A dense cover of fog on Wednesday lowered visibility to just 50 metres in Delhi, disrupting movement of vehicles and trains. A layer of dense to very dense fog extended from Punjab to Bihar across Haryana, Delhi and Uttar Pradesh, an IMD official said.

The Palam observatory near the Indira Gandhi International Airport recorded a visibility level of 50 metres.

Twenty trains were delayed by an hour to 10 hours due to the foggy weather, a Northern Railways spokesperson said.

The Delhi International Airport Limited tweeted that low-visibility procedures were in progress at the IGI airport.

Passengers have been advised to contact the airline concerned for updated flight information.

According to the weather office, very dense fog is when visibility is between 0 and 50 metres, between 51 and 200 metres is dense, between 201 and 500 metres moderate, and between 501 and 1,000 metres shallow.

The Safdarjung observatory, Delhi's primary weather station, recorded a minimum temperature of 5.8 degrees Celsius.

The minimum temperature in the capital is likely to rise to 9 degrees Celsius on Thursday amid cloudy weather and light rain. Clouds trap the heat, which gets through during the day, keeping the night time temperatures above normal.

However, cloudy weather reduces daytime temperature by preventing exposure to the Sun. Cold wave conditions are predicted to return to the plains of northwest India after the WD retreats and frosty northwesterly winds start blowing from the snow-covered mountains.

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