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Describe thermal inversion and its relationship with pollution.

Thermal Inversion

Earth’s atmospheric layers, showing their respective temperatures and altitudes. — **Each layer of the atmosphere has a different "normal" temperature.**

Under normal atmospheric conditions, temperature decreases in the troposphere (the lowest atmospheric layer) with increasing height.

Things are not always “normal” in the atmosphere, however. Temperature inversions occur when a layer of warmer air overlies cooler air near the earth’s surface. These occur when normal temperatures decrease with height but then switche to temperatures increasing with height, even if it is only for a short distance. The cold, dense air above the inversion can act like a lid, preventing convection currents from breaking through the layer of warm air.

Temperature inversions occur most frequently in summer months on afternoons when the temperatures are hot, humidity is high, and winds are minimal. It is on these days when inversions are most like to form and ozone action alerts occur. Weather scientists have gotten much better at predicting when a temperature inversion might occur, so ozone action alerts are typically sent out a day or two in advance.

A comparison between “normal” atmospheric conditions and an inversion. Under “normal” conditions, warm air rises above a cityscape, with cooler air above it. When an inversion occurs, warm air is trapped beneath cooler air. — **Temperature inversions prevent warm air from rising because the air near the surface is already cool.**

There are some atmospheric signs that you can spot that will help you predict when an inversion might occur. Work through the interactive below, taking into consideration what you've already learned about thermal inversions, and collect all the environmental signs that suggest an inversion is possible:

When the conditions above are in place, you are likely to experience a thermal inversion, which can look like this:

A captivating landscape featuring a lush green field with perfectly aligned rows of crops. There is a thin layer of fog over the field. — **You can see a thermal inversion as a layer of low-lying "fog".**

It is on these days that you are most likely to hear and/or read words like “Ozone Alert Day” or “Ozone Action Day” on news sites. These are days when certain actions are to be put into place to reduce primary air pollution in the hopes that secondary air pollution can be largely avoided.

Temperature inversions will continue to impact the formation and residual staying time of air pollution and it’s wise to keep an eye and ear on the weather in your area to be more aware of the potential pollution problems associated with them.

Essential Knowledge

During a thermal inversion, the normal temperature gradient in the atmosphere is altered as the air temperature at the Earth's surface is cooler than the air at higher altitudes.

Health Impacts

Thermal inversions can trap smog close to the ground, causing a slew of health effects. Below are some of the most common effects of thermal inversions. Have you ever experienced these effects? I have asthma and, as a high-schooler, my family visited Los Angeles. My eyes were itchy, and I coughed the whole time we were there!

🫁 Respiratory Irritation (click to reveal)

Inversion-related air pollution can lead to an increased incidence of respiratory infections. Vulnerable populations, such as children and the elderly, are particularly at risk.

Those who have respiratory ailments and cardiac conditions are the most at risk. In the zinc mining town of Donora, Pennsylvania in 1948, a prolonged temperature inversion event caused the deaths of 20 people and sickened dozens more.

A black and white historical scene depicts an industrial area surrounded by houses. Smoke rises from factories, illustrating the integration of industrial and residential spaces. — **In 1948 in Donora, Pennsylvania, a prolonged temperature inversion event caused the deaths of 20 people.**

This was, and remains, the most significant air pollution event in the history of the United States. This event caused many to sit up and take notice and led to new rules concerning air pollution.

COPD, a progressive lung disease, is another common health issue worsened by poor air quality during inversions. Breathing in pollutants can aggravate COPD symptoms.

🫀 Coronary Artery Disease (click to reveal)

The majority of deaths related to air pollution are directly attributed to CAD. The pollutants trapped during inversions can exacerbate heart conditions and increase the risk of heart attacks.

😮‍💨 Asthma Aggravation (click to reveal)

Individuals with asthma may experience more frequent and severe asthma attacks during inversions. The stagnant air allows pollutants to accumulate, triggering respiratory distress.

😷 Autoimmune Diseases (click to reveal)

People with autoimmune conditions face an elevated risk of flare-ups during inversion episodes.

Essential Knowledge

Thermal inversion traps pollution close to the ground, especially smog and particulates.

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