Dense fog for nearly nine hours in the early hours of Tuesday reduced visibility to as low as 50 metres in parts of the national capital, impacting both flight and rail operations.

Experts said the fog was due to the moisture from the last western disturbance, the impact of which will be over by Wednesday.

The lowest visibility recorded in the city was at Safdarjung at 50 metres, followed by 100 metres at Palam.“Visibility began dipping rapidly on Monday night itself and by 11:30 pm, had already touched 100 metres. It remained in this range till 8:30 am on Tuesday,” said an IMD official

The IMD classifies fog as “shallow” if visibility is between 1,000-500 metres; “moderate” between 500 and 200 m; “dense” between 199 and 50 m; and “very dense” below 50 m.

While no flights at Delhi’s Indira Gandhi International (IGI) airport had been cancelled or diverted, the flight tracking website Flight Radar 24 showed there were over 500 flights delayed through the day, primarily departures. The average delay time was around 30 minutes.