New Delhi: For a brief moment in early February, residents of the National Capital Region (NCR) enjoyed a rare respite—clear blue skies and breathable air. However, as the mercury begins its gradual climb toward the high 20s, the “clean air” dividend has vanished. On Sunday, February 15, the Air Quality Index (AQI) slipped back into the ‘poor’ category, clocking a reading of 221.
This regression raises a critical question: If the biting cold of January is over, why is Delhi’s deteriorating air quality becoming a headline once again? To understand this, one must look at the intersection of shifting wind patterns, local emissions, and the “boundary layer” physics of the atmosphere.
The February Flip: Why Delhi’s Air Quality is Tanking as Winter Fades
Understand the Meteorological Trap: Low Wind Speed and Mist
According to the India Meteorological Department (IMD), the current transition phase between winter and spring creates a specific set of atmospheric conditions that favor pollution. While daytime temperatures are hitting a comfortable 27°C, the nighttime lows remain near 11°C.
The IMD has forecast “misty mornings” for the coming week. Mist and high humidity act as a physical trap for particulate matter (PM2.5 and PM10). When moisture hangs in the air, pollutants bind to the water droplets, making them heavier and keeping them suspended near the ground rather than dispersing. Furthermore, the “ventilation index”—the product of wind speed and the height of the mixing layer—has remained low. Without strong surface winds to flush out the city’s emissions, the capital becomes a stagnant bowl of pollutants.
The Rebound Effect: Resumption of Urban Activity
During the peak of the winter “Cold Wave,” certain construction activities were halted, and the extreme weather naturally curtailed some vehicular movement. As the cold spell eases, there is a visible rebound in urban activity.
Data from the Sameer app, the official portal of the Central Pollution Control Board (CPCB), indicates that 24 out of 40 monitoring stations in Delhi are now reporting ‘poor’ levels. This uniformity suggests that the pollution isn’t just coming from a single source but is a result of the city’s baseline emissions—transportation and dust—failing to disperse. Delhi’s deteriorating air quality during this period is a reminder that while stubble burning is a seasonal villain, the city’s internal combustion is a year-round crisis.
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Atmospheric Boundary Layer: The Invisible Ceiling
A key factor analyzed by environmental scientists is the ‘Mixing Height.’ In summer, the sun heats the ground, causing air to rise high into the atmosphere, taking pollutants with it. In February, despite warmer afternoons, the earth cools rapidly at night. This creates a “temperature inversion” where a layer of warm air sits above a layer of cooler air, acting like a lid.
Until the ground temperature remains consistently high throughout the 24-hour cycle, this “lid” stays low. This prevents the vertical dispersion of smoke and dust, leading to Delhi’s deteriorating air quality. The IMD’s forecast of a generally cloudy sky on February 18 may further exacerbate this, as cloud cover can trap heat and pollutants closer to the surface during the night.
Comparative Analysis: December (Peak Winter) vs. February (Transition)
The table below explains the shift from the “Severe” smog of December to the “Poor” lingering haze of mid-February.
| Feature | December (Peak Winter) | February (Current Transition) | Impact on Air Quality |
| Average AQI | 320 – 450 (Very Poor to Severe) | 180 – 240 (Moderate to Poor) | Delhi’s deteriorating air quality persists but at lower intensity. |
| Primary Driver | Heavy Stubble Burning + Extreme Cold | Urban Baseline + Meteorological Trap | Shift from regional to local pollution sources. |
| Temperature | Lows of 4°C – 7°C | Lows of 9°C – 12°C | Warmer soil, but high night-day fluctuation remains. |
| Mixing Height | Very Low (traps air like a tight lid) | Moderate (lid is higher, but still present) | Pollutants have slightly more room but can’t fully escape. |
| Wind Speed | Often “Calm” (0–2 kmph) | Variable (5–12 kmph) | Lower wind prevents the “sweeping” of urban dust. |
| Visibility/Mist | Dense Smog (Chemical) | Morning Mist (Moisture) | Mist binds to PM2.5, keeping it suspended near the ground. |
The Path Ahead: A Volatile Transition
The weather bulletin suggests that while temperatures will stay within the normal range, the fluctuation of 1 to 3 degrees Celsius in the coming days will keep the atmosphere unstable. For the residents of Delhi, this means the threat of respiratory ailments remains high despite the departure of the “Great Smog” of November.
The current trend serves as a policy warning. It highlights that Delhi’s deteriorating air quality cannot be solved by weather-dependent miracles. As the city moves toward March, the focus must shift from emergency “GRAP” (Graded Response Action Plan) measures to permanent structural changes in how the city breathes. Until then, the “poor” AQI remains an unwelcome companion to the city’s misty February mornings.
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