Over the past two days, Kolkata witnessed a fascinating and somewhat counterintuitive demonstration of how weather patterns directly influence the city's air pollution levels. The relationship between atmospheric conditions and air quality played out in a quirky manner, highlighting the complex dynamics that govern urban environments.
A Windy Tuesday Offsets the Winter Chill
On Tuesday, residents woke up to cooler conditions, a typical winter scenario that often traps pollutants and worsens air quality. However, the day told a different story. Moderate wind speeds ensured sufficient dispersion of pollutants, preventing them from accumulating near the ground. As a result, most air quality monitoring stations across the city reported air in the 'moderate' category throughout the day. This episode served as a clear example of how favourable wind conditions can effectively counteract the pollution-enhancing effect of lower temperatures.
Wednesday's Sunny Deception: The Inversion Effect
The script, however, flipped completely on Wednesday. The day dawned with clear skies and bright sunshine, which pushed daytime temperatures higher. Such conditions are generally associated with better air dispersion. Paradoxically, a strong thermal inversion layer was at its peak, acting like a lid that sealed pollutants close to the ground. Despite the pleasant weather, pollution levels climbed at several monitoring stations as the day progressed, creating a stark contrast between visual comfort and invisible atmospheric stress.
Divergent Local Trends Across the City
This quirky behaviour was clearly mirrored in the data from the continuous ambient air quality monitoring system (CAAQMS). The city did not experience a uniform trend. Instead, different localities reported vastly different patterns, defying simple expectations.
At Ballygunge, the Air Quality Index (AQI) showed a gradual but steady rise, moving from 264 at 6 am to 298 by 6 pm, remaining in the 'poor' category all day. A similar pattern was observed in Jadavpur, where the AQI crossed into the 'very poor' range by afternoon, climbing from 296 in the early morning to 329 at 2 pm, before easing slightly by evening.
In contrast, areas around the Rabindra Bharati University (RBU) recorded high morning values close to 295, but the AQI improved by evening, dropping to 275. This suggests localized ventilation later in the day, possibly aided by a change in wind direction or a reduction in emission intensity. Victoria also showed a modest evening improvement after an afternoon peak.
Salt Lake remained relatively stable, with its AQI fluctuating within the 'moderate' band, indicating consistently better dispersion conditions in that zone. Stations at Fort William and Rabindra Sarobar displayed mixed behaviour, with afternoon deterioration followed by a partial recovery in the evening.
Scientists Point to Micro-Meteorology
Scientists from the Pollution Control Board (PCB) explained that such divergence is a reflection of micro-meteorological differences across Kolkata's neighbourhoods. Factors like wind corridors, local traffic patterns, and the presence of green cover can override broader city-wide weather signals. Wednesday's pattern was a potent reminder that a sunny, warm day does not automatically guarantee cleaner air, as thermal inversion can effectively trap pollutants.
The two-day episode in Kolkata reinforces the intricate and often unpredictable relationship between urban weather and pollution, underscoring the need for hyper-local monitoring and tailored solutions.
