altocumulus (Ac) Similar to cirrocumulus, with patches of small clouds occasionally separated by thin breaks. Although altocumulus clouds also may be identified by a "mackerel sky" pattern, they are lower, at around 10,000 feet, and the clumps of white or gray water droplets or ice crystals are larger. The clouds may develop directly overhead, depending on the temperature of the atmosphere, and may produce a shower.

altostratus (As) Dull, drab gray or blue middle-level clouds that usually contain moisture in the form of water droplets. Altostratus clouds are often opaque, giving a "ground glass" view of the sun or moon behind them. They may be a source of virga, filaments of ice crystals or water droplets that fall toward Earth but evaporate before touching the ground.

cirrocumulus (Ce) Loosely packed sheets of small white cloud segments at altitudes of around 18,000 to 20,000 feet, forming a "mackerel sky" resembling scales on a fish. The clouds may consist of ice crystals or water droplets or both. The patchy appearance is caused by vertical air currents at the cloud level, indicating a lack of stability and a possible approaching storm.

cirrostratus (Cs) Translucent veils of white fibrous cloud that tend to occur at altitudes of around 20,000 feet or more. Cirrostratus clouds often cover the entire sky and may cause the appearance of halos or reflected images of the sun or moon. They may signal an approaching storm.

cirrus (Ci) Generally, the highest clouds, forming "mares' tails" at altitudes between 20,000 and 40,000 feet. The clouds may appear as delicate white filaments, featherlike tufts, or fibrous bands of ice crystals.

cumulonimbus (Cb) Thunderstorm clouds that may vary considerably in altitude from ominously dark lower portions below 5,000 feet to white anvil-shaped tops that may reach upward to 50,000 feet. They contain large amounts of moisture, some of which may be in the form of hail. The cumulonimbus cloud may appear alone or as part of a wall of advancing storm clouds.

cumulus (Cu) Low-level billowy clouds that are usually dark on the bottom while the top resembles a giant white cotton ball. A cumulus cloud may be relatively tall, extending from a base around 2,000 feet to a top near 10,000 feet above ground. It casts a dark shadow and may be a source of moisture but generally produces no more than a summer shower.

nimbostratus (Ns) Low, dark rain clouds with ragged tops that have bottoms only a few hundred feet above ground and may range upward to an altitude of 3,000 feet. They obscure the sun and are associated with continuous rain, sleet, or snow but are rarely accompanied by thunder or lightning.

stratocumulus (Sc) Dark, gray rolls of clouds that usually cover the entire sky at an altitude from 1,500 to 6,500 feet. The rounded segments may appear checkered or wavelike and there may or may not be breaks of blue sky between segments. Stratocumulus clouds contain moisture but are usually not rain producers.

stratus (St) Wispy foglike clouds that hover a few hundred feet above ground, sometimes obscuring hills or tall buildings. They may begin as ground fog and can be a source of drizzle.