About 15 flights were delayed by 2-3 hours while four others cancelled due to dense fog at the Indira Gandhi International (IGI) Airport in New Delhi early on Sunday morning.
One flight of Jetlite and Indigo each to Mumbai was cancelled while another flight to Kabul was grounded as heavy fog enveloped the city.
Another flight of Indigo from Delhi to Goa was also cancelled.
The visibility dropped to below 50 metre while the Runway Visibility Range (RVR) dipped down to 125 metres forcing the airport authorities to implement CAT-III B Instrument Landing System.
Both the runways, the new runway (29/11) and the main runway (28/10) were made open to air operations since 6 am, an airport official said.