A "very dense" fog that lowered visibility to less than 50 metres engulfed the city on Sunday morning, causing flight disruptions and delaying trains.
The Alipore Met office said this was the first time such dense fog developed in Kolkata this winter.
The weather office forecast fog on Monday morning, too, but it would not be as dense as Sunday's.
As the sun appeared, the fog started to clear. The day was bright and sunny.
"If the fog-induced visibility is less than 200 metres, it is called dense fog. When the visibility drops to less than 50 metres, like on Sunday morning, it is called very dense fog," said G.K. Das, director of India Meteorological Department, Kolkata.
"I woke up at 8am and could not see anything through the window. I thought it was 5am or 6am and was about to go to sleep again, but then my eyes fell on the clock," said a resident of Avenida, a residential complex in New Town.
"I could not even see the next tower in our complex, the fog was so dense."
Flight operations at the Kolkata airport was stalled for more than three hours on Sunday morning because of poor visibility. Airport officials said the departure of around 30 flights was delayed. Six incoming flights were diverted.
Flight operations stopped at 5.30am as visibility dropped below the permissible level. The operations resumed around 8.45am after the conditions improved, said an official.
IndiGo airline's flight from Singapore to Kolkata was diverted to Nagpur. The airline's flights from Delhi, Bangalore and Chennai to Kolkata were diverted to Bhubaneswar, said the official.
SpiceJet's flight from Bangkok was diverted to Guwahati, while Air Asia's flight from Bangalore was diverted to Bhubaneswar, said the official.
The city airport has Category III B instrument landing system, which allows flights to land even when visibility is as low as 50 metres. "However, visibility went down below 50 metres for more than three hours this morning, stalling aircraft landing and take off," said the official.
Twelve long-distance trains were delayed on Sunday morning, owing to the fog, said a railway official.
Forty-six local trains were delayed by about 20 minutes on an average in the Howrah division. In the Sealdah division, almost all local trains were delayed by 10 minutes on an average.
The maximum temperature on Sunday was 25.1 degrees Celsius, one degree below normal. The minimum was 17.5 degrees, three notches above normal.
The Met office has forecast "fog or mist in the morning" on Monday and mainly clear sky later. The maximum and minimum temperatures are likely to be around 25 and 17 degrees Celsius, respectively.
