Cloud Classifications

There are four basic cloud classifications: cumulus, stratus, cirrus, and nimbus. Clouds are generally grouped according to height, as follows:

• Low (bases below 6,500 feet): composed mostly of water droplets, but may also contain ice particles and snow when temperatures are sufficiently cold.
• Middle (between 6,500 feet and 20,000 feet): composed primarily of water droplets, but can also contain ice crystals when temperatures are cold enough.
• High (above 20,000 feet): since temperatures at this height are so cold, these are primarily composed of ice crystals.

Name	Symbol	Description	Name	Symbol	Description	Name	Symbol	Description
Low Clouds			Middle Clouds			High Clouds
Cumulus		Little vertical development; flattened	Altostratus		Thin, semi-transparent	Cirrus		Filaments ("mares tails")
Cumulus		Considerable development; towering	Altostratus		Thick; may hide sun or moon	Cirrus		Dense, patchy
Cumulonimbus		Tops lacking clear-cut outlines	Altocumulus		Thin, semi-transparent	Cirrus		Dense, anvil-shaped
Stratocumulus		Spreading from cumulus	Altocumulus		Thin, patchy	Cirrus		Hook-shaped; thickening
Stratocumulus		Not spreading from cumulus	Altocumulus		Bands; thickening	Cirrus and cirrostratus		Not reaching 45° altitude
Stratus and/or fractostratus		Not bad weather	Altocumulus		Spreading from cumulus	Cirrus and cirrostratus		Exceeding 45° altitude
Fractostratus/ fractocumulus		Bad weather (scud)	Altocumulus		Double layered	Cirrostratus		Veil covering sky
Cumulus and stratocumulus		Not spreading from cumulus	Altocumulus		Tufts or turrets	Cirrostratus		Not increasing; not covering entire sky
Cumulonimbus		Clear top; often anvil-shaped	Altocumulus		Chaotic sky	Cirrocumulus		Main cirriform cloud