Temperature inversion

An inversion is an atmospheric layer where the temperature INCREASES with altitude instead of the usual decrease. The normal behaviour (ISA: −1.98 °C/1 000 ft) is reversed.

Effect: an inversion acts like a "lid" — it suppresses vertical motion because rising air immediately becomes colder than the surroundings (negative buoyancy). Pollutants, moisture and fog become trapped below the inversion.

Types of inversion

1. Radiation inversion

Formation: at the surface, on calm, clear nights. The Earth's surface cools rapidly by IR emission; air just above cools with it. Air higher up stays warm → inversion at the surface.

Properties:

Thickness: a few metres up to ~300 m.
Typical on clear nights in autumn and winter.
Dissolves at sunrise as ground heating resumes.

Consequences:

Ground fog often forms (radiation fog) — see Fog.
Pollutants (industry, traffic) accumulate.
Ice on runways possible.
Calm winds at the surface despite wind aloft (decoupling).

2. Subsidence inversion

Formation: in high-pressure regions air sinks at large scale; adiabatic warming makes it warmer than the air below (often cooler surface air).

Properties:

Altitude: 600–2 000 m AGL, often as an "inversion aloft" over cooler air.
Very stable — can persist several days.
Classic in anticyclones in summer (heatwaves) and winter (continental highs).

Consequences:

Smog accumulation below the inversion.
High visibility above the inversion (clear), poor visibility below (haze).
Cumulus clouds grow to the inversion and are "cut off" there (cumulus humilis instead of congestus).

3. Frontal inversion

Formation: at a warm front warm air rests on cold air → at the frontal surface temperature increases with altitude (from cold surface air into the warm frontal air).

Properties:

Sloping inversion along the frontal surface (typically 1
slope).
Stratiform clouds and steady precipitation in the warm frontal air.
Freezing rain risk: warm air above cold → rain drops into cold subzero zone → freezes.

4. Turbulence (mechanical) inversion

Formation: strong near-ground turbulence mixes cold air downward; aloft a warmer layer can persist.

Rare type, mostly academic.

Pilot consequences of inversions

Visibility

Below the inversion: haze, smog, possibly fog → reduced visibility.
Above the inversion: clear, often excellent visibility.
On climbing through the inversion: sudden visibility improvement ("punching through").

Wind

Wind shear at the inversion: wind below and above is often very different (speed and direction).
Low-level wind shear possible → risk on take-off/landing.

Stability and clouds

Vertical motion damped → no convection through the inversion.
Cumulus grows only to the base of the inversion → no thunderstorms while the inversion holds.
Breakup of the inversion on heating often produces sudden thunderstorm formation in the afternoon (severe weather).

Pollutant concentration

Pollutants (industry, vehicles) trapped → poor air quality.
For GA pilots: higher CO risk during taxi behind running engines in inversion layer.

Acoustic effect

Inversion reflects sound → sounds carried further (aircraft noise audible in valleys despite altitude).

Recognition in METAR/TAF

Temperature deviation in upper-air reports (WINTEM).
Haze (HZ) in METAR with clear sky aloft.
Observations of low stratus and clear visibility aloft.

Operational relevance

For VFR:

Ground fog may delay departure (radiation inversion).
Valley flight in inversion conditions: poor visibility in valleys.
On climbing through the inversion: beware wind shear.

For IFR:

Inversion often associated with IMC — descent from clear air into dense cloud.