At least 36 flights are reported to have been delayed as dense fog enveloped Delhi and parts of northern India.
Visibility at runways at the airport in Delhi dropped to below 50 metres, prompting flight disruption.
Dozens of trains were also cancelled or delayed, and road traffic on Monday morning was crawling.
Traffic disruption due to fog in winter has become routine in northern India, the BBC reported.
A number of cold-related deaths have also been reported.
Seven people died in two separate incidents in Delhi and neighbouring Uttar Pradesh state on Sunday after inhaling carbon monoxide from coal fires in their rooms to keep themselves warm.
Reports from the Kashmir Valley say Srinagar city recorded a minimum temperature of minus 1.8C on Sunday.
Cold weather is also continuing in most parts of the northern states of Punjab, Haryana, Uttar Pradesh, Madhya Pradesh and Rajasthan.