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Altocumulus clouds are clouds that belong to a class characterized by globular masses or rolls in layers or patches. The individual elements are larger and darker than those of cirrocumulus clouds and smaller than those of stratocumulus clouds. Like other cumulus clouds, altocumulus clouds signify convection.
Altocumulus clouds are usually white or gray, and often occurs in sheets or patches with wavy, rounded masses or rolls. Altocumulus are often seen preceding a cold front, and their presence on a warm, humid, summer morning frequently signals the development of thunderstorms later in the day. These clouds are known to cause rain if they are higher up.
One form of altocumulus clouds, altocumulus lenticularis, or lenticular cloud, is frequently reported as an unidentified flying objects because of its shape. The approach of altocumulus clouds can sometimes be disturbing, because the cloud has a dark and somewhat frightening appearance. These clouds can range from 1,200 to 6,100 meters above ground level.
Since altocumulus clouds are by definition just high cumulus clouds, it might help to tell you what a cumulus cloud is. Cumulus clouds have noticeable vertical development and clearly defined edges. They are often described as puffy or cotton-like in appearance. Cumulus clouds may appear alone, in lines, or in clusters. They are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Cumulus clouds are part of the larger family of cumuliform clouds. The most intense cumulus and cumulonimbus clouds may be associated with severe weather phenomena such as hail, waterspouts, and tornadoes.
There is a good article about clouds here. Here on Universe Today we have a great article about all of the different clouds types including altocumulus clouds. Clouds are an effect of the atmosphere. Astronomy Cast offers a good episode about atmospheres