Dense fog enveloped the IGI airport on Thursday morning, delaying over 20 flights and leading to diversion of 10 others.
Around 20 flights scheduled to depart this morning were delayed as the runway visibility dropped to 50 metres, far below the required low visibility take off minima of 125 metres, an airport official said.
Ten incoming flights were diverted to nearby airports, he said.
Fog started to descend on the airport after midnight and become dense at around 4am, forcing the authorities to implement the low visibility procedures at 4.44am on both the runways.
The runway visibility started to drop below the required LVT of 125 metres around 7am, delaying many flights.
Passengers had to stay in the plane for more than three hours as their aircraft could not take off due to poor visibility.
"We were at the runway since 6.50am and at 9.40am, the pilot announced that we need refuelling now. The pilot also said that we may not be able to take off for the next one hour," said Rajesh Nagar, a passenger who boarded a Jet Airways flight to Chennai.