Cirrus uncinus
Ci unc
Cirrus uncinus
© Wetterkurs.ch
Description
The name "uncinus" (Latin: hook-shaped) describes the characteristic form of this cirrus cloud, whose fibers are curved into small hooks or comma shapes at their upper ends. Cirrus uncinus forms when ice crystals fall from small generating cells and are drawn out into hook-shaped veils by differing wind speeds at various altitudes. The cloud appears as fine, often isolated white threads or streaks high in the sky, with ends that are distinctly curved upward. Colloquially, these clouds are also known as "mares' tails" due to their typical shape. Cirrus uncinus is one of the most easily recognizable types of cirrus and is a classic representative of high-level ice clouds.
Identification Features
- Fine, white fibers with distinctly hook-shaped, upward-curved ends
- Individual or loosely grouped comma-shaped cloud elements
- Often featuring a thickened "head" and a downward-hanging, curved "tail"
- Very high in the sky, translucent and of a silky, delicate appearance
- Cast no shadows on the ground due to their great height and thinness
- Clearly distinguishable from Cirrus fibratus by their characteristic curvature
- Ice crystal fallstreaks (virga) are often visible below the generating cells
- Frequently move slowly and gracefully across the sky
Occurrence by genus
Highlighted: all genera with which uncinus occurs.
Meteorological Significance
Cirrus uncinus is meteorologically significant as it frequently indicates an approaching warm front or an advancing low-pressure system. The hook shape is produced by strong wind shear in the upper troposphere and signals the presence of jet stream activity at high altitude. When Cirrus uncinus progressively thickens and consolidates into cirrostratus, a deterioration in weather and precipitation can be expected within 12 to 36 hours. However, if the clouds appear in isolation and subsequently dissipate, no significant change in weather is to be expected. The direction in which the hooks are trailing also provides information about the prevailing upper-level flow and can be used for weather forecasting.
Facts
- WMO-Code
- Ci unc
- Height layer
- High clouds
- Typical height
- 6.0 - 12.0 km
- Genus
- CIRRUS