Originally Posted by
FlightDetent
UK differences from ICAO Annex 3 (my PDF copy may not be the latest)
"The reporting of clouds in the METAR of operational significance will be introduced on 6 October 2005. From this date, the abbreviation NSC (no significant cloud) will be used if there are no clouds of operational significance, no TCU or CB; the abbreviations CAVOK and SKC are not appropriate."
NSC refers to no cloud below 5,000ft aal.