C_H = 4

(a) TECHNICAL SPECIFICATION

Cirrus uncinus, Cirrus fibratus or both, progressively invading the sky; they generally thicken as a whole.

(b) EXPLANATION

(i) The main characteristic of Cirrus corresponding to C_H = 4 is that they invade the sky progressively. The collection of clouds extends to one part of the horizon and its forward edge is moving towards the opposite part of the horizon.

(ii) The clouds occur most frequently in the form of strands trailing from a small hook or tuft (Cirrus uncinus); less frequently, they are in the form of straight or irregularly curved filaments (Cirrus fibratus).

(iii) The clouds usually seem to fuse together in the direction of the horizon from which they first appeared, but no Cirrostratus are present.