The reason behind such poor air quality is the phenomenon called winter inversion. Photo: Ninara/Flickr

Pollution levels high across north India in absence of winter rains

Indo-Gangetic Plains continue to witness bad air quality days made worse by absence of winter rains, with the Air Quality Index oscillating between ‘poor’ and ‘severe’ categories

Even though the season of stubble burning is over after the sowing of the rabi crop across the northwestern states of Punjab and Haryana, the state of air quality remains abysmal. Weather conditions in the form of rains could bring some immediate relief against the emissions from industrial activities, transport, and regional pollution corridors. However, there has been an absolute absence of winter rains across the plains.

Meteorologists say that a stable wind pattern can be seen over the region and speed is also very slow. “With unabated cold north-westerly winds reaching the plains, minimum temperatures will now drop and settle in single digits. With this, dispersing pollutants from the atmosphere would be very difficult. The more the minimum temperatures dip, thicker will be the inversion layer. And the thicker the inversion layer, it would be more difficult for sun rays or winds to penetrate through this layer and disperse the pollution level,” said Mahesh Palawat, vice president, Meteorology and Climate Change, Skymet Weather. 

In the past days, minimum temperatures have dropped. Delhi’s minimum temperature dropped to 4.4°C, while Ladakh witnessed -25.1 °C and minimum temperature was -10 °C in Gulmarg, Jammu and Kashmir. A cold wave is likely to prevail over the next couple of days. 

As the temperatures dip, cold north-westerly winds become heavier because of the increased moisture content. This also increases the capacity of the winds to capture pollutants close to the earth’s surface. 

The case of winter inversion

The reason behind such poor air quality is the phenomenon called winter inversion. During winters, the air in the planetary boundary layer (the lowest part of the atmosphere) is thinner as the cooler air near the earth’s surface is dense. The cooler air is trapped under the warm air above that forms a kind of atmospheric ‘lid’. Since the vertical mixing of air happens only within this layer, the pollutants released lack enough space to disperse in the atmosphere. 

The region usually experiences at least one or two bouts of winter rain and snow at this time of year. However, since there hasn’t been a significant Western Disturbance (WD) up in the Himalayas, entire plains haven’t received any rain. While sometimes weak Western Disturbances have been present, they haven’t been strong enough to cause any notable weather changes.

According to a study by IMD, on an average during the months of November and December, 2 moderate to severe WD’s cases were observed and 3 cases during January, February, March and April, respectively. 

All year long, Western Disturbances continue to affect the weather in the Western Himalayas. However, it isn’t until November that the frequency and intensity of Western Disturbances begin to steadily increase. Additionally, they begin to move toward mountainous states at lower latitudes, causing the weather to change. By January and February, their intensity and frequency are highest.

It’s the arrival of a strong Western Disturbance that triggers rain and snowfall across the hilly states and Indo-Gangetic Plains. It is only after the passage of this active system, which pushes icy winds across the plains, the onset of the Winter season is declared. 

However, with the pattern missing this season so far, pollutants in the atmosphere are not able to clear up on account of no weather activity or change in wind pattern (increase in wind speed). 

Compared to previous years, this year saw less rain in the final days of December. Jammu and Kashmir, Uttarakhand, Himachal Pradesh, Assam & Meghalaya saw isolated to scattered rainfall towards the end of  the year. 

“It is not only about the Indo-gangetic plains but most of the country saw air quality deteriorating. PM 10 was not the only contributor but carbon monoxide levels were also high. This shows construction activities were not alone but combustion was also high. Besides this, large scale meteorological phenomena like La Nina are also contributing by slowing down the circulations. We need more ‘Early Warning Systems’ to predict air quality. These systems could let us know what is the relative contribution of slowed weather systems,” said S N Tripathi, professor, Civil Engineering dept, IIT Kanpur.  

Instead of northwestern winds from Punjab and Haryana this year, the Indo-Gangetic plains have been experiencing westerly winds from Rajasthan. This was caused by the lack of any weather system that may have changed the direction of the wind. As a result, winds bringing pollution caused by stubble burning were unable to reach the plains as much as they could have. Additionally, the region has experienced below-normal winter rainfall thus far, which has kept temperatures above average. As a result, humidity levels were not extremely low, which prevented the contaminants from being trapped in the atmosphere.

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