This cloud's name derives from Latin altus, meaning "high", and Latin cumulus, meaning "heaped". Altocumulus is a middle-altitude cloud genus that belongs mainly to the stratocumuliform physical category characterized by globular masses or rolls in layers or patches. The individual elements of that cloud are larger and darker than those of cirrocumulus and smaller than those of stratocumulus.
Altocumulus generally forms about 2,000 to 6,100 meters above ground level and can create formations that can stretch for thousands of square miles. Extensive altocumulus formations, particularly if they take the form of undulatus are often referred to as altocumulus mackerel sky.
There are four species: The stratiformis species is composed of sheets or relatively flat patches of stratocumuliform cloud. The altocumulus lenticularis species is a lens-shaped middle cloud which can resemble flying saucers. The altocumulus castellanus is a turreted middle cloud that occurs seldomly. And the floccus species is a tufted middle cloud which is also rather seldom.
Source: en.wikipedia.org/wiki/Altocumulus_cloud - 23.10.2017
Altocumulus cloud forming a dense layer during the day.