Altocumulus (Ac) - the name derives from the latin words altus = high and cumulus = mass or heap. Altocumulus is a member of the ten fundamental cloud types (or cloud genera). It is a middle-level cloud, thus they usually form between 2 to 7 kilometers (6,500 to 22,000 ft).
Altocumulus cloud usually occur as a layer or patch of more or less separate cloudlets in the form of heaps, rolls, billows or pancakes. They are white and/or gray in colour with generally shadowed parts or undersites and often show a waved aspect. They may be well shaped by high winds into lentels or rippled wave patters and might appear as a pancake tower or like an UFO, often sharply outlined, but they may also become partly fibrous and diffuse. They may or may not be merged.
Altocumulus clouds chiefly consist of super-cooled water droplets of minus 10C, but ice crystals are often present. Usually they do not produce rain, but might indicate a weather change within a day or so. Altocumulus are extremely varied. They might occur in the
castellanus like pinnacles on a wall, floccus like little heaps, lenticularis in the lee of a mountain range and stratiformis as bumpy horizontal layers; and the
duplicatus in several layers, lacunosus , opacus obscuring sun or moonlight, perlucidus , radiatus , translucidus and undulatus as the famous Kelvin-Helmholtz waves. Last but not least they might also appear as Ac mammatus the rounded clouds that appear to hang on the underside of clouds.
What do altocumulus tell about the weather?
Ac castellanus and/or Ac floccus indicate a high risk for afternoon thunderstorms when observed on a summer morning.
Ac lenticularis the Föhn cloud usually indicates a weather deterioratiuon within the next 12 to 36 hours.
Ac stratus often indicates the margins of a warm sector and no significant weather change. However, if they show ripples and waves as Ac stratus undulatus then the weather will become worse within the next 12 hours.