With smog covering Delhi, air quality turns ‘severe’ as city gripped by cold wave

Delhi's air quality deteriorated to 'severe' category on Saturday, December 22, as a cover of smog surrounded the national capital and prevented dispersion of pollutants with the minimum temperature dropping to 4 degrees Celsius, four notches below average. At 8.30am, visibility was recorded at 600 metres and humidity was recorded at 97%.

"The sky will remain clear throughout the day, with mist or shallow fog in the morning and haze or smog thereafter," an India Meteorological Department (IMD) official said.

This is a fourth time this season that the air quality has worsened to severe category. The first time it dipped to the severe category this year was just two days before Diwali.

A day after Diwali, which fell on December 7, the air quality again slipped to severe category on November 8. The third time it slipped into severe category was on December 12.

The overall air quality index (AQI) of the city stood at 408, which falls in the 'severe' category, the Central Pollution Control Board (CPCB) data showed.

An AQI between 100 to 200 comes under the 'moderate' category, 201 and 300 is considered 'poor', 301 and 400 'very poor', while that between 401 and 500 is 'severe'.

Twenty-five areas of the national capital recorded severe air quality while 11 recorded very poor air quality, the CPCB said.

In Delhi, the overall PM2.5 level - fine particulate matter in the air with a diameter of less than 2.5 micrometre - was recorded at 290 and the PM10 level at 458, the CPCB said. In the National Capital Region, Ghaziabad recorded the worst air quality at an AQI of 451. Noida recorded severe air quality while Faridabad's pollution level remained in the very poor category, the CPCB said.

The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the air quality might improve over the next two days but would still remain in the very poor category.

Unfavourable weather conditions and prevalence of a thick smog cover are further worsening the conditions and preventing dispersion of pollutants, authorities said.

According to the Indian Institute of Tropical Meteorology, the maximum ventilation index on Wednesday, December 19, was 2,000 sqm/second which is extremely unfavourable for dispersion of pollutants.

A ventilation index lower than 6,000 sqm/second with an average wind speed of less than 10 kmph is unfavourable for dispersion of pollutants.

The cold wave that started midweek with the temperature dipping to 4 degree Celsius on Thursday, December 20. It was the coldest temperature recorded this year in December.

Temperatures across northwest India has been similarly low, said an India Meteorological Department official. Punjab, Haryana and Rajasthan were also experiencing biting cold conditions.

Friday's maximum temperature was 20.6 degrees Celsius, a notch below season's average while the minimum the minimum temperature was 4.7 degrees Celsius, three notches below average.