Delhi on Thursday recorded an air quality of ‘very poor’ category but government agencies expected it to improve with the wind speed picking up.

The Air Quality Index (AQI) at 9 am was 374 on Thursday, while the 24-hour average on Wednesday was 413. It was 379 on Tuesday and 295 on Monday, according to the Central Pollution Control Board data.

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".

However, the air quality in Delhi-NCR is likely to improve to the lower end of the ‘very poor’ category over the day as the wind speed picks up, said the central government's Air Quality Early Warning System for Delhi.

The predominant surface wind direction was northerly and the maximum wind speed is likely to be 10 kmph on Thursday, it said, adding that the air quality is likely to reach the poor category on Friday.

Ministry of Earth Sciences' air quality monitor, SAFAR, said stubble burning share in PM2.5 in Delhi's air was just 2 per cent on Wednesday.

The minimum temperature was 10.4 degrees Celsius on Thursday. The maximum temperature is likely to settle around 23 degrees Celsius, the weather department said.

Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.

Delhi's ventilation index -- a product of mixing depth and average wind speed was around 8,000 m2/s on Thursday and is likely to be 7,000 m2/s on Friday.

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.