On Wednesday, Delhi woke up under a familiar gray haze that blanketed the city in a dull, smoggy pall. To the naked eye, the conditions resembled the city’s notorious pollution peaks when the Air Quality Index (AQI) exceeds 350. Yet, according to the Central Pollution Control Board (CPCB), the 24-hour average AQI at 4 p.m. was 279, falling in the “poor” category. The previous two days had slightly higher readings — 294 on Tuesday and 301 on Monday — but none reached the extreme levels one might expect given the visible smog.
The apparent disconnect between what residents see and what sensors report has left many puzzled, and experts point to a combination of meteorological phenomena, atmospheric stagnation, and measurement quirks as the underlying factors.
Temperature Inversion: The Hidden Culprit
One of the main explanations is a temperature inversion, a meteorological phenomenon where a layer of warm air traps cooler air near the surface. Under normal conditions, air near the ground rises and mixes with upper layers, dispersing pollutants. During an inversion, however, this vertical mixing is restricted, leaving particulate matter suspended close to the surface, making the haze visually dense even if AQI readings are moderate.
“The inversion effect is what leads to a visible haze, even though pollution exists all throughout the year,” explained Shahzad Gani, assistant professor at the Indian Institute of Technology Delhi’s Centre for Atmospheric Sciences. He added that suspended solid and liquid aerosol particles scatter light, reducing visibility and giving the impression of worse air quality than sensor data indicates.
Stagnant Air and Weak Winds
Meteorological conditions over the past few days have further compounded the effect. Wind speeds fell below 5 km/h during evening and night hours, according to the Air Quality Early Warning System (EWS). Low wind speeds limit the horizontal dispersion of pollutants, effectively trapping them over the city.
The EWS reported a ventilation index of just 2,200 m²/s, far below the 6,000 m²/s generally considered necessary for adequate pollutant dispersion. As a result, even with AQI numbers in the high 200s, particulate matter accumulates near ground level, producing thick, gray haze reminiscent of winter smog.
“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.”
Shallow Mixing Layer Height
Another factor influencing the visible haze is the mixing depth, or the height of the atmospheric layer in which pollutants can disperse. On Wednesday, this was measured at just 1,500 metres, and forecasts indicate a further drop to 1,350 metres on Thursday. A shallower mixing layer concentrates pollutants closer to the ground, intensifying the visible haze and reducing air clarity.
Temperature Fluctuations Worsen the Haze
Daily temperature variations also play a role. On Wednesday, Delhi experienced a minimum temperature of 18.2°C, down from 20°C the previous day, and a maximum of 29°C, up from 26.4°C. Such widening diurnal gaps are typical in late October and exacerbate 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. He explained that these pre-winter conditions naturally concentrate particulate matter, even when emission levels haven’t drastically increased.
Are AQI Readings Accurate?
While meteorology explains much of the haze, some experts caution that AQI numbers themselves may not always fully reflect ground realities.
“There might be some amount of gap or even slight lack of transparency in this regard,” said Dahiya. He noted that water sprayers or misting near monitoring stations, as occasionally observed, could affect sensor readings, resulting in AQI numbers that understate actual pollution levels.
Madhavan Nair Rajeevan, former secretary of the Ministry of Earth Sciences, also acknowledged that local weather dynamics and measurement discrepancies can occasionally distort AQI readings. “The AQI trends we are seeing and the pollution spikes that we are noting might theoretically not correspond to each other,” he said, stressing that more comprehensive data would be needed to confirm systematic measurement issues.
Forecast and Outlook
The Air Quality Early Warning System predicts that air quality will remain in the poor category through Friday, deteriorating to “very poor” by Saturday. The India Meteorological Department (IMD) expects haze to persist during early morning and late evening hours, when particulate matter accumulates near ground level.
Temperature forecasts suggest that maximum temperatures will remain 2–4°C below normal through Thursday before approaching seasonal averages over the weekend, while minimum temperatures will stay 1–3°C above normal. These conditions, combined with stagnant winds and temperature inversion, create the familiar pre-winter pollution scenario that Delhi experiences every year.
The Pre-Winter Pollution Crisis
The atmospheric conditions observed in Delhi reflect the annual pre-winter pollution phenomenon. As temperatures fall, northwesterly winds carry smoke from crop residue burning in neighboring states into the National Capital Region. Stagnant air near the surface, low mixing depths, and weak winds exacerbate the effect, producing a haze that can visually resemble AQI readings well above 350.
Experts emphasize that even if AQI numbers appear moderate, the combination of temperature inversion, low wind speeds, and accumulated particulate matter can create hazardous visibility conditions, underscoring the need for continued vigilance and pollution mitigation measures.
In short, Delhi’s current gray skies are not necessarily indicative of catastrophic emission levels but rather a perfect storm of atmospheric conditions that trap pollutants near the surface. As pre-winter stagnation intensifies, the city can expect days where the haze looks worse than the numbers suggest, until winds pick up and temperatures stabilize.
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