Ten Calcutta-bound flights were diverted late on Thursday and early on Friday because of dense fog at the city airport, officials said on Friday.

On Friday morning, a few flights from Calcutta were cancelled as the inbound flights had been diverted to other cities and the aircraft were unavailable, an official said.

A trough of low pressure extending from Haryana to Bangladesh had brought cloud and rain to the city on Friday. The system started clearing from Thursday evening but some moisture remained in the atmosphere, leading to a foggy morning on Friday. Fog happens when the moisture in the air condenses and mixes with dust particles early in the morning, a weather scientist said.

Visibility had started dropping after 10.30pm and around midnight it was below 50m, the official said. Calcutta airport has Category III-B Instrument Landing System that allows operations at 50m visibility.

“But the visibility had dropped to almost zero, making it impossible for flights to land. Now there are several latenight domestic flights, along with international ones, and 10 flights got diverted,” airport director Kaushik Bhattacharya said.

According to officials, this was the biggest disruption because of fog, this winter.

Among the four international flights which could not land in Calcutta were Air India’s flight from Dubai that was diverted to Nagpur, and Thai AirAsia flight from Bangkok, diverted to Bhubaneswar. Two flights from Kuala Lumpur, operated by Malindo Air and AirAsia, were diverted to Dhaka, the official said.

Six domestic flights were diverted. GoAir flights from Pune and Mumbai, AirAsia flights from Chennai and Bangalore and IndiGo’s Chennai flight were all diverted to Bhubaneswar. IndiGo’s Delhi-Calcutta flight was diverted to Lucknow.

Visibility started improving after 3am but in the morning, a few flights had to be cancelled because of unavailability of aircraft, the official said.