20-hour fog spell delays 200+ flights, 73 trains in Delhi; haze likely today: IMD

Forecast for Tuesday shows dense fog is likely to continue, while Wednesday will see moderate to dense fog over the Capital. Met officials say the fog intensity will reduce after Wednesday, owing to a western disturbance impacting the region.

The Capital recorded this season’s longest fog spell for 20 hours, from 8.30pm on Sunday to 4.30pm on Monday, with visibility lifting over 1,000 metres briefly for one hour, before returning below 1,000 metres from 5.30pm. The fog, officials, said is likely to persist till Tuesday afternoon.

Passengers check arrival and departure time of flights at Indira Gandhi International Airport in New Delhi on Monday morning. (PTI)

According to officials, Delhi’s Indira Gandhi International (IGI) Airport was under a layer of dense fog for at least nine hours between 1am and 11am on Monday, leading to over 200 flights getting delayed, and five flights being diverted to the nearby Jaipur airport. At least 73 trains were also delayed by one hour or more, data shared by northern railways on Monday showed.

At Shivaji Bridge in New Delhi on Monday. At least 73 trains were also delayed by one hour or more, data shared by northern railways on Monday showed. (Sanchit Khanna/HT Photo)

Delhi had recorded a similar long spell of fog between 6pm on December 26 and 1pm on December 27, but the fog had lifted during the day, with visibility improving beyond 1,000 metres.“Visibility fell below 1,000 metres at Palam around 8.30pm and under 100 metres at 1am, after which it remained below 200 metres till 11am. Even during the day, when the sun came out, visibility remained between 500-800 metres, only rising above the 1,000 metre fog threshold for a brief period between 4.30pm and 5.30pm, before returning below 1,000 metres. At 20 hours, this is so far the longest spell of fog this season,” said a Met official.

The India Meteorological Department (IMD) classifies fog as “shallow” when visibility dips below 1,000 metres, as “moderate” when visibility is between 200 and 500 metres and as “dense” when it is below 200 metres. It is “very dense” when visibility is below 50 metres.

The first of the five flights diverted was an Air India Express flight - IX 136 from Sharjah to Delhi, which was diverted at 5.45am due to low visibility. This was followed by four flights between 9am and 10am, which included three SpiceJet flights -- SG-194 from Ahmedabad to Delhi, SG-8937 from Pune to Delhi and SG-8130 from Kolkata to Delhi--along with another Air India flight, from Cochin to Delhi (AI-466). Airport officials said in all five cases, CAT-III procedures were in place, and the pilots not compliant with the standards.

The airport initiates low visibility procedures (LVP) -- measures aimed at making landing for flights easier-- when visibility dips below 800 metres. During this period, CAT-I procedures are in place, which is the most basic set of precautions to guide pilots in landing. Only flights and pilots that are CAT-II compliant are allowed to land when visibility is less than 550 metres, while CAT-III A pilots can land when visibility is between 175 and 300 metres. CAT-III B is the most stringent set of qualifications for a pilot to land a flight, allowing them to land even when visibility is 50 metres.

While flights can still land at the airport when visibility is 50 metres, no flight is allowed to depart unless runway visibility range (RVR) is 125 metres, leading to flight delays.

This meant several arrivals and departures were delayed from 1am onwards, affecting later flights as well. A deviation of over 15 minutes from the scheduled time of arrival or departure is classified as a ‘delay’, with over 200 such flights – both international and domestic-- impacted on Monday morning.

At 12.55am, the Delhi airport tweeted that low visibility procedures were in place, asking passengers to check with the airline for the latest flight information. “Low visibility procedures are in progress at Delhi Airport. All flight operations are presently normal. Passengers are requested to contact the airline concerned for updated flight information,” it tweeted.

Forecast for Tuesday shows dense fog is likely to continue, while Wednesday will see moderate to dense fog over the Capital. Met officials say the fog intensity will reduce after Wednesday, owing to a western disturbance impacting the region.

“The western disturbance will bring cloudy skies, which impacts fog formation. For fog to be dense, clear skies are generally needed,” an official added.