Explained: What is thermal inversion and how it affects air pollution

It acts like a 'cap' on the upward movement of air from the layers below. That 'cap', which traps cold air under the hot air, acts like a lid and keeps smoke and other pollutants from rising into the atmosphere and dispersing

FP Staff November 08, 2021 20:21:12 IST
Explained: What is thermal inversion and how it affects air pollution

Representational image. AFP

Have you noticed that during winter, pollution increases? While the blame can be laid at the feet of a slew of different reasons including stubble burning (at least for residents of Delhi NCR) or an increase in pollution output due to firecrackers burst during Diwali.

However, apart from these external phenomena, there's also some science at play.

Climatic features like the drop in temperature and slowing down of wind speed also determines how long will the pollutants hover in our immediate atmosphere.

Another key aspect of the natural climatic phenomenon that contributes to increased pollution in winters is thermal inversion.

But what is a thermal inversion? And what is its effect on pollution? Let's take a deeper look at this phenomenon:

What it is?

To begin, first let's understand that the higher you go in the atmosphere, the lower the temperature of the air is on the outside. But during a thermal inversion, the exact opposite occurs: cool air remains closer to the surface trapped under a layer of warm air.

How does inversion happen?

Inversion occurs under specific conditions and both weather incidents and the topography of the place has a role to play.

Inversion can happen, for example, when the air near the ground rapidly loses its heat on a clear winter night. The ground becomes cooled quickly while the air above it retains the heat the ground had radiated during the day.

Temperature inversions also occur in some coastal areas because upwelling of cold water can decrease surface air temperature and the cold air mass stays under warmer ones.

Inversions can also form in areas with significant snow cover because the snow at ground level is cold and the sheet of air in contact with it, cools down faster. However, this cooling down of temperature does not permeate to higher layers as the white color of the snow reflects almost all heat coming in from the sun. Thus, the air above the snow is often warmer because it holds the reflected energy.

What does it do?

It acts as a 'cap' on the upward movement of air from the layers below.

That 'cap', which traps cold air under the hot air, acts as a lid and keeps smoke and other pollutants from rising into the atmosphere and dispersing. This, obviously, negatively impacts air quality.

Inversion traps particulate matter and other pollutants, which mix with condensed water vapour to form smog.

The most famous example of this was the Great Smog of London in 1952 when a high-pressure weather system caused an inversion.

The result was that the emissions of factories and domestic fires could not be released into the atmosphere and remained trapped near ground level. This was the worst pollution-based fog in the city’s history.

Thermal inversion also lowers the vertical wind movement because of lower radiative temperature on the ground. If the wind speed goes below 2m/second, the dispersal of pollutants from the lower strata of the atmosphere becomes slow. "As pollutants pile up in the breathable height, people face serious health impacts," a West Bengal Pollution Control Board scientist told The Times of India.

India Today quoted Additional Director General of India Meteorological Department (IMD) Anand Sharma as saying: "Right now inversion is not taking place, but once extreme winter will set in, inversion of temperature will make the pollution worse. Then, pollutants will refuse to disperse and the mixing height also comes down."

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