Delhi's AQI improved to 234 from 271 due to sustained winds reaching 22 kmph, but forecasters warn air quality may slip back to 'Very Poor' by December 27.
Strong and sustained winds for the second consecutive day helped Delhi's air quality index (AQI) improve further on Thursday, bringing the average AQI down to 234 from Wednesday's 271, both in the 'Poor' category.
The winds helped clear fine particulate matter that usually lingers close to the ground during winter. The AQI fell in a similar range in some other regions of the National Capital Region (NCR) as well, with Noida's tally falling to 256 from Wednesday's 273. Gurgaon, meanwhile, at 239, saw a marginal fall in AQI, compared to 236 a day earlier.
This marked a sharp improvement from Tuesday, when Delhi and Noida's AQI had deteriorated to enter the 'Severe' category, while Gurgaon's was in the 'Very Poor' category, Central Pollution Control Board's (CPCB) data had shown.
However, forecasts suggest this relief may not last. Wind speeds are expected to weaken over the weekend, while foggy mornings will return, limiting the atmosphere's ability to disperse pollutants. The ventilation index, which measures how easily pollution can spread, is predicted to remain below the favourable threshold of 6000 m²/s. As a result, the IITM has warned that Delhi's air quality is likely to slip back into the 'Very Poor' category by December 27 and remain there for several days.
According to the India Meteorological Department (IMD), west-southwesterly winds, reaching speeds of up to 22 kmph, were the key factor behind this dip.
Such winds improve the ventilation capacity of the atmosphere, which refers to how effectively the air can mix and disperse pollutants. In winter, colder temperatures near the surface often create what experts call an "inversion layer" - a lid-like effect where warm air above traps cooler air below. This prevents pollutants from rising and dispersing, causing them to accumulate close to the ground.
Strong winds break through this lid, stirring the air vertically and horizontally, and carrying pollutants away from the city, thereby lowering their concentration.
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Delhi also saw a mainly clear sky, which allows more sunlight to reach the surface, warming the ground and promoting vertical mixing of air. This mixing helps dilute pollutants by lifting them higher into the atmosphere, reducing their density at the breathing level. In contrast, cloudy or foggy conditions suppress this process, keeping pollutants trapped near the surface.
Together, the combination of sustained winds and clear sky created favourable dispersion conditions over Delhi and its neighbouring areas. This is particularly important in Delhi, where vehicle emissions, industrial activity, and residential sources continue to pump out pollutants daily. According to IITM (Indian Institute of Tropical Meteorology) data, the transport sector alone contributed more than 17% of PM2.5 levels in Delhi on Wednesday, while industries added around 8.6%. Neighbouring districts also played a role in Delhi's pollution load. Jhajjar in Haryana accounted for over 15% of PM2.5 contribution, followed by Rohtak, Sonipat, and Gurgaon.
Further, temperature conditions supported this improvement, with the day temperature being recorded at 22 degrees Celsius in Delhi, around 2 degrees above normal. Together, these factors created a temporary window of cleaner air.
The Capital, meanwhile, recorded a minimum temperature of 6.5 degrees Celsius, one notch below normal, according to IMD.