Definition
In aviation weather reports, FG is the standard contraction used to indicate fog — a surface-based cloud of suspended water droplets that reduces horizontal visibility to less than 5/8 statute mile (1,000 meters). When visibility is greater than 5/8 statute mile but obscuration is still present, the report uses BR (mist) instead of FG.
Plain English
FG is the code that means fog is present at the airport. Fog is essentially a cloud sitting on the ground, and it is only reported as FG when visibility drops below 5/8 of a mile.
Context Anchor
Seen in METARs and other aviation weather reports when fog is affecting the airport or reporting station.
Derivation
FG comes from the first and last consonants of the word fog. METAR weather codes use a two-letter shorthand system based on the World Meteorological Organization standard so that reports stay short and machine-readable.
Why Pilots Care
Fog sharply reduces visibility and can prevent takeoff, landing, or require instrument procedures and flight delays.
Grounding Statement
If you are standing on the ramp and the airport seems swallowed by a low gray cloud at ground level, the report may show FG.
Intuition Check
Do not read FG as a cloud layer or a sky-cover amount. In aviation weather reports, FG specifically means fog that is reducing visibility near the surface.
Example Sentence 1
The 0600Z METAR showed FG with visibility of 1/4 mile, so we delayed our departure until the sun burned it off.
Example Sentence 2
With FG reported, the approach was delayed until conditions cleared.