Dum Dum: A blanket of fog evoking a bleak January morning stalled flight operations for two hours on Thursday by dragging down runway visibility below the level at which the CAT III-B instrument landing system can help aircraft touch down.

Three flights due to land during that period were diverted while five others scheduled for take-off were delayed, a senior airport official said.

But sources said several more flights were delayed by the dense fog that hung over the twin runways from 5.30am till 7.30am.

The trigger for the unusual appearance of fog in the middle of March was the "coupling" of two contrasting weather systems in the vicinity, the Met office said.

An anti-cyclonic circulation on the Odisha coast and adjoining Bay of Bengal and a cyclonic circulation over Jharkhand had formed on Wednesday, setting the stage for the fog march. The first system pushed and the second helped pull moisture into land.

"The density of moisture in the air led to condensation of vapour even at a relatively high minimum temperature of 22 degrees Celsius in Dum Dum on Thursday morning," a weather scientist explained.

The moment these water droplets became visible, the required visibility for flight operations declined rapidly.

According to the Met office at the airport, visibility dipped to less than 50m at 5.30am and did not improve for two hours. An increase in temperature from 7.30am onwards led to visibility quickly shooting up to 700m.

The CAT III-B instrument landing system installed on the main runway before the start of last winter enables aircraft to land safely in low visibility up to a minimum of 50m. The winter months did not see any significant disruption in flight operations because visibility was above that cut-off on most days.

The fog on Thursday morning forced a Jet Airways flight from Guwahati to return. An AirAsia flight from Delhi and SpiceJet flight from Bangkok were also diverted to Guwahati.

The first aircraft to take off after visibility improved was IndiGo's Bangalore flightat 7.32am. The scheduled take-off was 5.30am.

Fog isn't a rarity in March - the average for the month is three days - but it is invariably thin and does not disrupt flight operations. In the peak of summer and then the monsoon, fog doesn't occur despite abundant moisture swimming in the air because the temperature does not drop enough for condensation of vapour to happen.

The Met office said fog was unlikely to return on Friday morning because the cyclonic circulation over Jharkhand had weakened.