Temperature inversion
Temperature & humidity thermal inversion inversion layer
In a temperature inversion, air becomes warmer with height through a layer instead of colder. This stable arrangement can restrict vertical mixing.
Reversing the usual pattern
In the lower atmosphere, temperature usually decreases with height. An inversion reverses this pattern within a layer: warmer air lies above colder air. A rising parcel then enters surroundings where it has less buoyancy, restricting upward exchange.
How an inversion develops
- Near the ground at night: Under clear skies, the surface loses heat and cools the air immediately above it more than the air higher up.
- Under high pressure: Sinking air warms and can form a warm layer above cool surface air.
- In hollows: Cold air can collect and strengthen the stable layer near the ground.
Fog below, sunshine above
If trapped air is moist enough, fog or low cloud may persist. Above it, conditions can be sunny and milder. Weak mixing also limits the upward dilution of pollutants, although concentrations still depend on emissions and wind.
What breaks up the layer
Heating from the ground, stronger turbulence or a change in weather can weaken an inversion. A temperature profile from a weather balloon or model shows where it lies. Two ground stations at different elevations alone do not reliably prove that an inversion exists.