Due to thermal inversion, pollutants get trapped closer to the surface of the earth. AFP (Representational Image)
Winter in Delhi is known for bad air quality, often leading to a surge in health issues. After recording the cleanest day post-Diwali in years, the National Capital’s air quality again deteriorated to “very poor” on Thursday (27 October) with an air quality index (AQI) of 354, which is the worst since January this year.
As the wind direction has changed to northwesterly, Delhi is expected to log “very poor” AQI in the coming days, say weather experts.
“The winds had been mostly calm, so pollutants have accumulated in the atmosphere. The current pollution is mostly from local emissions, but fire counts in neighbouring Punjab and Haryana are increasing significantly. Over the last two days, there have been more than 1,200 fire counts,” said VK Soni, senior weather scientist at India Meteorological Department (IMD), told Times of India (TOI).
Soni also said that even though its share is less, farm fires are expected to account for around 10 per cent of Delhi’s pollution.
While stubble burning, vehicular emissions and dust contribute to the toxic air in Delhi, factors such as change in weather conditions, temperature inversion, and wind direction also play a major role in the surge in pollution levels during winter.
What is the temperature or thermal inversion that affects Delhi in winter? How does it cause pollution in the National Capital? Let’s take a look.
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Thermal inversion
Under normal conditions, the higher the altitude the lesser the temperature. But this phenomenon can change due to changes in weather and even topography.
Temperature or thermal inversion is when the temperature increases along with the higher altitude.
The cool air gets “capped” close to the surface under a layer of a warm layer.
“The cap is a layer of relatively warm air aloft (above the inversion). Air parcels rising into this layer become cooler than the surrounding environment, which inhibits their ability to ascend,” says US’ National Weather Service.
The dip in wind speed during winter also leads to the concentration of pollutants in the air. AFP (Representational Image)
The phenomenon mostly occurs in winter when “mist and fog become trapped in the cooler air low down”, but inversions can happen throughout the year, says Met Office, United Kingdom’s national weather service.
How thermal inversion affects pollution?
As warm air rises, the air under the inversion cannot escape as it is cooler, and thus, it traps the pollutants closer to the ground, instead of dispersing them higher in the atmosphere.
It usually happens when the sky is clear and the wind is light and calm.
With a dip in temperature, the inversion height- the layer beyond which pollutants cannot diffuse into the upper layer of the atmosphere – is lowered, as per Indian Express.
The layer with “higher temperature acts as a cover on the colder air that is in contact with the ground and in which pollutants concentrate. This situation gives rise to the smog or “pollution beret”, visible from several kilometers around and that usually takes prepared with a descent in the levels of air quality,” as per Envira IOT website.
This temperature inversion affects the pollution levels in Delhi every year during winter.
Additional Director General of India Meteorological Department (IMD) Anand Sharma, told India Today that as temperature declines in Delhi, thermal inversion will make air quality worse as then “the pollutants will refuse to disperse and the mixing height also comes down”.
Besides temperature inversion in the National Capital, the dip in wind speed during winter also leads to the concentration of pollutants in the air.
“There’s nothing new about the winter pollution situation of Delhi, NCR and entire northern India at this time because North India is land-locked and every year this mix of weather conditions and pollution results in smog,” Anumita Roychowdhury, executive director of research and advocacy at Centre for Science and Environment (CSE), told Outlook last year.
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Delhi’s pollution woes
The calm wind conditions are likely to continue in the National Capital and combined with the pollutants from the burning paddy fields in Punjab and Haryana, the air quality is expected to remain “very poor” till the end of October.
With the increase in farm fires and other factors, the air quality is expected to further dip in Delhi. AFP File Photo
“The air quality is likely to remain in the very poor category from 28 to 30 October. The outlook for the subsequent six days is that air quality is likely to remain largely in the very poor to poor categories,” IITM’s early warning system forecast said, as per TOI.
Anumita also told Hindustan Times that weather conditions can worsen in Delhi in the coming days. “Till now, Delhi was seeing good wind speed, which was controlling the pollution levels. As soon as winds become calm, local emissions are enough to cause the AQI to reach ‘very poor’,” the CSE executive director said.
With inputs from agencies
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