NEW DELHI: The city experienced its most pristine first half of Dec on record, with only one ‘very poor’ air quality day so far this month. Delhi’s average air quality index (AQI) during Dec 1-15 this year was 238, by far the lowest for this period since AQI measurements began in 2015 and the first time that the average was below 300.
This was also the coldest first fortnight of Dec in at least 14 years, with the average minimum temperature at 8.2 degrees C, 1.1 degrees below normal. While the previous lowest since 2011 was 8.4 degrees C in 2022, the average minimum in all other years has been above 9 degrees C.
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The main reason for both – relatively clean air and colder nights – is strong northwesterly winds that have persisted for most days of Dec so far.
Dec AQI averaged over 300 in previous years
In previous years, the average AQI in the first half of December has consistently exceeded 300, with the previous lowest being 301 in 2022. The corresponding period last year registered 327.
The CPCB categorises AQI values as follows: 0-50 “good”, 51-100 “satisfactory”, 101-200 “moderate”, 201-300 “poor”, 301-400 “very poor”, and above 400 “severe”.
Weather specialists noted that extended gaps between western disturbances in December enabled sustained northwesterly winds to cleanse the air. Strong, steady winds across northwest India, combined with reduced moisture and fog, contributed to improved AQI readings.
The cold winds and absence of fog also led to clear days and a sharp fall in night temperatures.
“In Nov, we had a few feeble western disturbances which nevertheless did influence the weather pattern. Thus, there were lots of variation in winds and speeds were slow. In Dec, there was a prolonged gap between WDs. This allowed winds to catch up and ventilate the pollutants,” said Mahesh Palawat, vice-chairman, climate change and meteorology, Skymet.
Weather experts said December typically experiences moderate to dense fog, hindering pollutant dispersal. Wind speeds have averaged 8-10kmph, occasionally reaching 20kmph. Delhi-NCR has experienced predominantly clear skies, with limited moisture in the plains resulting in reduced fog formation compared to mid-November.
Historical data shows average AQI readings between December 1-15 were: 322 (2021), 335 (2020), 321 (2019), 330 (2018), 304 (2017), 367 (2016), and 323 (2015).
This December has seen six moderate air quality days, setting a new record. Previously, the highest count was three days each in 2022, 2019, and 2015. December has never recorded air quality better than moderate.
The sole ‘very poor’ air day this month occurred on Dec 8, when the AQI rose to 302. It was in the ‘poor’ zone on eight days, including Sunday (294). Saturday recorded ‘moderate’ air quality at 193.
The Union govt’s Early Warning System predicts AQI deterioration to ‘very poor’ by Monday due to decreasing wind speeds.