Dense fog over Delhi for a second consecutive morning reduced visibility to zero in several areas, leading to 81 trains being delayed and 15 flights getting diverted, officials said on Saturday.

The Met office said there was zero visibility since 11:30 pm on Friday at Palam and 12:30 am on Saturday at Safdarjung.

The fog was accompanied by mainly calm surface winds that prevented it from dissipating.

At 7 am, general visibility at the Indira Gandhi International Airport was zero in very dense fog. The Runway Visual Range (RVR) at its runways was in the range of 100-250 metres, with CAT III conditions.

Such very dense fog in zero visibility with CAT III conditions at the airport has been prevailing since 11:30 pm on Friday, the Met office said.

"While landing and takeoffs continue at Delhi airport, flights that are not CAT III compliant may get affected. Passengers are requested to contact the airline concerned for updated flight information. Any inconvenience caused is deeply regretted," the Delhi International Airport Limited, which operates the airport, said in a post on X at 6:56 am.

CAT III facility allows aircraft to operate in low-visibility conditions.

The Northern Railway said a total of 59 trains were running late by up to six hours and 22 by around eight hours.

In a related development, air quality in the national capital was in the 'very poor' category, with an Air Quality Index (AQI) reading of 380 at 9 am, according to Central Pollution Control Board data.

An AQI between zero and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.

The minimum temperature settled at 7.8 degrees Celsius, a notch above normal.

The Met office has predicted dense to very dense fog during the day. The maximum temperature is likely to settle around 21 degrees Celsius.