Cloud Types - Tropospheric Class: Families, Genera, Species, Varieties, and Supplementary Features - High Cirriform, Stratocumuliform, and Stratiform - Genus Cirrus

Genus Cirrus

Abbreviation: Ci

Cirrus clouds form in the highest and coldest region of the troposphere from about 16,500 to 40,000 ft (5 to 12 km) in temperate latitudes. At this altitude water almost always freezes so clouds are composed of ice crystals. The clouds tend to be wispy, and are often transparent. Isolated cirrus clouds often do not bring rain, however, large amounts of cirrus clouds can indicate an approaching storm system eventually followed by fair weather.

There are several variations of clouds of the cirrus genus based on species and varieties:

WMO species:

• Cirrus fibratus
  High clouds having the traditional "mare's tail" appearance. These clouds are long, fibrous, and curved, with no tufts or curls at the ends.
• Cirrus uncinus
  Filaments with up-turned hooks or curls.
• Cirrus spissatus
  Dense and opaque or mostly opaque patches.
• Cirrus castellanus
  A series of dense lumps, or "towers", connected by a thinner base.
• Cirrus floccus
  Elements which take on a rounded appearance on the top, with the lower part appearing ragged.

WMO varieties; opacity-based:

• None (always translucent).

WMO varieties; fibratus pattern-based:

• Cirrus intortus
  Irregularly curved or tangled filaments.
• Cirrus vertebratus
  Elements arranged in the manner of a vertebrae or fish skeleton.

WMO varieties; fibratus and uncinus pattern-based:

• Cirrus radiatus
  Large horizontal bands that appear to converge at the horizon.
• Cirrus duplicatus
  Sheets at different layers of the upper troposphere, which may be connected at one or more points.
• Varieties are not commonly associated with Ci species spissatus, castellanus, or floccus.

Non-WMO variant:

• Cirrus aviaticus
  Persistent condensation trails (contrails) formed by ice crystals originating from water vapor emitted by aircraft engines.