The 12 Basic Cloud Classifications

Cumulus Congestus >25,000 ft (top) 3 - 6,000 ft (base) Very active separated heaps with flat bottoms and growing towers Cumulus (Heap) Family
Swelling Cumulus 15 - 25,000 ft (top) 4 - 5,000 ft (base) Active separated heaps with flat bottoms and bumpy cauliflower tops Cumulus (Heap) Family
Cumulus of Fairweather 3 - 5,000 ft (top) 2 - 3,000 ft (base) Small heaps of clouds with flattish bottoms and rounded tops Cumulus (Heap) Family
Cirrostratus > 25,000 ft High level veil of ice crystal cloud, frequently producing a halo around Sun or Moon Stratus (Layer) Family
Altostratus 15 - 20,000 ft Thickly layered water droplet cloud. Sun's disk diffused as viewed through sheet of ground glass Stratus (Layer) Family
Stratus 0 - (Fog) 1 - 8,000 ft Low-lying formless cloud, called fog when base lies on the ground Stratus ( Layer) Family
Cirrocumulus > 25,000 ft High level layered cloud with wavelike or fine, dappled cumuliform structure Heap/Layer Family
Altocumulus 12 -- 20,000 ft Mid-level layered cloud with globular structure, sometimes called 'buttermilk sky' Heap/Layer Family
Stratocumulus 3 - 10,000 ft Low layered cloud with bumpy, heap-like structure Heap/Layer Family
Cirrus > 25,000 ft An ice crystal cloud showing filaments of ice crystals precipitating and evaporating Precipitation Family
Cumulonimbus > 25,000 ft (top) 3 - 6,000 ft (base) A massive pile of cloud penetrating the freezing level and sometimes forming an anvil at the base of the stratosphere Precipitation Family
Nimbostratus 10 - 25,000 ft Generally formless dark mass of low storm cloud associated with general continuous rainfall or snowfall