Altocumulus which in Latin means mid-heap, are mid-range clouds that are created between the heights of 2000 metres and 6000 metres. The clouds are typically grey or white in colour and form in patches, layers, bands, rolls, waves, etc. The clouds are present in groups and have a distinctly fluffy cotton like appearance. Altocumulus clouds are similar in appearance to Cirrocumulous clouds, but appear much darker and are at a higher altitude than Cirrocumulus.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
Formation occurs when the solar radiation heats the surface of the Earth. Hot air, or thermals rise upward as they are lighter than he surrounding air mass; while rising the thermals undego convection, until neutral bouyancy is reached. This means that the warm air mass continues to rise until the temperature of the air parcels reaches a thermal equilibrium with the surrounding air mass. As the parcel air dilutes into the surroundsing atmosphere, it loses heat and its buoyancy . In this case the transision zone where warm air is replaced by cold air is called a cold front. At that point the warmer air parcel had been cooled enough for the moisture to condense and turn into clouds.
Most commonly, Altocumulus clouds form via convection in unstable atmospheric layers, which could be the result of gradual lifting of air mass before an approaching cold front. As the air currents rise like waves, water vapour once again condenses into cloud like form. However, water tend to evaporate in wave trough patterns, thus forming Altocumulus clouds in patches and bands.
Effect may not be apparent with Altocumulus clouds, as they are not known to produce weather effects like, rain or snow; but may in fact indicate approaching weather changes, that lies within a time frame of up to more than a day. On warm and humid days, or especially in the mornings, Castellanus clouds may indicate that there is enough instability in the atmosphere, for an afternoon storm to be created. In general Lenticularis clouds, may be indicative deterioration in the stability of the atmosphere between half a day and right up to two days in advance.
There are many different types of Altocumulus clouds, which are identified by species, variety which predominantly based on shape, size and other specific atributes. These are:
Ac Stratiformis, that have horizontal and bumpy layers.
Ac Castellanus, which have tall and pointed formation.
Ac Floccus, with pile like formation,
Ac Nammatus, appear to be attached to the underside of other clouds.
Ac Opacus or lacnuous, that are known to obstruct moon and sunlight.
Ac Translucidus, with a transulscent appearance.
Ac Lenticularis, that are clear-edged and are very dark in appearance.
Ac Radiatus, are made up of parralel bands that intercept at a distance.
Ac Duplicatus, appear in mutiple layers.
Ac Perlucidus, are thick clouds layers with random gaps, that allow sunlight through.
Ac Undulates, is due to oppositely moving air masses.
Altocumulus Clouds fritter away and disappear, when they absorb access radiation and their structure becomes fibrous and gradually dissipates as well as the entrainment of dry air, which creates a similar fate. When precipitation occurs, which is rare in Altocumulus case, the clouds disband and eventually disappear. With changing temperatures or instability, the clouds move to lower altitudes and take on the characteristics of other clouds.