Altocumulus
Altocumulus clouds belong to a class distinguished by globular masses or rolls in layers or patches. These clouds are larger and darker than cirrocumulus and smaller than stratocumulus. Like other cumulus clouds, altocumulus indicates a vertical and upward transference of atmospheric conditions. Altocumulus are often seen preceding a cold front, and their presence on a warm, humid, summer morning usually signals the development of thunderstorms later in the day.
A form of altocumulus, the Lenticular clouds, are sometimes mistakenly reported as UFOs. The approach of the altocumulus can sometimes be disturbing, as it has a dark and somewhat frightening appearance. These clouds are found from about 8,000 to 20,000 feet in altitude. Higher altocumulus clouds can sometimes produce rain.