Delhi records best December AQI since system began in 2015

New Delhi: With an average Air Quality Index of 294, Delhi has recorded its cleanest December air since the introduction of the AQI system in 2015.

This improvement can be attributed to strong, consistent winds in the first half of the month and record-breaking rainfall in the second, officials said.

According to data from the Central Pollution Control Board (CPCB), December marked Delhi's cleanest first half ever, with only one 'very poor' air day recorded between December 1 and December 15.

On Monday, the AQI stood at 173, in the "moderate" category, a significant improvement from Sunday's 225 (poor). This marked the eighth moderate air quality day this month, with the capital previously recording no more than three moderate days in December.

The lowest AQI of this month was recorded on December 28, at 139, marking the cleanest December day on record.

The average AQI during this period stood at 238, significantly lower than the usual AQI of over 300 for this time of year.

In comparison, Delhi's average AQI was 348 last December, 319 in 2022, and 336 in 2021. The previous lowest December AQI was 300, recorded in 2015.

Additionally, the month is set to conclude with at leats 53.5 mm of rainfall, making it the fifth-wettest December since 1901. The city also recorded its highest single-day December rainfall in 101 years, with 41.2 mm on December 28 (Friday).

This year, Delhi experienced six rainy days in December, compared to just one in 2023, none in 2022, four in 2021, and a single rainy day in December 2020.

In total, Delhi recorded 53.5 mm of rainfall this month, with 41.2 mm occurring in a single day. In contrast, December 2023 and 2022 saw no rainfall, while 2021 recorded 9.6 mm, and 2020 recorded 1.6 mm.

According to the India Meteorological Department (IMD), the highest-ever single-day rainfall in December was 75.7 mm, recorded on December 3, 1923.

While the nights were pleasant, with an average minimum temperature of 8.4 degree celsius, consistent with the long-period average (LPA) for December, the days were slightly warmer, with an average maximum temperature of 23.1 degree celsius -- 0.3 notches above the LPA.

This marks the coldest December since 2021, when the average maximum temperature was 22.4 degree celsius, according to IMD officials.

Experts attributed the favourable air quality in December to the favourable dispersion of pollutants, particularly in the first two weeks of the month.

However, the third week saw a slowdown in winds, causing six severe air quality days between December 17 and 23. This was due to a western disturbance that brought added moisture, which led to stagnation in the air.

Mahesh Palawat, vice president at Skymet, explained that winds in early December helped disperse pollutants effectively, preventing dense fog, and maintaining relatively clean air.

On Monday, Delhis AQI stood at 173, in the "moderate" category, a significant improvement from Sunday's 225 (poor).

This marked the eighth moderate air quality day this month, with the capital previously recording no more than three moderate days in December.

The lowest AQI of the month was recorded on December 28, at 139, marking the cleanest December day on record.

The highest AQI recorded in December was 451, which occurred on December 19.

The CPCB categorises AQI as follows: 0-50 (Good), 51-100 (Satisfactory), 101-200 (Moderate), 201-300 (Poor), 301-400 (Very Poor), and above 400 (Severe).