Definition
In a METAR, MI is a descriptor used with fog (FG) or, occasionally, mist, to indicate that the obscuration is shallow — meaning it lies close to the ground, generally less than six feet deep, with clearer air above it. It is reported as MIFG when paired with fog.
Plain English
MI tells you the fog is low to the ground — like a thin layer hugging the surface — with better visibility above it.
Context Anchor
Seen in METAR present-weather groups, especially when shallow fog is reported near an airport.
Derivation
MI comes from the French word 'mince,' meaning thin or shallow. The international weather codes used in METARs draw on French-language origins, which is why the abbreviation does not match the English word.
Why Pilots Care
Shallow fog can reduce visibility for taxiing and takeoff while conditions improve just above the surface, influencing go or no-go choices.
Grounding Statement
Picture a thin blanket of fog lying over the runway while the air just above it is much clearer.
Intuition Check
Shallow does not mean unimportant here. It means the weather layer is low and thin, close to the ground.
Example Sentence 1
The METAR reported MIFG, so the pilot expected a thin layer of fog along the runway with clearer air just above.
Example Sentence 2
With MI conditions present, the pilot could see the runway lights but stayed alert for surface visibility changes.