Type IV Whirls

(Section 2.6.3.3.6)

These are partial or, on rare occasions, complete rings of cloud with dark centres. They are sometimes seen in veil, band and billow forms. Three groups may be observed:

IVa: Comprised of whirls of small radius of curvature (0.1°−0.5°), and may appear as small bright crests looking somewhat like light ripples on a water surface

IVb: Have the form of a simple bend of one or several bands with a radius of curvature of 3°−5°

IVc: Have a large-scale ring structure