Dense fog disrupted flight operations for nearly four hours at Calcutta airport on Sunday morning.

The departure of 27 flights was delayed. Three flights bound for Calcutta were diverted, while three others had to circle overhead before landing.

The recently installed Category II Instrumental Landing System could not help, as visibility dropped to 50 metres.

Domestic flight operations at the airport start around 5am, but on Sunday, the first plane — Deccan’s flight to Raipur —took off at 8.40am. It was followed by Air India’s Port Blair flight, whose scheduled departure was 5.35am.

“An anti-cyclonic circulation (clockwise movement of air) over the sea led to heavy incursion of moisture, which condensed into fog early in the morning after coming in contact with the cold North Wind,” said G.C. Debnath, the director of the weather section at Regional Meteorological Centre, Alipore.

Visibility started dropping from 5am, said an airport Met department official. Around 5.30am, the visibility dipped below 350 metres, making it impossible for flights to operate even by using the CAT-II landing system.

For two hours from 5.55am, the visibility was about 50 metres. Even after the conditions improved, there was runway congestion, leading to cancellation of Jet Airways’s Calcutta-Port Blair flight.