Take-offs from the IGI Airport require a minimum visibility range of 125 metres.
However, as Delhi airport has advanced technology for low-visibility landings, called the CAT IIIB system (short for category three), aircraft qualified to land in the visibility of 25-50 metres could arrive.
There were, however, nearly 50 diversions to nearby airports as many pilots deployed by airlines were not trained in CAT IIIB instrument landing system, the source said.
"Runway visibility since 5.30 AM has been between 50-75 metres. This is so far the worst fog we have experienced this year," RK Jenamani, Director, India Meteorological Department, Delhi area and IGI Airport said.
An official from the Met department termed it the "worst" fog of the season.
While visibility improved to 2,000 metres later in the day and operations were restored to normal, there were consequential delays throughout the day and a waiting time of up to six hours was common.
Many took to Twitter to share pictures of the overcrowded domestic terminal where passengers could be seen sitting on floors and occupying staircases.
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