ICAO Annex 3 – Meteorological Service for International Air Navigation
4.2 Criteria related to phenomena included in SIGMET and AIRMET messages and special air-reports (uplink)

4.2.1 Recommendation. – An area of thunderstorms and cumulonimbus clouds should be considered:

a) obscured (OBSC) if it is observed by haze or smoke or cannot be really seen due to darkness;
b) embedded (EMBD) if it is embedded within cloud layers and cannot be readily recognized;
c) isolated (ISOL) if it consists of individual features which affect, or are forecast to affect, an area with a maximum spatial coverage less than 50 per cent of the area concerned (at a fixed time or during the period of validity); and
d) occasional (OCNL) if it consists of well-separated features which affect, or are forecast to affect, an area with a maximum spatial coverage between 50 and 75 per cent of the area concerned (at a fixed time or during the period of validity).