Thermal Inversion & Pollution (College Board AP® Environmental Science): Study Guide

What is a thermal inversion?

• Thermal inversion is a meteorological event

• Occurs when a layer of warm air traps cooler air near the ground

• Pollutants accumulate in the trapped cool air, leading to high concentrations

• This typically occurs during winter nights or early mornings

How thermal inversion occurs

• During a thermal inversion, the normal temperature gradient in the atmosphere is changed as the air temperature at the Earth’s surface is cooler than the air at higher altitudes

• Thermal inversions occur when cold and warm air masses, high-pressure systems, and topographic characteristics prevent warm air from rising

• They can happen anytime, although they are more common in winter when the ground cools faster than the air above it

• Inversions are more likely in valleys or areas with minimal wind

Smog and thermal inversion

• Thermal inversion traps pollution close to the ground, especially smog and particulates, leading to poor visibility and air quality

• During the day the ground heats up, warming the air above

  • Warm air rises, allowing pollutants to disperse

• During the night the ground cools rapidly, cooling the air near the surface

  • A layer of warm air above traps the cooler air and pollutants