Explained: Why is it unusually foggy over north India this winter?

For several days during the last two months, zero visibility and dense fog has engulfed parts of Delhi and all of Punjab and Haryana. What is behind this intense fog which has stayed on for several days this season?

Delhi, Delhi fog, Delhi weather, Punjab fog, Haryana weather, North India fog, Indian Express

A residential area is shrouded in smog in New Delhi. (Reuters Photo: Danish Siddiqui)

4 min readPuneFeb 20, 2021 12:23 PM IST First published on: Feb 20, 2021 at 12:23 PM IST

For several days during the last two months, zero visibility and dense fog has engulfed parts of Delhi and all of Punjab and Haryana keeping up with cold weather conditions. Last week alone, multiple road accidents due to poor visibility killed close to a hundred people. What is behind this intense fog which has stayed on for several days this season?

What is fog?

Fog is a phenomenon of small droplets remaining suspended in the air. Fog develops normally during late evening, night or early morning hours of the day, severely affecting visibility. Poor visibility, falling to less than a kilometre disrupts the smooth flow of vehicular and air traffic. Road accidents, delays in flight take-offs and landings are linked to poor visibility caused by fog. Foggy conditions prevail over the plains of north India during the winter season and can prolong for days and sometimes even for weeks.

What factors led to dense fog over north India this winter?

Fog developed over Delhi-Haryana-Punjab belt during February 2-6, due to the passing of an active western disturbance, which caused light rain and brought along fresh moisture over these regions. Though western disturbances continued to pass through the extreme northern hilly terrains affecting weather over Jammu and Kashmir and Ladakh during February 8-19, the plains largely remained unaffected.

“In the absence of an active western disturbance, an anticyclone formed and stayed put over the extreme north of the plains. This coincided with the dominant easterly waves pumping in moisture into the region, favouring the fog formation,” said RK Jenamani, senior scientist at National Centre for Weather Forecasting, New Delhi.

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Clear sky conditions accompanied by calm winds during the day allowed the fog to persist for longer than normal duration.

The Punjab-Haryana-Delhi belt is infamous for possessing a high concentration of sources causing air pollution, but this season, the pollutants had little role to play with respect to fog, meteorologists said.

With the persistent prevalence of an easterly trough across Central India after February 8, the easterly winds continued to remain active for 9 to 10 consecutive days. This resulted in continuous moisture being fed and the water droplets contributed towards the fog development and its persistence all these days.

Satellite images of fog during February 11-16, 2021 over Punjab, Haryana and Delhi. (Source: IMD)

Why is the fog unusual this year?

Even though fog over the plains of north India is common during December to February months, fog along the Indo-Gangetic plains this season has been unique in more than one way.

It is mainly due to these three reasons:

1. The prolonged persistence of foggy conditions recorded continuously between 7pm to 10.30am, especially at Amritsar airport, Punjab.

2. The growing geographical expanse of zero visibility with very dense fog conditions engulfing Punjab-Haryana-Delhi belt.

3. The timing of very dense fog for 9 to 10 days, as such conditions occur limited to 2 or 3 days within the first week of February.

For how many days in February has fog been reported so far?

The airport data on fog shows that very dense fog was reported for 12 nights and days over Amritsar airport which accounted for a total of 156 hours in February (till February 19) alone. This was record sort, as, on average, Amritsar airport experiences fog for four nights and days equivalent to 15 hours, that too during the entire month. A similar trend was noticed over Amritsar in January with 16 nights/days equal to 110 hours and in December with the fog affecting 17 nights/days totalling 161 hours.

Whereas, over New Delhi, the total number of fog days has surpassed the February average, noted the Met experts. As opposed to an average three nights/days equal to 12 hours in all of February, the Indira Gandhi International (IGI) airport till February 19 has experienced fog for four nights/days measuring 13 hours.