A thermal inversion is an atmospheric condition where a layer of warm air settles above a layer of cooler air. This reversal creates a stable atmospheric condition. Understanding this phenomenon is important for comprehending episodes of severe air pollution, as it significantly impacts air quality.
How Air Usually Behaves
Under typical atmospheric conditions, the air near the Earth’s surface is warmer than the air above it. This occurs because the sun heats the ground, which then warms the adjacent air. As this warm air rises, it expands and cools.
This process of warm air rising and cooler air sinking creates vertical air currents, known as convection. These movements promote atmospheric instability and facilitate the vertical mixing of air, allowing pollutants released at the surface to disperse upwards and spread throughout a larger volume of the atmosphere.
What is a Thermal Inversion?
A thermal inversion, also called a temperature inversion, is a meteorological phenomenon where the normal atmospheric temperature gradient is reversed. In this condition, air temperature increases with altitude within a specific layer, rather than decreasing. This forms an abnormal temperature profile where a warm air layer acts as a “lid” over the cooler air below.
How Thermal Inversions Develop
Thermal inversions can form through several mechanisms. One common type is a radiation inversion, which often occurs on clear, calm nights. The ground rapidly cools by radiating heat into space, cooling the air directly above it more quickly than the air higher up.
Another way inversions develop is through subsidence. This happens when a large mass of air slowly sinks over a wide area, typically associated with high-pressure systems. As the air descends, it is compressed and warms, creating a warm layer aloft that can trap cooler air below.
Frontal inversions also occur when different air masses interact. When warm air overrides a cooler air mass, such as in a warm front, the less dense warm air slides up and over the denser cold air, forming an inversion layer.
Why Pollutants Get Trapped
The warm air layer in a thermal inversion creates a highly stable atmospheric condition. This stable layer acts like a cap or lid, preventing the cooler, denser air beneath it from rising and mixing vertically. Because of this stability, normal convective air currents, which would typically carry pollutants upwards and away from the surface, are suppressed.
Pollutants emitted at ground level, such as those from vehicles or industrial activities, become trapped within this stagnant, cooler air layer near the Earth’s surface. They cannot escape into the upper atmosphere and accumulate, leading to increased concentrations. This lack of vertical dispersion significantly reduces air quality, as harmful substances remain concentrated where people breathe.