The maximum temperature in Delhi took a big tumble on Monday to settle at 9.4 degrees Celsius, making it the coldest December day since 1901.
It was also the coldest day in the national capital during December-February period since 1951.
According to the Safdarjung Observatory, the minimum temperature settled at 2.6 degrees Celsius, four notches below the season's average; humidity level was 100 per cent.
The minimum temperature at Palam was 2.9 degrees Celsius, Lodhi Road was 2.2 degrees Celsius and Ayanagar 2.5 degrees Celsius.
"The Safdarjung Observatory recorded a maximum of 9.4 degrees Celsius, making it the coldest December day since 1901," said Kuldeep Srivastava, head of IMD's regional weather forecasting centre. He said the normal temperature at this time of the year is 20.8 degrees Celsius.
Before this, 11.3 degrees Celsius was the lowest maximum for the month recorded on December 28, 1997.
"Considering all the months, it has surpassed the previous lowest maximum temperature of 9.8 degrees Celsius recorded on January 2, 2013," the IMD said in a statement.
The average maximum temperature for December 2019 till Monday is 18.6 degrees Celsius, the lowest since December 1997 (when it was 17.3 degrees Celsius) and the second-lowest since 1901, when IMD started keeping the records.
The factors responsible for such a cold December are a "very long spell" of icy winds and a layer of fog lingering over vast swathes of the northern plains for 10-12 days, the experts said.
As the frigid air tightened its grip, a blanket of thick fog reduced the visibility to near zero at many places, disrupting train and flight operations.
The IMD said the national capital witnessed the season's worst "dense fog" with "visibility of 0-200 metres for a very prolonged period from 0230 to 1200 hours". Thereafter shallow to moderate fog covered the city.
Around 530 flights got delayed, 40 cancelled and 21 diverted.
Flights are operating at the airport under CAT III B conditions, which means the runway visual range (RVR) is between 50 and 175 metres.
Seventy-one trains were delayed by 2-7.5 hours, according to a Railways official.
The Delhi-Bhubaneswar Rajdhani was delayed by seven and a half hours.
Falling temperatures, low wind speed and high humidity led to a further dip in air quality index, recorded in the severe category at 446 at 4 pm.
Foggy conditions will continue to prevail on Tuesday with the minimum temperature likely to settle around three degrees Celsius.
The national capital on Saturday had recorded its coldest day of the season as the minimum temperature plummeted to 2.4 degrees Celsius in the morning.