AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/7/7-3)
Unit 7 · Topic 7.3
7.3 Thermal Inversion
Normally air gets cooler with height, so warm, polluted air near the ground rises and disperses. In a thermal inversion, a layer of warm air sits on top of cooler surface air like a lid, trapping pollution near the ground, sometimes with deadly results.
Key terms
- thermal inversion
- inversion layer
- troposphere
- air temperature gradient
Normal conditions
In the troposphere, the lowest layer of the atmosphere, air temperature usually drops as you go up, by about 6.5 °C per kilometer on average. The ground absorbs sunlight and warms the air right above it. Warm air is less dense than cool air, so it rises, carrying pollutants up and away where they spread out and become diluted. This vertical mixing keeps air near the ground fairly clean even in busy places.
What an inversion is
In a thermal inversion, the normal pattern flips over part of the atmosphere: a layer of warmer air sits above cooler air near the surface. The cool surface air is denser than the warm air above it, so it can't rise through it. The warm layer acts like a lid, called the inversion layer, and pollutants from cars, chimneys and factories build up underneath it.
Common causes include:
- Clear, calm nights, when the ground loses heat quickly and chills the air just above it, while air higher up stays warmer.
- Valleys and basins, where cold, dense air drains down the slopes and pools at the bottom.
- High-pressure weather systems, where sinking air warms as it is compressed and forms a warm layer aloft.
- Winter, with long nights and weak sunlight, which makes inversions more frequent and longer lasting.
How inversions end, and what they cause
Most inversions break up during the day as sunlight warms the ground and the surface air rises again, or when a storm or front brings wind that mixes the air. Some last for days when a stagnant high-pressure system stays put.
While they last, inversions raise concentrations of particulates, smog, SO₂ and other pollutants, causing breathing problems, worsening asthma and heart and lung disease, and reducing visibility.
The Donora smog of 1948
Donora, Pennsylvania, is a small town in a river valley that was home to a steel mill and a zinc smelter. In late October 1948, an inversion settled over the valley for about five days. Emissions from the plants, including sulfur dioxide, fluoride compounds and particulates, built up under the lid. About 20 people died during the event, and roughly half the town's 14,000 residents got sick. Donora helped convince the public and lawmakers that air pollution could kill, and it contributed to the first US federal air pollution laws, which led eventually to the Clean Air Act. London's 'Great Smog' of December 1952, also caused by an inversion over a city burning coal, killed thousands.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Spotting an inversion in data
A weather balloon records these morning temperatures over a valley town: 0 m, 4 °C; 200 m, 6 °C; 400 m, 8 °C; 600 m, 7 °C; 800 m, 5 °C; 1,000 m, 3.5 °C. (a) Is there an inversion? If so, between what heights? (b) What would you predict about air quality in town that morning?
Show the solutionHide the solution
- Step 1: In normal conditions temperature falls with height. Look for any layer where it rises instead.
- Step 2: From 0 m to 400 m, temperature rises from 4 °C to 8 °C. That is an inversion.
- Step 3: Above 400 m, temperature falls again (8 → 7 → 5 → 3.5 °C), so the normal pattern resumes. The warm layer around 400 m is the lid.
- Step 4: (b) Cool air at the surface can't rise through the warmer air above, so pollutants from traffic and chimneys stay trapped in the lowest 400 m. Air quality would be poor until the sun warms the ground or wind mixes the air.
Answer: (a) Yes, from the ground up to about 400 m, where temperature increases with height. (b) Pollutants would be trapped near the ground, so air quality would be poor until the inversion breaks.
Common mistakes
- Describing an inversion as 'cold air on top of warm air'. It is the reverse: warm air on top of cooler surface air.
- Saying inversions are caused by pollution. Inversions are weather events; they trap pollution that is already being emitted.
- Thinking inversions only happen in summer. They are common on clear winter nights and mornings, especially in valleys.
On the exam
- Expect a described temperature-versus-altitude graph or table. Identify where temperature increases with height and explain why that traps pollutants.
- If asked about health effects, link the trapped pollutants (particulates, SO₂, ozone) to respiratory problems such as asthma.
Connected topics
Videos
Check yourself
5 questions on 7.3 Thermal Inversion. Pick an answer to see if you got it, and why.
| Altitude above valley floor (m) | Air temperature (°C) |
|---|---|
| 0 | 2 |
| 200 | 5 |
| 400 | 8 |
| 600 | 6 |
| 800 | 4 |
Hypothetical measurements on a still winter morning over a valley city
Between which altitudes is the air temperature profile inverted?
Which outcome is most likely in the valley city while these conditions last?
A city of more than a million people sits in a basin surrounded by mountains. In winter, snow often covers the valley floor, and calm, clear high-pressure weather can last for a week or more.
During these spells, air near the valley floor stays near −5 °C while air a few hundred meters up stays several degrees warmer. Haze from vehicles, home wood stoves and industry builds up day after day, and hospital visits for asthma rise.
Described scenario
Which conditions described in the passage best explain why the inversion forms?
Which action would most directly reduce health harm during an inversion episode?
Which weather change would most likely end the inversion and clear the air?
0 of 5 answered