Thousands of passengers had a tough time as a dense fog stalled operations at the city airport on Friday morning.

Two flights arriving in Calcutta were diverted and 25 planes had delayed take-offs because of the fog, an airport official said.

Visibility had dropped below 50m and no flights could operate between 6.03am and 8.05am, the official said.

The Met office termed the fog unusual at this time of the year. It was caused by a high-pressure belt over the Bay of Bengal, an official said.

“The high-pressure area is led to air surrounding the weather system to be dispersed in various directions away from it and a lot of it entered Calcutta and neighbouring areas. Temperatures have already started to dip in the city because of a northwesterly wind blowing into the city from the plains. As the moisture in the air condensed and turned into water droplets with the drop in temperature, visibility dropped,” the official said.

He said there was a chance of a similar fog on Saturday morning as well. Although temperature tends to drop around this time of the year, so much moisture is normally not present, which is why a fog is rare, he said.

An AirAsia flight from Delhi could not land and was diverted to Bhubaneswar. An IndiGo flight from Bhubaneswar had to return to Bhubaneswar. Both landed in the city later, the airport official said.

While some flights scheduled for take-off were stuck during the fog, others were delayed because of the congestion once operations resumed.

The Category III-B Instrument Landing System was installed at Calcutta airport on January 4 this year to allow flights to land safely in low visibility of up to 50m. At times, visibility improved but again dropped. So flights could not be operated, the official said.