Flights at Calcutta airport were delayed early on Sunday morning as dense fog enveloped the runway, punching holes in the airport officials’ claims that the beast had been tamed.

Two early morning flights were diverted after failing to land, while 10 flights were delayed by 30 minutes on an average. There was no flight movement at the airport from 1.15am to 6.10am.

The London-Calcutta British Airways flight, scheduled to land at 1.15am, could not do so and was diverted to Dhaka. It is scheduled to touch down in the city late on Sunday.

“This is a normal phenomenon at this time of the year. Condensation of moisture fed by a southerly wind from the Bay of Bengal causes the fog,” said a Met department official.

The Air India Express flight, which flies from Mumbai to Calcutta and then proceeds to Bangkok, hovered over the runway for 45 minutes.

The flight was scheduled to land at 5.45am but had to return to Mumbai. Another flight from Mumbai replaced it, which finally landed at 9.10am and took off for Bangkok at 10.20am.

Visibility started dropping after midnight and around 1am, it was below 350 metres. The Cat II Instrumental Landing System, installed last year to fight fog delays, was of little help.