Definition
A visible mass of tiny water droplets or ice crystals suspended in the air at or near the earth's surface, reducing horizontal visibility to less than five-eighths of a statute mile (1 kilometer). Fog forms when the air near the surface is cooled to its dew point, or when sufficient moisture is added to the air to raise the dew point to the existing temperature.
Plain English
A cloud sitting on the ground. The air has cooled enough, or picked up enough moisture, that water droplets form right where you're trying to see and fly, cutting visibility down sharply.
Context Anchor
Pilots encounter fog in preflight weather planning, airport weather reports, forecasts, and during taxi, takeoff, approach, and landing.
Derivation
From the Old Norse word 'fok,' meaning spray or snowdrift — something carried in the air that obscures vision. The aviation meaning keeps that core idea: fine particles in the air that block sight.
Why Pilots Care
Decides if VFR flight is allowed and whether instrument procedures or delays are needed.
Analogy
Fog is like walking into a cloud that has settled onto the airport. The air may be calm, but the view ahead can disappear very quickly.
Grounding Statement
Picture driving through a thick morning mist where you can barely see the next streetlight — that same mist sitting over a runway is fog.
Intuition Check
Fog does not mean any kind of murky air. Smoke, dust, and haze can also reduce visibility, but fog specifically involves tiny water droplets or ice crystals near the ground.
Example Sentence 1
The pilot delayed departure when morning fog dropped visibility at the field below approach minimums.
Example Sentence 2
Fog formed over the runway during the approach, forcing a go-around and diversion.