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While examining a UK Metform 215 (F215) chart, you notice the abbreviation OCNL associated with a weather phenomenon. What does this indicate?

  • A

    It is an occlusion in the northern latitudes of the UK.

  • B

    It is only affecting northern latitudes.

  • C

    It is overcast cloud with no discernible level.

  • D

    It is occasional.

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 percent 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 percent of the area concerned (at a fixed time or during the period of validity).

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