7.3 Thermal Inversion

What is a thermal inversion?

Normally, air temperature decreases with altitude, allowing warm air near the ground to rise and disperse pollutants. In a thermal inversion, a layer of warm air sits above a layer of cool air near the surface, trapping the cool, dense air (and pollutants) close to the ground. This 'lid' prevents vertical mixing.

Where and when inversions form

Inversions are common in valleys (cool air sinks and pools) and during clear, calm nights when the ground radiates heat quickly, cooling surface air. Cities surrounded by mountains, like Los Angeles or Mexico City, are especially prone because topography blocks air from escaping horizontally too.

Consequences for air quality

Because pollutants can't rise or disperse, thermal inversions cause smog and pollutant concentrations to build up near the surface, worsening respiratory problems and reducing visibility. Inversions typically break up once the sun heats the ground enough to warm and lift the trapped surface air layer.

Key terms

Thermal inversion

A layer of warm air trapping cooler, denser air (and pollutants) near the ground, preventing vertical mixing.

Normal temperature profile

Air temperature normally decreases with altitude, allowing warm surface air to rise and disperse pollutants.

Why valleys are prone to inversions

Cool air sinks and pools in valleys, and surrounding mountains block horizontal escape of trapped air.

Conditions favoring inversion formation

Clear, calm nights when the ground rapidly radiates heat, cooling the surface air layer.

How inversions break up

Daytime solar heating warms the ground and surface air, restoring normal convection and mixing.

Try an AP-style question

What is a thermal inversion?

1. Cold air rising above warm air
2. Warm air sitting above cooler air
3. Winds getting faster with height
4. Temperature falling steadily with height