Definition
In a METAR, BC is a descriptor used with an obscuration (most commonly fog, as in BCFG) to indicate that the phenomenon is occurring in patches rather than as a continuous layer across the airport area. Patches typically cover parts of the aerodrome with clear areas in between, and visibility can vary significantly from one patch to the next.
Plain English
BC means the weather phenomenon — usually fog — is showing up in scattered patches around the airport, not as one solid blanket. Some areas are clear; others are not.
Context Anchor
Seen in METAR weather reports, especially in codes such as BCFG for patches of fog.
Derivation
BC comes from the Latin and French root for 'patch' or 'piece' (via banc, meaning a bank or patch of something). In METAR shorthand it was assigned the two-letter code BC to distinguish patchy coverage from partial (PR) or shallow (MI) variants of the same phenomenon.
Why Pilots Care
Alerts the pilot that visibility or ceiling conditions may vary across the airport, requiring caution on approach or departure.
Intuition Check
Do not read patches as a repair or a marked area on a surface here. In METAR weather reports, patches means the weather is present in separate spots, not everywhere at once.
Example Sentence 1
The METAR reported BCFG, so the pilot expected patches of fog along the runway with clearer areas in between.
Example Sentence 2
With BC in the report, the pilot expected clear visibility in some directions and reduced visibility in others.