India, Dec. 30 -- Dense fog enveloped Delhi on Tuesday, delaying over 150 flights, even as the air quality improved marginally and was in the higher end of the "very poor" category. The visibility dropped to 100 metres at Palam by 10:30pm on Monday, and remained 50 metres until 5:30am on Tuesday. At Safdarjung, visibility fell to 50 around midnight and remained in this range until around 8am. At 8:30am, it improved to 200 metres.

Fog is classified as shallow when visibility is between 500 and 1000 metres, moderate between 200 and 500 metres, dense between 50 and 200 metres, and very dense when it dips below 50 metres.

A red alert was issued around 10:30pm for Delhi as visibility dipped to 100 metres at Palam. A yellow alert is in place ...