Definition
A temperature inversion that forms at or near the ground, in which a layer of cold air lies trapped beneath a layer of warmer air. It typically develops on clear, calm nights as the ground rapidly radiates heat away, cooling the air immediately above it while the air higher up remains warmer.
Plain English
A shallow layer of cold air sitting on the ground with warmer air just above it -- the opposite of how the atmosphere normally behaves, where air is warmest near the surface and gets colder with height.
Context Anchor
Seen in weather briefings and forecasts when discussing fog, haze, smoke, poor visibility, stable air, or early-morning conditions near an airport.
Derivation
Inversion comes from the Latin invertere, meaning 'to turn upside down.' The normal pattern of warm-low, cool-high is flipped, so the temperature profile near the surface is inverted.
Why Pilots Care
Trapped cooler air can produce fog, frost, and reduced visibility, altering takeoff and landing performance and requiring adjusted departure procedures.
Analogy
Think of a lid sitting over a pot. The cooler air near the ground is held down under warmer air above, so moisture and haze can stay trapped near the surface.
Grounding Statement
Picture a calm, clear morning with fog or smoke pooled in low ground -- that pooling happens because cold, heavy air has settled at the surface under warmer air above.
Intuition Check
Do not think of "inversion" as just any weather change. Here it means the normal temperature pattern is reversed: colder air is below warmer air near the ground.
Example Sentence 1
The hazy layer over the runway at sunrise was a sign of a surface inversion that had formed overnight.
Example Sentence 2
A strong surface inversion kept the airport below freezing while the air a few hundred feet up remained above zero.