Plain English Meaning

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.

Full 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.

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 (PR) or shallow (MI) variants of the same phenomenon.