CIRRUS

One of the three basic cloud forms (the others are cumulus and stratus). It is also one of the three high cloud types. Cirrus are thin, wispy clouds composed of ice crystals and often appear as veil patches or strands. In the mid-latitudes, cloud bases are usually found between 20,000 to 30,000 feet, and it is the highest cloud that forms in the sky, except for the tops, or anvils, of cumulonimbus, which occasionally build to excessive heights.

Deriving their name from the Latin for "a lock of curly hair," cirrus clouds are white fibrous clouds composed of ice crystals. These ice crystals may be large and may fall out of the cloud before evaporating. This fall out of ice crystals extends the cloud in the vertical. A cirrus cloud, one of the three major cloud types that comprise the ten main groups or genera, is not a cloud layer but a single cloud element.

High-level clouds (16,000 feet or more), composed of ice crystals and appearing in the form of white, delicate filaments or white or mostly white patches or narrow bands. Cirrus clouds typically have a fibrous or hairlike appearance, and often are semi-transparent. Thunderstorm anvils are a form of cirrus cloud, but most cirrus clouds are not associated with thunderstorms.