Shallow fog wrapped parts of the national capital on Tuesday morning, reducing visibility and affecting vehicular movement.
A layer of dense to very dense fog over the Indo-Gagetic plains and adjoining central and eastern parts of the country affected rail and air traffic.
At least 21 trains to Delhi were delayed by one-and-a-half to five hours due to the foggy weather, a Railways spokesperson said.
An airport official said five flights were diverted to Jaipur on Monday night due to bad weather in Delhi.
The Safdarjung observatory, Delhi's primary weather station, recorded a minimum temperature of 8.5 degrees Celsius, a notch above normal.
The maximum temperature is likely to settle around 17 degrees Celsius.
Moderate to dense fog is predicted in Delhi for the next two days. Coldwave conditions are set to return on the weekend and the mercury may drop to 4 degrees Celsius.
According to the India Meteorological Department (IMD), 'very dense' fog is when visibility is between 0 and 50 metres, 51 and 200 metres is 'dense', 201 and 500 metres 'moderate', and 501 and 1,000 metres 'shallow'.
In the plains, the Met office declares a cold wave if the minimum temperature dips to four degrees Celsius or 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 the normal is more than 6.4 degrees Celsius.
The Air Quality Index (AQI) of the national capital was recorded at 384 at 11 am.
An AQI between 201 and 300 is considered 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.
The Commission for Air Quality Management had on Monday said curbs under Stage III of the Graded Response Action Plan, including a ban on non-essential construction and demolition work, would remain in force in Delhi-NCR as pollution levels were showing an upward trend.
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