Definition
A shallow layer of radiation fog that forms close to the surface on clear, calm nights when the ground cools rapidly by radiation, chilling the air immediately above it to its dew point. It usually extends no more than about 50 feet above the surface and typically dissipates a few hours after sunrise as the ground warms.
Plain English
A thin layer of fog that sits low to the ground overnight when the earth cools quickly under clear skies. It tends to burn off shortly after the sun comes up.
Context Anchor
Pilots encounter ground fog in weather reports, preflight weather briefings, and during early-morning departures or arrivals when low areas near a runway cool overnight.
Derivation
“Ground” means the surface of the earth, and “fog” means tiny water droplets suspended in the air near the surface. Together, the words point to fog whose main effect is close to the ground, where taxiing, takeoff, landing, and runway visibility matter.
Why Pilots Care
It can reduce visibility to near zero at the airport surface even when conditions a few hundred feet above are clear, creating hazardous takeoff and landing situations.
Analogy
It is like a thin blanket lying on the ground: from above it may look shallow, but when you are inside it at runway level, it can block what you need to see.
Grounding Statement
Picture a cool, calm morning where the runway surface and nearby grass are covered by a thin white layer, while the tops of trees and buildings are still visible above it.
Intuition Check
Ground fog does not mean all fog touching the ground is harmless or minor. It means the fog is concentrated near the surface, exactly where runway and taxi visibility are most important.
Example Sentence 1
The 6 a.m. departure was delayed an hour while ground fog burned off the field.
Example Sentence 2
The pilot waited for sunrise because ground fog often lifts once the surface warms.