The visibility, which was 800 meters at 5.30 AM, decreased significantly to 50 meters at 8.30 AM with vehicles plying on the roads with their headlights on.
"The humidity was 100 per cent at 8.30 AM," said a MeT official.
Thirty flights were delayed due to fog, airport officials said.
Services of 55 trains including premiere trains like Rajdhani, Shatabdi Express and Duronto Express were severely affected due to bad weather conditions.
"Also trains like Punjab Mail, Bharmhaputra Mail and Orissa Sampark Kranti Express were running late by several hours due to dense fog," said a Northern Railway official.
Yesterday, the maximum and minimum temperatures had settled at 20 and 8.2 degrees Celsius respectively.