Satellite imagery showed fog developing over parts of Punjab, Haryana, north Rajasthan, and west Uttar Pradesh starting at 8 pm on Thursday.

An official at the India Meteorological Department (IMD) said dense to very dense fog blanketed parts of Punjab, Rajasthan, Haryana, Delhi, and Tripura. Moderate fog prevailed in parts of Uttarakhand, east Uttar Pradesh, Bihar, Jharkhand, and Odisha.

At 5.30 am, visibility levels stood at zero metres in Bathinda and Bikaner; 25 metres in Amritsar and Patiala; 50 metres in Ganganagar, Churu, Hisar, Palam, Safdarjung, Jaipur, and Agartala; and 200 metres in Ambala, Dehradun, Sultanpur (east UP), Purnea, Bhagalpur, Ranchi, and Jharsuguda.

A spokesperson for the Indian Railways said 22 trains arriving in Delhi were delayed by up to six and a half hours due to the foggy weather.

The IMD reported that the visibility level plummeted to zero metres at the Amritsar airport by 8 pm, while it abruptly dropped from 1400 metres at 9.30 pm to 400 metres at 10 pm at the Palam Observatory, near the Indira Gandhi International Airport.

It further dropped to 50 metres by 11.30 pm and zero metres by 4.30 am, before improving to 300 metres by 7.30 am.

Early morning foggy weather in north and northeast India has heavily impacted road, rail, and air traffic since the end of December.

IMD scientists say the absence of active western disturbances -- weather systems originating in the Mediterranean region and bringing unseasonal rainfall to northwest India -- is the reason behind the blinding layer of fog persisting over the plains in northwest India since December 25.