Delhi's air quality turned "severe" on Tuesday with slow wind speed and low temperature allowing accumulation of pollutants, according to weather department officials.
The air quality is likely to remain "severe" on Wednesday too. No major improvement is predicted till December 26, Kuldeep Srivastava, the head of the India Meteorological Department's regional forecasting centre, said.
The city's 24-hour average air quality index (AQI) was 418, which falls in the severe category. It was 332 on Monday, 321 on Sunday and 290 on Saturday.
Air pollution in the neighbouring cities of Faridabad (407), Ghaziabad (468), Greater Noida (458) and Noida (450) also entered the "severe" zone.
An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".
Srivastava attributed the dip in the air quality to slow wind speed, low temperature, and high humidity due to a Western Disturbance.
Low temperature makes the air heavier, trapping pollutants close to the ground, he said.
On Tuesday, the minimum temperature was 5.3 degrees Celsius and the maximum wind speed 8 kmph.
The central government's Air Quality Early Warning System for Delhi said the city's ventilation index, a product of mixing depth and average wind speed, was 2,000 m2/s on Tuesday and is likely to be 2,500 m2/s on Wednesday.
Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed.
A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.
