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.
