The fog that enveloped Patna since Tuesday morning did not clear out for the better part of the day.

Though it was the first foggy day this season, the India Meteorological Department (IMD) has forecast shallow to moderate fog across Bihar in the next 24 hours that would intensify over the next couple of days, dragging down day temperatures. The maximum temperature in Patna settled at 24.9ºC on Tuesday, two degrees below normal for this time of the year, while the minimum temperature was 15.6ºC.

Patna Met director Sumendu Sengupta attributed the sudden fog (as seen in the picture from Patna by Ashok Sinha, clicked around 7am on Tuesday), which brought down the day temperature, to a western disturbance. "The disturbance over north Pakistan and Jammu and Kashmir has brought down the temperature in Patna," he said.

A western disturbance is defined as an extra-tropical storm originating in the Mediterranean region that brings sudden winter rain and fog to the northwestern parts of the Indian subcontinent along with sudden intrusion of cold air.

The fog will continue on Wednesday as well, the Met director said.

"People have already started getting the wintry feel, especially during the early morning and night. Though there has been a significant drop in the morning and night temperatures , heavy snowfall has not occurred in the western Himalayas. That is why the northwesterly winds are not as cold as they should have been," said a weatherman.

He, however, said scattered rain and snow were forecast in Jammu and Kashmir between November 30 and December 1, which may lead to gradual decrease in temperatures in Bihar. Visibility in Patna dropped to 500m on Tuesday morning. Some flights were delayed. Visibility up to 800m is must for a flight to take off or land. "The first flight that lands in Patna at 7.55am was delayed by more than two-and-a-half hours. But no flight was cancelled," said an airport source.

All airlines operating from Patna airport have switched to their winter schedule from October 30.