First dense smog of the season in Delhi-NCR

The fog fused with particulate pollutants suspended in the city’s air on Monday when the air quality index (AQI) worsened into the “severe” zone,

A thick smog wrapped Delhi and the National Capital Region (NCR) late on Sunday and early on Monday, bucking contrary predictions by the Met department and severely impairing visibility for commuters across large swathes of the city.

Vehicles move slowly amid low visibility due to fog on a cold winter morning, at NH24 near Akshardham Temple in New Delhi, on Monday. (Raj K Raj/HT Photo)

The fog fused with particulate pollutants suspended in the city’s air on Monday — when the air quality index (AQI) worsened into the “severe” zone, bathing much of the region in a dense, dark smog that made trips difficult for commuters.

The Capital recorded an AQI of 410 on Monday, showed the Central Pollution Control Board’s (CPCB) 4pm bulletin, worse than 353 (“very poor”) on Sunday and the first time pollution levels were in the “severe” category since December 4.

An AQI between 301 and 400 is classified as “very poor” and one above 400 as “severe”. CPCB’s pollution monitors max out at 500.

The city on Monday also recorded a minimum temperature of 7.2 degrees – a degree below normal for this time of the year, but higher than 6.2°C on Sunday. The maximum was 23.2°C — one degree above normal and 0.5 degrees lower than Sunday.

Experts at the India Meteorological Department (IMD) said the visibility dropped precipitously overnight as fog enveloped Punjab and pushed its way to Delhi-NCR, catching commuters as well as weather experts off guard.

IMD on Sunday predicted that Delhi was unlikely to record an episode of dense fog this year, citing the relative lack of moisture in the city’s air.

A fog is like a low-lying cloud and is formed of suspended water droplets, which have condensed in the presence of low temperatures and calm winds. Smog is just fog that has combined with smoke and other pollutants to turn a thick grey. It forms when moisture in the air traps pollutants closer to the ground, as the low temperature and calm winds make it difficult for the pollutants to disperse.

So far, Delhi only saw isolated spells of shallow fog, owing to the relatively dry air and strong winds.

Experts said that while the overall phenomenon across the Indo-Gangetic plains where the visibility dipped below 200 metres can be classified as a “dense fog formation”, in Delhi-NCR, pollution levels were already fairly high on Sunday evening. Generally, fog, when accompanied by an AQI above 300, is categorised as smog.

“Fog is the natural phenomenon, which upon interacting with emissions and pollutants in the air, can create smog. If pollution is already very high, then it is generally termed as smog,” said Dipankar Saha, former head of CPCB’s air laboratory.

Fog typically appears as a white layer in satellite images, compared to smog, which is grey.

An IMD official said that the dense layer of fog started spreading from Punjab and blanketed even eastern Uttar Pradesh, draping Haryana and Delhi in the process.

To be sure there were pockets within these regions as well where the visibility was over 1,000 metres. Experts said this was because the extent of fog could vary wildly between one area and the next.

“Fog is the only meteorological event with high spatial variability, meaning the fog cover may be extremely thick in one part of the city and very shallow in another. It is thicker closer to water bodies and canals,” said RK Jenamani, scientist at IMD.

Jenamani added that factors such as wind speed, humidity levels, low temperature and pollutants, all play a role in the fog forming near the surface.

“For fog to become thick, one needs adequate moisture, calm winds and low temperatures. Delhi was already seen calm winds at night and low temperatures, with additional moisture also possibly getting added through the northern plains, coming all the way from Punjab. The pollution in the air can also make it dense, which is generally referred to as smog,” Jenamani said.

Mahesh Palawat, vice-president at Skymet meteorology, said that while forecasts showed that Delhi was unlikely to experience a dense fog till next year, meteorological conditions changed within a matter of hours.

“This fog began forming in Punjab and was gradually pushed towards UP, as the wind direction during this season is northwesterly. Local winds in Delhi also became calm late at night and with sufficient moisture and low temperatures, the moisture began to condense and form fog,” he said.

The Indira Gandhi International (IGI) airport, where the IMDs Palam observatory is located, saw visibility oscillate between 150 to 200 metres between 3:30 am and 6am. The visibility gradually improved to 700 metres after 7am, officials said.

The airport initiated low visibility procedures from midnight and kept it in place till 4am, which led to some flights getting delayed.

“This was initiated as visibility dipped, but no flights were diverted or cancelled due to the fog. Only slight delays occurred,” said an airport official.

IMD has also issued a yellow alert until December 23 now, forecasting moderate to dense fog in the city till then.

A yellow alert is generally issued to ask the public to “be aware” of a weather situation developing in the next 24 to 48 hours. An orange alert asks residents to “be prepared and alert” for a weather event and a red alert – the most severe form — asks the public to be prepared to ‘take action’ against a weather event.