'Severe' air quality and dense fog: Double whammy for Delhi in winters

Delhi's air quality hit "severe" levels again, with fog reducing visibility and causing flight and train delays. Conditions are expected to remain poor.

A double whammy hit the national capital on Tuesday as air quality deteriorated into the “severe” category for the fourth time this month and then combined with dense fog to reduce visibility, impacting various travel operations.

People seen out on a cold and foggy morning at Indraprastha Extension Road in New Delhi. (RAJ K RAJ / Hindustan Times)

Winds calming overnight caused the air quality to plummet, experts said. Delhi’s 24-hour average air quality Index (AQI) stood at 412 at 4 pm on Tuesday — up from a reading of 373 at the same time on Monday.

However, an increase in surface wind speed — up to 20 km/hr — during the day led to a marginal improvement, and by 8 pm, the AQI was back in the ‘very poor’ (398) category.

This is the fourth “severe” air day this month and the seventh overall this year. Last year, in December, Delhi recorded six “severe” days; three in December 2023; one in December 2022; and seven in 2021. The highest number of “severe” days recorded in December is eight – in both 2018 and 2019.

Forecasts show AQI is likely to fall but still stay in the “very poor” category in the coming days. “Delhi’s air quality is likely to be in the ‘very poor’ category from December 24 till December 26. The outlook for the subsequent six days from December 27 shows the AQI is likely to be in ‘very poor’ again,” said Centre’s Air Quality Early Warning System (EWS) for Delhi. To be sure, the EWS failed to accurately forecast Tuesday’s air quality. It had only forecast ‘very poor’ air during the day.

Meanwhile, as a dense fog shrouded the region, visibility dropped to as low as 50 metres at both Palam and Safdarjung.

India Meteorological Department (IMD) officials said moderate fog prevailed in Delhi till around 4:00 am, with visibility at 350m at Safdarjung and 450m at Palam. It then dipped sharply over the next hour with visibility at 100m by 5:30 am.

FlightRadar 24, showed over 500 flights were delayed at Delhi’s Indira Gandhi International (IGI) airport, with the average delay time around 34 minutes. A total of 10 flights were cancelled too —six arrivals and four departures.

Airport officials said this was also down to impact at other airports in north India, where visibility was worse. IMD data showed multiple airports in Northern India, including Amritsar, Agra, Jaisalmer, Gwalior, Kanpur, Hindon and Bareilly, saw zero visibility.

Northern Railways data meanwhile showed that over 100 trains were late under the Delhi division, with delay time ranging from 30 minutes to over 5 hours.

“As winds remained calm, visibility fell further and was 50 metres at both Palam and Safdarjung between 7:30 am and 8 am, before improving marginally to 100 and 150 m respectively at 8:30 am,” said an IMD official, adding moderate fog was then witnessed from 9 am onwards as the sun began to come out.

To be sure, the IMD had no colour coded alert for Delhi in place. It had forecast a shallow to moderate fog for the Capital. It has similarly forecast shallow to moderate fog on Wednesday morning.

Fog is classified as shallow when visibility is between 500-1,000m; moderate when it is between 200-500m; and dense below 200m. If it dips below 50m, then it is defined as ‘very dense’ fog.

The IMD forecast said dense fog will persist in isolated pockets across the northern plains for the next few days, but strengthening northwesterly winds from Wednesday will reduce its overall intensity and spread.

However, dense to very dense fog is again expected by the end the year, with a feeble western disturbance expected to influence North India between December 28-31.

“We should see some marginal relief as wind speeds pick up from Wednesday as fog formation is less when wind speeds are high. However, a mild western disturbance is again approaching around December 28. This will slow down wind speed and also add moisture, thus creating dense to very dense fog,” said Mahesh Palawat, vice President at Skymet.

After the sun came out on Tuesday, temperatures rose during the day. The maximum stood at 23.2°C – three degrees above normal and 1.4°C higher than Monday’s maximum. The minimum was 8.8°C, a degree above normal but 2.2°C lower than Monday. The maximum is expected to hover between 18-20°C till Friday now amid icy-cold northwesterly winds. The minimum should hover between 6-8°C during this period, the IMD has said.