Stratocumulus
Stratocumulus clouds usually form between 1,000 and 6,500 feet (300 and 2,000 metres), whereas altocumulus clouds form between 6,500 and 23,000 feet (2,000 and 7,000 metres). Clouds at these levels are referred to as low cloud and medium cloud respectively.
There is no great difference in the properties of stratocumulus and altocumulus since both are composed of water droplets and are normally limited in vertical extent, so the distinction is rather an arbitrary one.
Stratocumulus will often give a sheet of almost total cloud cover, with perhaps one or two breaks. The cloud elements are rounded and almost join up. Occasionally, the sheet is composed of a series of more or less parallel rolls which often, but not always, lie 'across the wind'. Also stratocumulus sometimes produces light falls of rain or snow.
Altocumulus also provides a sort of dappled pattern, but since it is at a greater altitude the cloud elements look smaller. There are many variations on the theme including altocumulus castellanus which is like a vigorous medium-level cumulus (this type of cloud is sometimes an indication that thunderstorms will follow).
Both stratocumulus and alto cumulus are formed by weak convection currents, perhaps triggered by turbulent airflows aloft. The convection affects a shallow zone because dry, stable air above the cloud sheet prevents further upward development.
Sometimes there are huge sheets of stratocumulus covering thousands of square kilometres around the flanks of a high pressure system, especially over the oceans. The weather below such sheets tends to be dry, but it may be rather dull if the cloud is two or three thousand feet thick.
Related links:
- Cumulus
- Cumulonimbus
- Stratus
- Cirriform Clouds
- Stratocumulus and Altocumulus
- Nature of Clouds
