Altocumulus cloud formation
|Genus||Alto- (middle), -cumulus (heap)|
|Classification||Family B (Medium-level)|
|Appearance||Similar to cirrocumulus, but individual segments are larger and darker.|
Altocumulus (Alto, "high", cumulus, "heaped") is a middle altitude cloud genus that belongs to the stratocumuliform physical category characterized by globular masses or rolls in layers or patches, the individual elements being larger and darker than those of cirrocumulus and smaller than those of stratocumulus. Like other cumuliform and stratocumuliform clouds, altocumulus signifies convection. It is usually white or grey, and often occurs in sheets or patches with wavy, rounded masses or rolls. A sheet of partially conjoined altocumulus perlucidus is sometimes found preceding a weakening warm front, where the altostratus is starting to fragment, resulting in patches of altocumulus perlucidus between the areas of altostratus, Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds. Towering altocumulus, known as altocumulus castellanus, frequently signals the development of thunderstorms later in the day, as it shows instability and convection in the middle levels of the troposphere, the area where towering cumulus clouds can turn into cumulonimbus. It is therefore one of three warning clouds often recorded by the aviation industry, the other two being towering cumulus and cumulonimbus. Altocumulus generally forms about 6,500 feet to 20,000 feet (2,000 to 6,100 meters) above ground level, a similar level to altostratus formations and satellite photography has revealed that the two types of cloud 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.
One form of altocumulus, altocumulus lenticularis (lenticular cloud) can resemble flying saucers and may occasionally be mistaken for "unidentified flying objects". This is formed by uplift usually associated with mountains. There may be another rare type of altocumulus, altocumulus undulatus asperatus, but this has not been officially named yet.
See also 
|Wikimedia Commons has media related to: Altocumulus clouds|
- International Cloud Atlas – Altocumulus
- Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genus
- National Science Digital Library – Altocumulus
- WW2010: University of Illinois