Visibility was zero at 7.30 a.m. at Palam, the Met Office said. Humidity was 100 per cent at 8.30 a.m.

"There was dense to very dense fog in parts of Delhi. The sky will remain partially cloudy during the day with haze and smog thereafter," an India Meteorological Department (IMD) official said.

The intense fog made the Delhi Traffic Police issue an advisory which asked drivers to use low beam head lights, avoiding overtaking and checking weather updates before leaving house.

Low visibility contributed to the delaying of 27 Delhi-bound trains.

The Delhi International Airport Limited (DIAL) said there have been some "dynamic delay" in some flights leaving the Indira Gandhi International Airport but not a single flight has been cancelled despite the low visibility.

"There was not a single cancellation or diversion of flight at the Delhi Airport on Monday. There could be of up to 15-20 minutes delay in departures as per the Air Traffic Control (ATC).

"The runway visibility had improved to 150 metres after sometime," a DIAL spokesperson told IANS.

The visibility improved to 2,000 metres by 10.30 a.m. at Safdarjung, the IMD said, however, it was poor at Palam adjacent to the airport, where it was 300 metres.

One of the reasons cited for the sudden dense fog was calm winds and rains in the preceding days.

"Rains and calm winds contributed to the forming of the fog. It was expected to happen on Sunday too, but winds with speed around 10-15 kmph scattered them," Mahesh Palawat of Skymet said.

The fog will continue for the next couple of days due to a western disturbance approaching on Tuesday, which will lead to a change in the direction of the wind.

There was no rainfall in the past 24 hours. The IMD forecast light rains accompanied by thunderstorm or hail from Tuesday till Thursday that will reduce the likelihood of fog, he added.

On Sunday, the maximum temperature was recorded at 19.9 degrees Celsius, two degrees below normal for this time of the year and the minimum was at 9.4 degrees Celsius, one above average.