Weather, the saviour until now, turns rogue

The Capital's relative reprieve from post-Diwali smog this year came to an abrupt end on Wednesday, with two critical weather patterns - a delayed winter and surface winds - suddenly turning against the city.

- Aheli Das

As dawn broke on Wednesday, Delhi's residents found themselves enveloped in a noxious haze, the air thick and acrid. This one-two punch of cooling temperatures and calm winds left residents choking on the season's first "severe" air day, as AQI levels surged past 400, plunging into hazardous territory.

Late Tuesday night, AQI in Delhi was hovering in the lower end of the "very poor" zone - the 24-hour average was around 314 at around 10pm. But as the night wore on, winds eased and temperatures dropped, leading pollutants to settle over the city like a dark grey veil.

It started with a drop in wind speed, leading to pollutants accumulating in the night sky. And as temperature dropped in the early hours of Wednesday, the two factors combined to create the noxious smog.

The smog also kicked off, for the first time this season, the cyclical phenomenon known as "inversion" - where pollutants get trapped close to the surface due to low temperature.

Inversion takes place when the surface air is cooler than the air at higher altitudes - the inverse of what usually happens. For this, a combination of both calm winds and low temperature is needed, leading to the mixing height-an invisible layer where pollutants interact-dipping and coming closer to the surface.

The lower the mixing height, lesser the space for pollutants to move freely and escape. This created a "lid-like" layer over the Capital.