The minimum temperatures in the national capital rose marginally though.
Delhi saw cold wave conditions for the fifth consecutive day on Monday and the season's longest fog spell which crippled road, rail, and air traffic movement.
India Meteorological Department officials said the Palam observatory, near the Indira Gandhi International (IGI) Airport, recorded visibility levels below 1,000 metres from 8.30 pm on Sunday to 4.30 pm on Monday.
The visibility levels were reduced to 50 metres at Palam and 25 metres at the Safdarjung observatory and the Ridge weather station early in the morning, an official of the IMD said.
A total of 267 trains were delayed due to bad weather, a railway official said.
Five flights were diverted and 30 were delayed due to the foggy conditions, officials at the IGI Airport said.
Satellite images showed a fog layer extending from Punjab and adjoining northwest Rajasthan to Bihar through Haryana, Delhi, and Uttar Pradesh.
The minimum temperatures in the national capital rose marginally though.
The Safdarjung observatory, Delhi's primary weather station, recorded a minimum of 3.8 degrees Celsius as against 1.9 degrees on Sunday, which is the lowest in January in two years and the second lowest in the month since 2013.
The weather stations at Lodhi Road, Ayanagar, and Ridge recorded a minimum temperature of 3.6 degrees Celsius, 3.2 degrees Celsius, and 3.3 degrees Celsius.
The Safdarjung observatory had logged a minimum temperature of 1.9 degrees Celsius on Sunday, 2.2 degrees Celsius on Saturday, 4 degrees Celsius on Friday, 3 degrees Celsius on Thursday, and 4.4 degrees Celsius on Wednesday.
Monday was the fifth consecutive day that Delhi's minimum temperature was lower than most hill stations in Himachal Pradesh and Uttarakhand, including Chamba (8.7 degrees), Dalhousie (9 degrees), Dharamshala (9.2 degrees), Shimla (10.3 degrees), Manali (6 degrees), Kangra (8.9 degrees), Dehradun (6.5 degrees), Mussoorie (11.3 degrees), Nainital (6 degrees), Mukteshwar (7.6 degrees) and Tehri (9.2 degrees), according to the IMD.
The chilly weather has prompted the Delhi government to extend winter vacation in schools till January 15.
Meteorologists attribute the long spell of intense cold to a large gap between two western disturbances, which meant frosty winds from the snow-clad mountains blew in for a longer-than-usual period.
Mahesh Palawat, vice president (meteorology and climate change), Skymet Weather, said, "Usually, there is a gap of three to four days between two western disturbances. This time, the gap increased to seven days. A western disturbance retreated from the region by December 30 and the next one came on January 7." A senior IMD meteorologist said short-term relief is likely after a couple of days under the influence of back-to-back western disturbances.
When a Western disturbance -- a weather system characterised by warm moist winds from the Middle East -- approaches a region, the wind direction changes. Hence, the chilly northwesterly winds from the mountains will stop blowing for a few days leading to an increase in temperatures, he said.
The IMD said the foggy weather may lead to road accidents and advised people to drive slowly and use fog lamps on their vehicles. It said there are chances of tripping of power lines in areas witnessing very dense fog.
Long exposure to dense fog may cause respiratory problems for people having asthma bronchitis and other lung-related health problems, it said.
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