Thermal inversion, the phenomenon that traps air pollution.

The snowfalls in early December 2021 seemed to foreshadow a winter season like the ones before.

However, for several weeks now, a powerful high-pressure system has remained stationary northwest of the peninsula, causing storms to veer northward. This is known as an anticyclonic blocking.

The consequences are long-lasting atmospheric stability, frosts, one of the driest starts to the hydrological year in the last 60 years, and a series of thermal inversion phenomena that come with an unwanted “gift”: an increase in air pollution levels.

GFS model for January 25th, projected for February 1st, 2022.. Source:

What is a thermal inversion?

To understand this phenomenon, you first need to understand the normal behavior of the atmosphere.

When the atmosphere is functioning normally:

The air temperature decreases with altitude, at a rate of approximately 6.5°C per 1000 meters. That’s why as you climb a mountain, for example, the sensation of cold intensifies. The usual process is as follows:

  • During the day, the sun heats the Earth’s surface. As a result, the air in the lowermost layer of the atmosphere increases in temperature.
  • This warm air becomes lighter, facilitating its upward movement, similar to how hot air balloons ascend.
  • As it rises, the warm air cools down and becomes denser or heavier. This change causes the air mass to be attracted by Earth’s gravity, leading to convective motion.

What happens during a thermal inversion?

Although the previous section describes the normal functioning of the atmosphere, sometimes the opposite occurs.

During winter, when nights are colder and longer, and particularly under stable atmospheric conditions (clear skies, little wind, etc.), the valleys’ bottoms experience lower temperatures than the surrounding mountaintops. This phenomenon is the result of a layer of thermal or temperature inversion and is clearly visible when smoke, for example, spreads horizontally near the ground.

In the following tweet from the US National Weather Service, a thermal inversion can be clearly observed.

However, let’s take a closer look at how this temperature inversion occurs:

  • As the Sun sets, the Earth’s surface begins to cool down, causing the air in the lowermost layer to also lose temperature.
  • This layer of air becomes colder than the layer of air directly above it. Due to their different densities, they cannot mix.
  • The thermal inversion is corrected as the Sun heats the Earth’s surface, restoring normal operating conditions.

How are temperature inversions related to air quality?

Air pollution levels are often higher during episodes of thermal inversion. Why? As mentioned in the previous section, it’s because the atmospheric layers do not mix.

This situation causes pollutants to become “trapped” in the air closest to the Earth’s surface, unable to rise and disperse. In urban areas, for example, emissions from traffic or heating systems remain stagnant in these layers, exposing residents to episodes of air quality that can range from moderate to very unfavorable (according to the classification used in the National Air Quality Index).

The Great Smog of London in 1952, a textbook example of thermal inversion

One of the most well-known episodes of thermal inversion accompanied by high levels of pollution is the infamous Great Smog of London in 1952, which claimed the lives of around 12,000 people over five days. The description provided by the correspondent of ABC newspaper at that time was striking:

These thick, almost solid fogs that engulf buses, with a man walking ahead carrying a torch of resin in his hand; that silence sound; that force movie theaters to announce to the audience that “visibility of the screen doesn’t go beyond the fourth row”; that cancel performances, like what happened last December 8th when a representation of La Traviata was halted due to sudden laryngitis of the tenor and the two sopranos, and because the choirs couldn’t even see the conductor’s baton; that enters homes and lungs; that dirties furniture and blackens clothes and saliva, sticking to glass, curtains, and paintings; it is the scourge of those with heart problems, asthma, and severely compromised bronchial health, causing death. Sometimes they die without assistance because doctors cannot arrive in time through “the blanket” that reduces the horizon to two yards.

How to address these temperature inversion and high pollution phenomena?

Beyond some risky projects with unknown consequences that aim to control the climate by “seeding” clouds through the injection of silver iodide, there is no magical solution to immediately correct this situation.

However, we can implement measures such as monitoring air quality using devices like those offered by Arantec, with Smarty Air being one of their main products. Measuring pollution levels in a hyper-local manner will always allow us to take measures that help protect the health of the most vulnerable groups, including the elderly, children, people with respiratory or cardiac conditions, and pregnant women.


Thermal inversions, in themselves, do not pose any direct risks to human health, apart from the inherent cold sensation of winter.

However, they contribute to trapping anthropogenic pollutants in the lower layers of the atmosphere.

Real-time air quality monitoring provides valuable information that can help take appropriate measures. When it comes to protecting human health, any solution that can be implemented to minimize the harmful effects of pollution should be considered.

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