Delhi's air quality was recorded in the “very poor” category on Thursday and government agencies said it is likely to improve as the wind speed picks up.

The city's air quality index was 374 at 9 am on Thursday. The 24-hour average AQI was 413 on Wednesday, 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".

The central government's Air Quality Early Warning System for Delhi said Delhi-NCR's air quality is likely to improve to the lower end of the “very poor” category on Thursday.

The predominant surface wind direction was northerly and the maximum wind speed is likely to be 10 kmph on Thursday, it said.

The central agency said the AQI is likely to reach the “poor” category on Friday.

The contribution of stubble burning in neighbouring states to Delhi's PM2.5 levels was just 2 percent on Wednesday, according to the Ministry of Earth Sciences' air quality monitor, SAFAR.

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.