A stratocumulus cloud belongs to a class of clouds characterized by large dark, rounded masses, usually in groups, lines, or waves, the individual elements being larger than those in altocumuli, and the whole being at a lower altitude, usually below 2,400 m (8,000 ft). Weak convective currents create shallow cloud layers because of drier, stable air above preventing continued vertical development. Vast areas of subtropical and polar oceans are covered with massive sheets of stratocumuli. These may organize into distinctive patterns which are currently under active study. In subtropics, they cover the edges of the horse latitude climatological highs, and reduce the amount of solar energy absorbed in the ocean.
When these drift over land the summer heat or winter cold is reduced. 'Dull weather' is a common expression incorporated with overcast stratocumulus days, which usually occur either in a warm sector between a warm and cold front in a depression, or in an area of high pressure, in the latter case, sometimes persisting over a specific area for several days. If the air over land is moist and hot enough, stratocumuli may develop to various cumulus clouds, or, more commonly, the sheet of stratocumulus may become thick enough to produce some light rain. On drier areas they quickly dissipate over land, resembling cumulus humilis.
This often occurs in late morning in areas under anticyclonic weather, the stratocumulus breaking up under the Sun's heat and often reforming again by evening as the heat of the Sun decreases again. Most often, stratocumuli produce no precipitation, and when they do, it is generally only light rain or snow. However, these clouds are often seen at either the front or tail end of worse weather so may indicate storms to come, in the form of thunderheads or gusty winds. They are also often seen underneath the cirrostratus and altostratus sheets that often precede a warm front as these higher clouds decrease the Sun's heat and therefore convection, causing any cumulus clouds to spread out into stratocumulus.
These are same in appearance to altocumuli and are often mistaken for such. A simple test to distinguish these is to compare the size of individual masses or rolls: when pointing one's hand in the direction of the cloud, if the cloud is about the size of the thumb, it is altocumulus; if it is the size of one's entire hand, it is stratocumulus. This often does not apply when stratocumulus is of a broken, fractus form, when it may appear as small as altocumulus. Stratocumulus is also often, though not always, darker in colour than altocumulus.Stratocumulus clouds are the main type of cloud that can produce crepuscular rays. Thin stratocumulus clouds are also often the cause of corona effects around the Moon at night.