An altocumulus cloud is a type of cloud that belongs to the middle zone: in temperate latitudes, its base is located between 1,500 and 6,000 m from the ground and its thickness is in the order of 1,500 m. It is composed of white or grey strips, shingles, bands, rolls etc., generally produces its own shadows and appears quite evenly in a bank, sheet or a layer of which the elements may or may not be joined together and have an apparent width of between approximately 1° and 5° (1° corresponds to the width of the little finger on an outstretched arm, and 5° to that of three fingers on an outstretched arm); this formation often gives the sky a dappled effect. The passage of a bank of altocumulus clouds in front of the sun or moon can create a halo around it.
Altocumulus clouds indicate a certain level of air instability in the layers where they appear. They may form in a clear sky when a significant air mass rises to mid-height and causes the partial condensation of the water vapour it contains (due to a land formation or the approach of a disturbance). There are a wide variety of other processes that lead to the formation of altocumulus clouds which are associated with the evolution of other types of clouds: for example, the subdivision of the upper regions of stratocumulus clouds or the staggering of the upper regions or the sides of large cumulus clouds.
Among castellanus altocumulus clouds (castellanus means crenellated), the cumuliform appearance is enhanced by the formation of ascendant protuberances in the shape of small turrets, announcing the development of cumulonimbus storm clouds. Another remarkable variety of altocumulus clouds is the lenticularis type (lenticularis means lenticular). These clouds are lens or almond shaped with very well defined edges. They appear under a wind of a land formation that the air flow cannot move around and stay under the ascending "bellies" of waves maintained by this air flow after it has crossed the obstacle; in some cases, these lenticular clouds are stacked on top of one another in successive layers of air and look like "stacks of dishes".