Delhi experienced thick, gray hazy skies on Wednesday, creating an impression of severe pollution, even though the 24-hour average Air Quality Index (AQI) was recorded at 279, falling into the ‘poor’ category. This is a notable improvement from the previous days, which saw AQI levels of 294 and 301. Experts attribute this apparent contradiction between the AQI reading and the visible haze to specific atmospheric conditions and potential measurement nuances.
According to meteorologists, stagnant air and temperature inversions are key culprits. A temperature inversion occurs when a layer of warm air traps cooler, polluted air closer to the ground. As temperatures drop seasonally, this effect intensifies, leading to a visible layer of haze formed by suspended particulate matter.
Shahzad Gani, an assistant professor at IIT Delhi’s Centre for Atmospheric Sciences, explained, “The inversion effect is what leads to a visible haze, even though pollution exists all throughout the year.” He added that reduced visibility is a direct consequence of these suspended particles scattering light.
Compounding the issue, wind speeds have decreased, falling below 5 km/h during evening and night hours. The ventilation index, a measure of the atmosphere’s capacity to disperse pollutants, was a mere 2,200 square metres per second on Wednesday, significantly lower than the 6,000 threshold needed for adequate dispersion.
Mahesh Palawat, Vice President at Skymet Weather, noted, “Wind speed has decreased further in the last two days, which naturally leads to a higher build-up of pollution.” He further elaborated that while sunlight can temporarily clear the haze in the morning, it re-accumulates as the sun sets.
The mixing depth, which indicates the height pollutants can spread, was only 1,500 metres and is expected to drop further, concentrating pollutants near the ground. A widening diurnal temperature gap, typical for late October, also exacerbates the inversion effect.
Sunil Dahiya, founder of Envirocatalysts, commented, “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.”
However, some experts also raised concerns about potential measurement anomalies affecting the reported AQI figures. Dahiya pointed out that if data reflects discrepancies, it might not accurately represent the current pollution levels. He cited instances where water sprayers were used near weather monitoring stations.
Madhavan Nair Rajeevan, former secretary of the Ministry of Earth Sciences, acknowledged that local weather dynamics and measurement issues can sometimes skew air quality readings, suggesting that current AQI trends might not always align perfectly with observed pollution spikes, though more data is needed to confirm systematic issues.
The Air Quality Early Warning System predicts that the air quality will remain ‘poor’ until Friday, likely worsening to ‘very poor’ by Saturday. The forecast for the coming week indicates a continuation of poor to very poor air quality conditions due to deepening pre-winter atmospheric stagnation.
The India Meteorological Department expects the haze to persist in the early mornings and evenings throughout the week as pollutants continue to accumulate. While maximum temperatures are expected to be slightly below normal initially, they should normalize by the weekend, with minimum temperatures remaining slightly above normal.
These atmospheric conditions signify the beginning of Delhi’s annual pre-winter pollution cycle, characterized by temperature inversions and the influx of smoke from farm fires in neighboring regions carried by northwesterly winds.