The Capital on Wednesday was cloaked in gray hazy skies, reminiscent of days at this time of the year when the AQI crosses 350. But it wasn’t. The capital’s 24-hour average air quality index stood at 279 in the “poor” category at 4 p.m. Wednesday, down from 294 on Tuesday and 301 the previous day, according to the Central Pollution Control Board.
Experts say this could be because of meteorological conditions that trap pollutants at low altitudes, and measurement anomalies.
Experts said the apparent contradiction could stem from stagnant atmospheric conditions that prevent pollutant dispersion regardless of emission levels. Temperature inversion — a phenomenon where warm air traps cooler, polluted air near the surface — has intensified as seasonal temperatures drop, creating a visible haze from suspended particulate matter.
“The inversion effect is what leads to a visible haze, even though pollution exists all throughout the year,” said Shahzad Gani, assistant professor at the Indian Institute of Technology Delhi’s Centre for Atmospheric Sciences. “Visibility goes down because solid and liquid aerosol particles get suspended in the atmosphere and scatter light.”
Wind speeds too fell, dropping below 5 kilometres per hour during evening and night hours, according to the Air Quality Early Warning System (EWS). The system reported a ventilation index of just 2,200 square metres per second on Wednesday — well below the 6,000 threshold considered necessary for adequate pollutant dispersion.
“Wind speed has decreased further in the last two days, which naturally leads to a higher build-up of pollution,” said Mahesh Palawat, vice president at Skymet Weather. “The haze somewhat dissipates in the morning hours as more sunlight penetrates the atmosphere, but as soon as the sun sets, the haze starts accumulating again in the evening.”
The mixing depth — the height of the atmospheric layer where pollutants can disperse — measured just 1,500 metres on Wednesday and is forecast to drop to 1,350 metres on Thursday, further concentrating particulates near ground level.
Another phenomenon likely influencing the haze is a drop in temperatures. Wednesday’s minimum temperature dropped to 18.2 degrees Celsius from 20 degrees the previous day, while the maximum climbed to 29 degrees from 26.4 degrees — a widening diurnal gap typical of late October that exacerbates temperature inversion.
“The further dip in minimum temperature helps in building up the layer of haze, as high levels of pollution settle near the lower part of the atmosphere,” said Sunil Dahiya, founder of environmental think tank Envirocatalysts.
But some experts also questioned whether recent air quality improvements reflect actual conditions or measurement anomalies.
“There might be some amount of gap or even slight lack of transparency in this regard,”admitted Dahiya. “If that has persisted, there might be a chance that the data we are seeing is not reflective of the pollution levels in the city currently.”
Dahiya cited recent incidents where authorities have been seen using mist and water sprayers close to weather stations.
Madhavan Nair Rajeevan, former secretary of the ministry of Earth Sciences, said measurement discrepancies and local weather dynamics can occasionally distort air quality representations. “The AQI trends we are seeing and the pollution spikes that we are noting might theoretically not correspond to each other,” he said, but cautioned that more comprehensive data would be needed to confirm any systematic measurement issues.
The Air Quality Early Warning System forecast air quality would remain in the poor category through Friday before deteriorating to “very poor” on Saturday. The outlook for the following six days indicates continued poor to very poor conditions as pre-winter atmospheric stagnation deepens.
The India Meteorological Department expects the haze to persist during early morning and late evening hours through the week as particulate matter continues to accumulate near ground level. Maximum temperatures are forecast to remain 2-4 degrees below normal through Thursday before approaching normal levels by the weekend, while minimum temperatures will stay 1-3 degrees above normal.
The atmospheric conditions reflect the onset of Delhi’s annual pre-winter pollution crisis, when falling temperatures create temperature inversions and northwesterly winds carry smoke from farm fires in neighbouring states.
