Cirrus uncinus
Abbreviation Ci unc
Family A
Genus Cirrus
Occurrence Worldwide
Altitude >7000m, in troposphere
Formation Saturation of upper-level air mass combined with strong wind immediately below cloud level
Precipitation None
Potential Hazards Indicates an approaching frontal system (usually warm or occluded)

Cirrus uncinus (Ci unc), popularly known as mares' tails, is a species of thin cirrus cloud which has a fibrous and hooked appearance. It forms at very high altitudes in unstable layers of the troposphere, usually at temperatures of approximately negative 50 degrees Celsius. Often, they herald the approach of a frontal system and possibly precipitation. When wind shear is positive (higher windspeeds at higher altitudes), the hook curves downward, whereas it curves upward when wind shear is negative.

High Clouds (Family A): Cirrus (Ci) • Cirrus aviaticus • Cirrus intortus • Cirrus radiatus • Cirrus uncinus • Cirrus Kelvin-Helmholtz • Cirrostratus (Cs) • Cirrocumulus (Cc) • Pileus • Contrail
Middle Clouds (Family B): Altostratus (As) • Altostratus undulatus • Altocumulus (Ac) • Altocumulus undulatus • Altocumulus mackerel sky • Altocumulus castellanus • Altocumulus lenticularis
Low Clouds (Family C): Stratus (St) • Orographic stratus • Fog • Nimbostratus (Ns) • Cumulus humilis (Cu) • Cumulus mediocris (Cu) • Stratocumulus (Sc)
Vertical Clouds (Family D): Cumulonimbus (Cb) • Cumulonimbus incus • Cumulonimbus calvus • Cumulonimbus with mammatus • Cumulus congestus • Cumulus castellanus • Pyrocumulus  • Pyrocumulonimbus

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