Dense to very dense fog engulfed the Indo-Gangetic plains, including Delhi, for the second morning on the trot on Tuesday, lowering visibility to 25 metres in the national capital and affecting traffic movement.
According to the India Meteorological Department (IMD), very dense fog is when visibility is between 0 and 50 metres, 51 and 200 is dense, 201 and 500 moderate, and 501 and 1,000 shallow.
The Met office had on Monday forecast that the Indo-Gangetic plains may see "dense to very dense" fog over the next five days, which may result in cancellation and diversion of trains and flights.
Satellite images showed a thick layer of fog over Punjab, Haryana, Delhi, north Rajasthan and Uttar Pradesh.
Visibility dropped to 25 metres at the Palam airport and 50 metres at the Safdarjung airport, an IMD official said.
"Due to moisture and light winds at lower tropospheric levels over Indo-Gangetic plains, dense to very dense fog in many/some pockets very likely over Punjab, Haryana, Chandigarh, Delhi, Himachal Pradesh and Uttar Pradesh in night/morning hours during the next three days and dense fog in isolated pockets for subsequent two days," the Met office had said in a statement on Monday.
The IMD also issued an advisory, saying difficult driving conditions on highways, some collisions and tripping of power lines are possible.
"Likely train delays, diversons and cancellations. Airport operations are likely to be affected with flight delays and cancellations," it said.
The weather department said long exposures to fog can cause wheezing, coughing and shortness of breath in people having asthma bronchitis. It could also lead to eye irritation or infection.
It suggested that travellers carry essential items like water and medicine on long journeys.