Altocumulus (Alto, "middle", cumulus, "heaped") is a cloud belonging to a class 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 cumulus clouds, altocumulus signifies convection. It is usually white or gray, and often occurs in sheets or patches with wavy, rounded masses or rolls. Altocumulus often are seen preceding a cold front, and their presence on a warm, humid, summer morning frequently signals the development of thunderstorms later in the day. Alto means high and these clouds may cause rain if they are higher up.
One form of altocumulus, altocumulus lenticularis (Lenticular cloud) is frequently reported as "Unidentified flying objects". The altocumulus approach can sometimes be disturbing, for the cloud has a dark and somewhat frightening appearance. It is about 8,000 feet to 20,000 feet above ground level.