0 a type of cumulus (= a tall, rounded, white cloud with a flat base) found at medium height, above the level of cumulus -- 高积云
It is a fairly uncommon phenomenon, most often observed in altocumulus, cirrocumulus, lenticular clouds and cirrus clouds.
The presence of altocumulus undulatus may indicate precipitation within the next 20 hours or simply an overcast day.
Extensive altocumulus formations, particularly if they take the form of undulatus are often referred to as altocumulus mackerel sky.
This often does not apply when stratocumulus is of a broken, fractus form, when it may appear as small as altocumulus.
Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds.
Cirrocumulus is distinguished from altocumulus in several ways, although the two stratocumuliform genus types can occasionally occur together with no clear demarcation between them.
Cirrocumulus generally occur at higher altitudes than altocumulus, and thus the cloudlets appear smaller as they are more distant from observation at ground level.
An example of this would be an opaque layer of altocumulus stratiformis arranged in seemingly converging rows.
