Height of base:

above 5000 to 15000 m (16500 - 45000 ft) (high-level clouds)

Description:

Wispy, threadlike clouds;

drawn out by high winds across the sky

often together with cirrostratus and cirrocumulus

highest and fastest moving clouds (although they may not appear thus)

Occurrence:

World-wide

Made of:

falling ice crystals

Precipitation:

None

Formation:

Ice deposition on deposition nuclei,

Falling through layers of different wind speed leads to drawn appearance

Evaporation if reaching layers with higher temperature

Species:

Fibratus, uncinus, spissatus, castellanus, floccus

Varieties:

Intortus, radiatus, vertebrates, duplicatus

How to distinguish from …

Cirrocumulus (Cc)

Cc has cloudlets, Ci does not have a dappled or grainy structure

Cirrostratus (Cs)

Cs is thin milky veil, while Ci has separate streaks and elements

Cirrus floccus

(Kerkhoven, March 2006)

Cirrus intortus

(Morris, Sep 07)

Cirrus fibratus radiatus

Morris, October 07

Cirrus floccus intortus

Morris, Sep 07

Cirrus intortus and altocumulus castellanus

Morris, March 28, 2008

Cirrus radiatus and altocumulus lenticularis

Morris, May 16, 2008