Explanatory remarks and special clouds

(Section 2.3.1.8)

Cirrus tufts with rounded tops often form in clear air. Fibrous trails may appear under the tufts; the tops then gradually lose their roundness. Subsequently, the tufts may disappear completely; the clouds are then in the form of filaments (species fibratus or uncinis).

Cirrus in the form of filaments may develop also from dense Cirrus patches, from Altocumulus castellanus and floccus and, occasionally at very low temperatures, from Cumulus congestus.

Cirrus may also form from aircraft condensation trails (contrails) that have persisted for at least 10 minutes (Ci homogenitus). No species, varieties or supplementary features are identified with Cirrus homogenitus as new or recently formed aircraft condensation trails usually undergo a fairly rapid state of change, and may display a variety of transient shapes.  

Persistent contrails (Ci homogenitus) can, over a period of time and under the influence of strong upper winds, take on the form of species and/or varieties of Cirrus. When this occurs, the Cirrus is identified by relevant species and varieties followed by 'homomutatus' (for example, Cirrus homomutatus)

Colour

At all times of the day, Cirrus not too close to the horizon is white, whiter than any other cloud in the same part of the sky. 

With the Sun on the horizon it is whitish, while lower clouds may be tinted yellow or orange. 

When the Sun sinks below the horizon, Cirrus high in the sky is yellow, then pink, red and finally grey. The colour sequence is reversed at dawn.

Cirrus near the horizon often takes a yellowish or orange tint, owing to the great thickness of air traversed by the light in passing from the cloud to the observer. These tints are less conspicuous in clouds in the low and middle levels.