Definition
Fog that forms when relatively warm rain or drizzle falls through cooler air near the surface. The cool air becomes saturated as moisture evaporates from the falling drops, producing fog that often develops along and ahead of warm fronts.
Plain English
Fog created when warm rain falls through colder air below it. The cold air soaks up moisture from the rain until it can't hold any more, and fog forms.
Context Anchor
Pilots may encounter precipitation fog in weather reports, forecasts, and approach planning, especially near warm fronts or steady rain.
Derivation
From Latin praecipitare, meaning 'to fall headlong.' In weather terms, precipitation is anything falling from the sky—rain, drizzle, snow. The fog is named after what causes it: the falling rain.
Why Pilots Care
It can reduce visibility to near zero with little warning, forcing sudden IFR conditions or missed approaches.
Grounding Statement
Picture warm rain falling from a layer aloft into cooler air near the ground. The cool air can't hold all the moisture evaporating off the drops, so fog forms in the space between the cloud base and the ground.
Intuition Check
Precipitation fog does not mean any fog that happens to occur during rain. It means fog formed because falling rain or drizzle evaporates into cooler air and adds moisture.
Example Sentence 1
The METAR showed visibility dropping to half a mile in precipitation fog as the warm front moved in.
Example Sentence 2
The pilot delayed takeoff because precipitation fog had formed over the runway in the cool night air.