The cool, quiet hour after dawn feels like the best time to step outside in October, yet AIIMS says pollution stays highest in the early morning until about 8:30 or 9 am.

Delhi's air shows how quickly things can turn. At 4 pm on Tuesday, 20 October, the Central Pollution Control Board put the Air Quality Index (AQI) at 179, which counts as moderate. By 7 am the next day, the Air Quality Early Warning System showed 219, which counts as poor, and it forecast 342 for 22 October.

The invisible lid above the city

The explanation lies in physics. Normally, temperature falls with height, so air near the ground mixes and carries pollutants upwards. University of Reading researchers Ankit Bhandekar and Laura Wilcox explain that a temperature inversion reverses this, placing warmer air above colder air like a lid on a pot. 

Pollutants stay near the ground, and the mixing height, the level up to which they can disperse, shrinks from about one kilometre in summer to a few hundred metres in winter.

The lid sits lowest after dark. Sarath Guttikunda, founder of UrbanEmissions.Info, told Frontline that in November and December the inversion height can fall below 50 metres at night and hover around 200 to 500 metres by day. As the sun climbs, the layer lifts and the smog thins, which is why the early hours feel the worst.

A bowl that holds on to smoke

Delhi's geography makes this even worse. The city sits in the Indo-Gangetic plains, with the Himalayas to the north and the smaller Aravalli range to the south, which help create a natural bowl. 

The Himalayas block air from flowing northward, so pollution must travel across northern India before it finds an exit over the Bay of Bengal. The monsoon rains that cleanse the air........

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