(a) TECHNICAL SPECIFICATION
(ii) Among the numerous possible cases covered by CL = 9, the following two are frequently observed:
(iii) The cirriform parts of Cumulonimbus capillatus may become invisible when the cloud passes over the point of observation. It should nevertheless be classified as Cumulonimbus capillatus on the basis of its history, coding CL = 9.
(iv) The same applies when the cirriform parts of Cumulonimbus capillatus become hidden by other clouds.
(v) The occurrence of lightning, thunder or hail sometimes provides the only indication of the presence of Cumulonimbus. In this case, it is not possible to decide whether the cloud belongs to the species calvus or capillatus, the coding is, by convention, CL = 9.
(vi) Sometimes, when the 0 °C (32 °F) level is low, the fibrous structure of the upper part spreads through the whole Cumulonimbus capillatus, which then degenerates into a cirriform cloud mass (CH = 3), the coding CL = 9 is maintained as long as at least one Cumulonimbus remains in sight or is known to be present.
(vii) Cumulonimbus capillatus sometimes produces cloud masses that may become detached from it and assume an independent identity. Very often, they have the appearance of Cirrus, Altocumulus, Altostratus or Stratocumulus.