Air in North India turns 'poor' again as rains play truant, winds go slow

Plunge in AQI attributed to 'winter inversion', where cooler air is trapped under a warmer layer, creating an atmospheric blanket that traps pollutants closer to the Earth's surface

Air pollution remains high across Indo-Gangetic Plains despite stubble burning from the states of Punjab and Haryana having subsided.

According to meteorologists, the absence of winter rains is keeping pollution levels high across northern states, with the Air Quality Index (AQI) oscillating between ‘poor’ and ‘severe’.

Apart from the absolute absence of winter rains across the plains, a stable/slow wind pattern has added to the causes, experts said.

The reason for the high rates of pollution detected over North India has been identified as a phenomenon called 'winter inversion', where cooler air is trapped under a layer of warm air creating an atmospheric blanket of sorts. This has trapped pollutants closer to the Earth's surface, prohibiting them from dispersing into the upper reaches of the atmosphere.

“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 the inversion layer be. And the thicker the inversion layer, the more difficult it would be for the sun;s rays or winds to penetrate through this layer and disperse the pollution,” said Mahesh Palawat, Vice President-Meteorology and Climate Change, Skymet Weather.

Since stubble burning has now reduced, the elevated levels of air pollution can be attributed to industrial activity, vehicular pollution and prevailing weather conditions.

Experts claim that in order to aid the dispersion of pollution, cities would need to drastically cut down emissions at source.

Experts claim that the lack of rainfall has been linked to the absence of Western Disturbances (WD) in the Himalayan region, which resulted in scattered snowfall in the Western Himalayas and rainfall in adjoining areas. Since rainfall already occurred earlier in the winter season, December has only been experiencing feeble WDs which has had no effect on the weather pattern thus far.

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.

Alongside WDs, factors such as the La Nina—an atmospheric phenomenon occurring over the pacific ocean—have contributed to the current weather pattern, say experts.

"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

According to the Early Warning System, “The air quality is likely to deteriorate marginally but remain in the Very Poor category on Friday. The air quality is likely to improve to the lower end of the ‘very poor’ category on Saturday and Sunday. The outlook for subsequent six days is that the air quality is likely to remain in the Very poor to Poor category.”

The India Meteorological Department (IMD) issued a yellow alert for Delhi till December 26, forecasting dense fog and a dip in both maximum and minimum temperature will continue.