Met office sources said, Patna registered a minimum temperature of 6.5 degrees Celsius while the maximum hovered around 17 degrees Celsius.
Such dense was the fog blanket in the state capital in the morning that it reduced visibility to 50 metres, though later in the day it cleared up to go above 1,000 metres.
Met sources said Gaya with 4.2 degrees C recorded the lowest minimum temperature in the state, five degrees below the normal temperature for this time of the year.
"It could turn into very severe cold wave if the temperatures plummet six degrees below the normal," Indian Meteorological Department (IMD) director at Patna R K Giri told PTI.
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Giri said the Patna airport had to reschedule its flights over fog advisory from the IMD as visibility was too low in the morning to allow operations of flights.
Patna airport Director-in-charge T K Dutta said, "Three to four flights got delayed in the morning due to dense fog, but operations resumed around noon as the visibility cleared."
Fog also disrupted railway traffic with almost all long distance trains including several Rajdhani Expresses running several hours behind the schedule. Departure of a number of trains at Patna Junction too got delayed by three to six hours.
"Fog is hampering the normal functioning of railways. Many trains are running late by several hours, which is causing much inconvenience," East Central Railway (ECR) Chief Public Relations Officer (CPRO) Arvind Kumar Rajak said.