Several flights from Calcutta flew hours behind schedule on Tuesday morning because of fog. Visibility went down to as low as 200 metres, making it difficult for flights to take off, officials at Netaji Subhas Chandra Bose International Airport said.

An Indian airlines flight for Mumbai was the first to leave Calcutta, at 6.50 am. The take-off was scheduled for 6 am. An Alliance Air flight for Port Blair left at 8.10 am, instead of 5.35 am.

Among the other affected airlines were Jet Airways, Air Sahara and Air Deccan. The Jet flight for Delhi was delayed by 50 minutes, for Mumbai by an hour and for other destinations by around 40 minutes.

?Visibility at 6 am was just 200 metres. The minimum required is 550,? said A.K. Das, director-in-charge of the airport?s meteorological office. Normal visibility was restored around 7 am.

According to Das, the fog is likely to continue till mid-February. ?We are talking to Airports Authority of India for installation of the mid-runway visual range machine to facilitate landing in poor visibility,? Das said. The gadget, Vaishala Transmisso Meter, is an essential component of the Category II Instrument Landing System. Officials said it could not be installed this winter.