Special observations made by aircraft operating on international air routes whenever

(a) severe turbulence or severe icing is encountered;

(b) moderate turbulence, hail or cumulonimbus clouds are encountered during transonic or supersonic flight; or

(c) other meteorological conditions, for example, the other en route weather phenomena specified for SIGMET messages, are encountered

that, in the opinion of the pilot-in-command, may affect the safety or markedly affect the efficiency of other aircraft operations.