Definition
A type of fog that forms when cold air moves over a warmer water surface. Water evaporates from the surface into the cold air directly above it, quickly saturates that air, and condenses into visible fog that appears to rise off the water like steam.
Plain English
Fog that looks like rising steam, formed when cold air passes over warmer water and the water evaporates straight into it.
Context Anchor
Pilots may encounter steam fog near lakes, rivers, coastal water, or wet runways when cold air has moved over a warmer surface.
Derivation
Called steam fog because it looks like the steam rising off a hot drink or a warm bath on a cold morning. The visual is the name.
Why Pilots Care
It can form quickly and reduce visibility to near zero over water, creating sudden hazards for low-level flight, takeoff, or landing near lakes and rivers.
Analogy
Think of a hot bath in a cold bathroom — the wisps you see rising are the same process: warm water evaporating into cold air and condensing on contact.
Grounding Statement
Picture a cold morning over a warm lake: moisture rises from the water and turns into thin fog as soon as it meets the colder air.
Intuition Check
Do not read “steam” as hot vapor from boiling water here. Steam fog is made of tiny liquid water droplets, like other fog, and it forms because cold air is passing over a warmer moist surface.
Example Sentence 1
On the cold November morning, steam fog drifted off the lake and reduced visibility along the shoreline approach.
Example Sentence 2
Steam fog reduced surface visibility to less than a mile along the river valley during the cold night.