New Delhi, Dec 22 (IANS): A fog blanket over large parts of northern India hit railway traffic, delaying 50 trains, but air traffic was largely unaffected.

"There is a heavy backlog in trains, leading to many delays and rescheduling of trains," a Northern Railways spokesperson said.

"Around 50 trains ran behind schedule by eight hours due to heavy fog all over northern India," the railway official said.

However, flight operations were largely unaffected at the Indira Gandhi International (IGI) airport which witnessed slight fog Wednesday morning.

According to the Delhi International Airport Limited (DIAL) statement, six flights were cancelled at the Indira Gandhi International (IGI) airport here due to low visibility at other airports where the flights were headed and technical reasons.

Visibility fell to 350 metres at 8.30 a.m. while the minimum temperature was recorded a notch below average at 6.8 degrees Celsius.

Four flights were cancelled due to low visibility at other airports where the flights were to take off and two services were curtailed due to technical reasons.

"Almost all of the flights departed on schedule," an airport official said.

Meanwhile, 87 flights were delayed due to technical reasons.

The maximum temperature was recorded at 23 degrees -- one notch above average Wednesday.

Sounding the fog warning, the India Meteorological Department (IMD) Tuesday said the condition is expected to last from Dec 20 to Feb 20.

"The fog season has started from today and it will last upto Feb 20," R.K. Jenamani, director, meteorological department (IGI airport), told IANS.

According to IMD, the coldest day of the season so far has been Dec 16 when minimum temperature dropped to 4.7 degree and maximum to 21.7 degrees.