Unseasonal fog at Calcutta airport has left officials foxed. The phenomenon occurred twice last week, and may recur in the days ahead, weather officials said.

Early on Saturday and a couple of days before that, dense fog enveloped the airport and reduced visibility to zero. Morning flights were disrupted. Met officials explained that the phenomenon was ?totally localised? and the fog was caused by radiation.

?After a smart shower at night around this time, the sky clears and the heat from the surface begins to radiate or escape upwards. This leads to a quick drop in atmospheric temperature in the area. This phenomenon occurs mainly in large open areas, like the airport,? an official said.

Officers said in the cooling process, the moisture or water particles in the air condense into fog. ?This kind of fog is known in meteorological parlance as radiation fog,? an official said.

On Saturday, dense fog delayed eight domestic flights for nearly two hours on an average, airport sources said.

?The first Calcutta-Mumbai flight of Indian Airlines was scheduled to leave at 6 am, but left at 7.45 am. The fog lasted for about one-and-a-half hours on Saturday, finally clearing at 7.20 am,? said an officer at Netaji Subhas Chandra Bose International Airport.

?We were surprised. At this time of the year, such dense fog is unusual,? said the official.