A cold wave swept through parts of Delhi on Monday and dense fog lowered visibility to 50 metres in some areas of the city, affecting road and rail traffic.

Ten trains were reported running late by 1:45 to 3:30 hours, a railway spokesperson said.

The Safdarjung observatory, Delhi's primary weather station, logged a minimum temperature of 5 degrees Celsius -- three notches below normal. The maximum temperature is likely to settle around 19 degrees Celsius, the weather office said.

On Sunday, the capital reported a cold day. According to the weather office, a cold day is when the minimum temperature is less than or equal to 10 degrees Celsius and the maximum temperature is at least 4.5 degrees Celsius below normal.

The maximum temperature at Safdarjung on Sunday had settled at 16.2 degrees Celsius, five notches below normal and the lowest so far this season.

The mercury had dropped to 3 degrees Celsius in the Ridge area, 4.9 degrees below normal, making it the coldest place in the capital.

On Monday, the Ridge and the Ayanagar weather stations recorded a minimum temperature of 4 degrees Celsius and 4.1 degrees Celsius respectively.

Dense to very dense fog prevailed in some parts of the National Capital Region, Haryana, Punjab, west Uttar Pradesh and north Rajasthan on Monday.

Bathinda in Punjab and Bikaner in Rajasthan reported zero visibility, while it dropped to 50 metres and below at Ambala, Hisar, Amritsar, Patiala, Ganganagar, Churu and Bareilly.

Foggy conditions will persist in these areas over the next few days, the India Meteorological Department (IMD) said.

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

In the plains, the IMD declares a cold wave if the minimum temperature dips to 4 degrees Celsius. A cold wave is also declared when the minimum temperature is 10 degrees Celsius or below and is 4.5 notches below normal.

A "severe" cold wave is when the minimum temperature dips to two degrees Celsius or the departure from normal is more than 6.4 degrees Celsius.

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