibrous appearance, Stratocumulus always appears non-fibrous, except at extremely low temperatures. The above criteria apply in addition to those based on the character of the precipitation and the nature of its particles, which sometimes provide a clue to the identity of the cloud.
Stratocumulus differs from Cumulus in that its elements usually occur in groups or patches and generally have flat tops; if however, Stratocumulus tops are in the form of domes, they rise, unlike those of Cumulus, from merged bases.
CLOUDS FROM WHICH STRATOCUMULUS MAY FORM
Stratocumulus may form from Altocumulus when the small macroscopic elements grow to a sufficient size (Sc altocumulomutatus).
Stratocumulus sometimes forms near the base of Altostratus, or more often of Nimbostratus, as a result of turbulence or convection in the layers moistened by evaporating precipitation (Sc altostratogenitus or Sc nimbostratogenitus); it may also form by transformation of Nimbostratus (Sc nimbostratomutatus).
Stratocumulus may develop as a result of the lifting of a layer of Stratus or as a result of the convective or undulatory transformation of Stratus, with or without change of height (Sc stratomutatus).
Stratocumulus is often formed by the spreading out of Cumulus or Cumulonimbus (Sc cumulogenitus or Sc cumulonimbogenitus). Ascending currents producing Cumulus or Cumulonimbus slow down as they reach a higher layer of stable air. When the convective clouds approach this layer, they tend to spread out, forming a patch of Stratocumulus which either surrounds the cumuliform columns like a shelf, or tops them. Whether the former or the latter case occurs, depends on the speed of the ascending currents and the degree of stability of the higher layer. Not infrequently, the convective clouds dissipate completely and only the Stratocumulus remains.
Note: In the cases described above, the Cumulus or Cumulonimbus always gradually widens into the Stratocumulus patch or sheet. Cumulus or Cumulonimbus clouds can, however, also enter or transpierce a preexisting layer of Stratocumulus formed independently of them. When this occurs, the convective clouds do not widen upward towards the Stratocumulus layer and a thinned or even a cleared zone frequently appears in the Stratocumulus around the cumuliform columns.
Stratocumulus may also form from Cumulus as a result of strong wind shear.
A particular form of Stratocumulus cumulogenitus often occurs in the evening when convection ceases and, in consequence, the domed summits of the Cumulus clouds flatten.
Stratocumulus is composed of water droplets, sometimes accompanied by raindrops or snow pellets and, more rarely, by snow crystals and snowflakes. Any ice crystals present are usually too sparse to give the cloud a fibrous appearance; during extremely cold weather, however, Stratocumulus may produce abundant ice crystal virga which may be accompanied by a halo. When Stratocumulus is not very thick, a corona or irisation is sometimes observed.
The appearance of Stratocumulus is similar to that of Altocumulus, buro