Delhi’s Longest September Streak of Moderate Air Quality Since 2016 Ends as Weather Shifts
Image: AI-Skymet
Delhi’s September 2025 was unlike the city’s usual late-monsoon rhythm. For 18 consecutive days, from September 12 to 29, the Capital’s air quality remained trapped in the “moderate” category — the longest uninterrupted September stretch since 2016. This period wasn’t marked by dramatic pollution spikes but rather by a stubborn plateau, where pollution levels hovered just above the 100 AQI mark, refusing to budge. A shift in weather patterns, showers, and gusty southeasterly winds finally broke the cycle.
A Meteorological Hold on Pollution
September’s weather in Delhi was unusually dry toward the end of the monsoon season. Total rainfall stood at 136.1 mm — about 10% above normal — but crucially, showers tapered off earlier than expected. Typically, monsoon rain acts as a natural cleanser, washing out dust and particulate matter. This year, however, the monsoon withdrew on September 24 — the earliest retreat since 2002. The sudden transition to dry conditions meant that routine emissions were no longer being flushed out effectively, allowing AQI levels to linger in the moderate range.
Data shows that September’s monthly mean AQI settled at 104.7, similar to previous years. What made 2025 stand out was not the overall pollution level but the consistency — nearly three weeks of stagnant air quality without a single break into the “satisfactory” category. Historical trends reinforce this pattern: the latter half of September is typically more polluted than the first. Since 2016, two-thirds of days between September 21 and 30 have breached the 100-AQI threshold, compared with about half in the first 10 days.
Heat, Clear Skies, and Chemistry
The weather further amplified pollution levels late in September. Temperatures soared, with Delhi recording 38.1°C on September 28 — the hottest September day in two years — followed by 37.5°C the next day. Clear skies and high solar radiation triggered photochemical reactions, converting nitrogen oxides from traffic into ground-level ozone. This pollutant emerged as a key driver of moderate air quality levels through the latter half of the month. Without rainfall or strong winds to disperse pollutants, the city’s atmosphere became a chemical laboratory, with ozone concentrations spiking during the afternoons.
Early October: Rains Return, Air Resets
Post-monsoon withdrawal, Delhi usually enters a calmer atmospheric phase with little rainfall. October is generally a benign month, averaging just 14.1 mm of rain. This year, though, the city has already clocked 52 mm of rainfall in the first week of October, including 38 mm on October 1 and another 14 mm the following morning. These showers, along with gusty winds, provided a much-needed atmospheric reset after weeks of stalled pollution.
Multiple weather systems are currently interacting over northern India. A low-level cyclonic circulation over north Rajasthan, a western disturbance approaching from the west, and deep depressions over both the Bay of Bengal and Arabian Sea are jointly steering moisture into Delhi’s atmosphere. Their combined influence has set up an east–west trough extending from north Rajasthan to southwest Uttar Pradesh, crossing the Delhi region. This configuration has triggered scattered rain and thundershowers, with heavier, widespread rainfall expected between October 5 and 7. Thunderstorms and gusty winds are likely to enhance vertical mixing and pollutant dispersion, helping improve air quality significantly.
Current AQI Status: A Transitional Phase
On October 3, Delhi recorded clear skies with temperatures between 25.9°C and 33.9°C, moderate air quality, and a light breeze around 13 km/h. Humidity hovered near 59%, adding a slight mugginess, but atmospheric ventilation improved. After the prolonged September plateau, the city is entering a transitional air quality phase — shifting from persistent moderate AQI levels toward more dynamic, weather-driven variations.
The cumulative impact of 18 days of uninterrupted moderate air quality has not been negligible. Prolonged exposure to AQI between 101 and 200 can aggravate respiratory conditions, especially among children, the elderly, and those with pre-existing heart or lung diseases. While pollution levels were not extreme, the absence of clean-air breaks magnified health risks over time. This kind of sustained moderate pollution is increasingly becoming a defining feature of Delhi’s late-monsoon season.
Why This Matters
Delhi’s September air quality story highlights how pollution levels are not driven by emissions alone but are closely intertwined with meteorology. Rainfall timing, wind direction, temperature swings, and atmospheric chemistry all shape the pollution profile. As climate variability intensifies and monsoon patterns become less predictable, these transitional windows may lengthen, making “moderate streaks” more frequent. This underscores the need for adaptive pollution management strategies that align closely with weather behavior.
