Cloud of operational significance
urn:aero:airm:1.0.0:ContextualModel:ATMBusinessTerms:Cloud_of_operational_significance
A cloud with the height of cloud base below 1 500 m (5 000 ft) or below the highest minimum sector altitude, whichever is greater, or a cumulonimbus cloud or a towering cumulus cloud at any height.
Source:
ICAO