H4 Clouds: Cirrus (Ci) progressively invading the sky
Cirrus in the form of filaments, strands or hooks, progressively invading the sky and generally becoming dense
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Notes: These are the same clouds as H1 but they progressively invade the sky and generally become denser as a whole. They often seem to fuse together in the direction from which they come. The forward edge moves towards the opposite part of the horizon. The clouds occur most frequently in form of strands trailing from a small hook or tuft and less frequently in the form of straight or irregularly curved filaments.
