What is thermal inversion?
Normally, air temperature decreases with altitude. However, during a thermal inversion, the opposite occurs: a layer of cold air becomes trapped near the ground by a layer of warmer air above. This phenomenon acts like a lid, preventing vertical air circulation.
This atmospheric stagnation is especially common during winter mornings, when clear nights and radiative cooling of the ground create this setup. Cities like Santiago de Chile, Bogotá, Lima, and São Paulo frequently experience this phenomenon during the colder months.
How does a thermal inversion form?
There are several types of thermal inversion, but the most relevant for urban air quality is radiation inversion. It occurs when:
- The ground cools rapidly at night, especially under clear skies.
- The air in contact with the ground cools, becoming denser.
- A layer of warmer air settles above, preventing the cold air from rising.
Other common inversions include subsidence inversions (associated with high-pressure systems) and warm front inversions. All share the same effect: the air becomes stagnant.
The direct link between thermal inversion and smog
When air cannot rise, pollutants emitted near the ground—such as those from vehicles, industrial chimneys, or wood-burning heaters—become trapped. This leads to a rapid increase in the concentration of fine particulate matter (PM2.5 and PM10), nitrogen oxides (NOx), and ground-level ozone.
The result is a dense, toxic haze known as smog. Unlike summer photochemical smog, winter smog from thermal inversion is often more persistent and dangerous, as pollutants accumulate for hours or even days.
Specific data in South America
In Santiago de Chile, thermal inversions are responsible for up to 70% of critical pollution episodes in winter. The city, surrounded by the Andes mountain range, suffers a "bowl" effect that exacerbates stagnation. During 2023, more than 30 environmental pre-emergencies were directly linked to this phenomenon.
In Bogotá, thermal inversions combined with altitude (2,600 m above sea level) create unique conditions: the cold nighttime air is denser, and pollutants concentrate in the city's valleys and low-lying areas.
Consequences for health and the environment
Prolonged exposure to smog from thermal inversion has serious effects:
- Irritation of eyes, nose, and throat.
- Worsening of respiratory diseases such as asthma or COPD.
- Increased risk of heart attacks and strokes.
- Reduced visibility and damage to sensitive crops.
The most vulnerable groups are children, older adults, and people with chronic diseases. During thermal inversion episodes, hospital emergencies can increase by up to 20%.
How to identify and prevent the effects of thermal inversion
Signs of an active thermal inversion
If you notice a layer of fog or dense haze in the morning that does not dissipate until late afternoon, you are likely under a thermal inversion. It is also common for the temperature near the ground to be colder than in nearby elevated areas.
Practical tips to reduce exposure
- Avoid outdoor activities during the early morning hours, when the inversion is strongest.
- Do not use wood or coal for heating; opt for gas or electricity.
- Ventilate your home only after midday, when the inversion layer has dissipated.
- Wear an N95 mask if you must go out during high pollution hours.
- Keep windows closed during the night and morning.
The role of technology and early warnings
Given the recurrence of this phenomenon, weather and alert apps like Contingencias have become essential tools. They allow users to receive real-time notifications about thermal inversion episodes, pollution levels, and personalized recommendations based on their location.
Additionally, current weather models can predict the formation of a thermal inversion up to 48 hours in advance, giving authorities time to activate vehicle restrictions or suspend school activities.
Conclusion: understanding the air we breathe
Thermal inversion is a natural phenomenon, but its consequences are amplified by human activity. Knowing how it forms and how it affects our health is the first step to protecting ourselves. Combining individual measures, public policies, and technological tools can make a difference in the quality of the air we all share.