Thermal Inversion & Pollution (College Board AP® Environmental Science): Revision Note

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

Effects of thermal inversions

• Pollutants such as smog, particulate matter and ground-level ozone build up to high concentrations near the ground

• This increases respiratory problems such as asthma attacks and bronchitis, leading to more hospital admissions

• Visibility is reduced

How inversions end

• Inversions usually break down when the sun warms the ground during the day, or when wind mixes the air layers

Case study: Donora, Pennsylvania (1948)

• A thermal inversion trapped emissions from the town’s zinc and steel works in a river valley for five days

• About 20 people died and around 6,000 of the town’s 14,000 residents became ill

• The disaster raised awareness of air pollution and helped lead to the first US federal air pollution laws

Reducing the impacts

• Air quality alerts warn vulnerable people to stay indoors

• Temporary bans on burning (e.g. wood stoves) and limits on traffic during inversion episodes

• Cutting emissions at source reduces how much pollution can be trapped