A girl goes to school on foggy Thursday. (Pradip Sanyal)

Dense fog on Thursday morning left thousands of passengers stranded as flight operations were stalled for several hours.

Rail passengers, too, had a harrowing time with two Rajdhani Express trains, one Howrah-bound and the other Sealdah-bound, delayed by nearly seven hours. Several local trains of the Howrah and Sealdah divisions also ran late.

Runway visibility dipped to just 50 metres at 5.20am and remained so for around three hours. The first flight of the day — a Qatar Airways flight to Doha — took off from the Calcutta airport only at 8.45am.

But before that, around 40 flights were delayed by three hours on an average. A Kingfisher Delhi-Calcutta flight was diverted to Bhubaneswar and two flights had to hover for around half an hour before landing. “The delay had a cascading effect as several flights later in the day were also delayed,” said an airport official.

The weatherman blamed excess moisture and the still air for the fog. “There is a lot of moisture in the air over Calcutta and its suburbs because of western disturbances over north India and the eastern regions around Uttar Pradesh,” said G.C. Debnath, director, India Meteorological Department, Calcutta.

“Fog forms when there is a drop in temperature in such humid conditions. The lack of wind added to it,” Debnath said. He said the fog was dense on Thursday despite an increase in minimum temperature because humidity was a phenomenon localised to north Calcutta.

The minimum temperature on Thursday was 19.1 degrees Celsius, a notch higher than Wednesday and five degrees more than the normal temperature at this time of the year. The moisture-laden air is blocking the free flow of the chilly North Wind.

As Calcuttans wonder whether to send their woollens off to the laundry, they would do well to carry their umbrellas on Friday with light shower predicted in the city. The Met office warned of fog on Friday too.