Definition
Fog is a surface-based cloud composed of tiny water droplets (or ice crystals) suspended in the air, reducing horizontal visibility to less than 5/8 statute mile (1,000 meters). In a METAR, it is reported with the code FG. When visibility is 5/8 statute mile or greater but less than 7 statute miles, the condition is reported as mist (BR) rather than fog.
Plain English
A cloud sitting on the ground that cuts visibility to less than about two-thirds of a mile. If you can see farther than that, it's called mist instead.
Context Anchor
Seen in the present weather section of a METAR, such as when checking airport weather before departure or arrival.
Derivation
FG comes from the first and last consonants of 'fog,' a pattern the METAR code uses for many weather phenomena (BR for mist/brume, HZ for haze). The two-letter code keeps reports short and standardized worldwide.
Why Pilots Care
Fog can force delays, require instrument procedures, or prevent takeoff and landing until it clears.
Grounding Statement
If you walked outside and could not clearly see a building two blocks away, that is the visibility threshold where mist becomes fog.
Intuition Check
Fog here does not mean just any cloudy or damp weather. In a METAR, FG points to fog at the reporting location that is reducing visibility to a specific, operationally important level.
Example Sentence 1
The 1200Z METAR reported '1/4SM FG OVC002,' so the pilot delayed departure until the fog burned off.
Example Sentence 2
With FG reported at one-quarter mile, the flight switched to an instrument approach.