Dense fog enveloped the Indira Gandhi International Airport in Delhi this morning with visibility dropping to near zero affecting flight operations. This led to delays, diversions and cancellations of around 75 domestic and international flights. Passengers were a harassed lot as many airlines delayed the departure of their flights due to dense fog. The general visibility at the main and the new runways were less than 50 metres. The runway visibility was below 125 metres. Last night, Air India rescheduled its flights to New York, Chicago and Toronto after visibility dropped below the accepted limit. The thick cover of fog in Delhi had its cascading effect on the flight operations at the Mumbai airport. Six halt flights between Delhi-Mumbai-Delhi were cancelled and a couple of other flights diverted. An airport official however said, the operation is now returning to normal.