Identifying the types of Cloud present

Optical Phenomena

The following optical phenomena may aid in the identification of certain cloud types:

  • Halo - a luminous ring usually white in colour, with the sun or moon at its centre. Sometimes the inner edge of the ring is faint red in colour, and in rare cases, the outer edge is a faint violet colour. Inside the ring the sky is darker than outside it. This feature is usual for Cirrostratus, and may occur with other types in certain species under specific conditions.

  • Corona - one or more sequences of coloured rings of relatively small diameter, centred on the sun or moon. The outer edge is red, the inner blue. This feature can occur with Altocumulus, and with other types in certain species under specific conditions.

  • Irisation - colours appearing on clouds, sometimes in the form of bands nearly parallel to the margin of the cloud. Green and pink predominate, often with pastel shades. This feature can occur with Cirrocumulus, Altocumulus and Stratocumulus.