Missing western disturbances, evading rains keep pollution levels high across north India

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Vibha Sharma

New Delhi, December 23

The absence of winter rains is keeping pollution levels high across Indo-Gangetic Plains, with the Air Quality Index oscillating between ‘poor’ and ‘severe’ categories even after the stubble burning season is over in Punjab and Haryana, according to experts.

Even the brightest days in Delhi-NCR during the first half of December kept air quality in the ‘poor’ category. Apart from an absolute absence of winter rains across the plains, a stable/slow wind pattern has added to the causes.

According to the experts, usually by this time of the year, the region witnesses at least one or two spells of winter rains and snowfall. However, Western Disturbances governing the weather pattern over the north-western plains, especially during the winters, have either gone missing or are too feeble to affect major changes.

Normally, the arrival of a strong WD triggers rain and snowfall across the hilly states and Indo-Gangetic Plains (IGP).

However, due to the absence of any strong WD in the Himalayas, rains have been evading entire plains, the result being that pollutants in the atmosphere are not able to clear up due to no weather activity or change in wind pattern.

Further in the IGP, the hub of winter air pollution, things seem to be getting worse with meteorologists predicting lesser rains in the remaining days of December.

According to the IMD, November saw the passage of five WDs across North India of which two (November 2-5 and 6-9) caused isolated to scattered rain or snowfall over Western Himalayan region and rainfall over adjoining areas.

The remaining three were feeble (November 13-15, 18-21 and 22-24) and did not affect the region, it says.

As stubble burning subsided after the sowing of the rabi crop, the burden of high air pollution levels rest on emissions from sources like industrial activities, transport, regional pollution corridors, and prevailing weather conditions.

“Weather conditions in the form of rains bring some immediate relief, but with increasing climate change these systems have also become inconsistent.

“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,” says Mahesh Palawat from Skymet Weather.

A significant correlation was also found between the elevated levels of air pollutants (PM10, PM2.5, sulphur dioxide, nitrogen dioxide, and carbon monoxide) and low ozone levels in winter months and meteorological parameters such as air temperature (low values), air humidity (high values), and wind speed (high values) at the same time.

“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, “says Prof S N Tripathi from IIT Kanpur.

High levels of air pollution are reflecting in major cities across the Indo-Gangetic Plain.

The delayed monsoon also pushed the harvesting of the Kharif crop and stubble burning episodes to November for 2021, like in 2022. However, meteorology thereafter played out differently in the two years. In 2021, the passage of active western disturbances on account of prevailing La Nina conditions brought some good spells of rain and snow across Northwest at regular intervals, the experts say.