Dense fog in the national capital on Wednesday morning disrupted flight and train operations with five flights diverted and 22 trains delayed by up to eight hours, officials said.

Heavy fog engulfed parts of the city with the minimum temperature settling at a notch below normal.

"Five flights diverted as the captain was not trained to land under CAT conditions," the official said.

When the runway visual range is minimum 200 metres, then only a pilot who is trained in instrument landing system Category 3A (CATIIIA) can land the aircraft.

If the RVR is minimum 50 metres, then a pilot who is trained in using CATIIIB landing system can land the aircraft at the airport.

"The minimum temperature was recorded at 7 degrees Celsius, a notch below normal,” a MeT official said.

The humidity level was 100 per cent and visibility 50 metres.

The Agartala-Anand Vihar Terminus Rajdhani Express was delayed by 1:30 hours while the Varanasi-New Delhi Kashi Vishwanath Express was delayed by 3:45 minutes.

The Rewa-Anand Vihar Express was delayed by six hours while the Azamgarh-Delhi Junction Kaifiyat Express was delayed by 5:45 hours.

The Ghazipur-Anandvihar Express was running late by eight hours, officials said.

"Delhi reported dense, very dense fog with visibility of 25-50 metres at 0530 hours IST on Wednesday and dense, very dense fog is likely to continue till 1000-1100 am and improve thereafter.

"Dense to very dense fog very likely to affect flight operations and transportation due to poor visibility," the India Meteorological Department said in a tweet.