Delhi has done remarkably well in air quality this December, recording an average Air Quality Index (AQI) of 294, the lowest for the month since the AQI system was introduced in 2015. Officials attribute this improvement to strong winds and record-breaking rainfall.

What caused this improvement?

According to the Central Pollution Control Board (CPCB), data shows that Delhi had the cleanest first half of December ever. The city reported only one day of "very poor" air quality from December 1 to December 15, courtesy of steady winds that carried away pollutants.

The last half of the month also helped as Delhi received 53.5 mm of rain and this is the fifth wettest December since 1901. Also, it was on December 28 when 41.2 mm of rainfall occurred in just one day. This happens to be the highest December single-day rainfall in the last 101 years.

How does this compare to previous years?

This December’s average AQI of 294 is a significant improvement from 348 in 2023, 319 in 2022, and 336 in 2021. The previous record for the lowest December AQI was 300, set in 2015.

The cleanest day during December was recorded on the 28th at just 139. In comparison, the month's high AQI reading was 451 on December 19, and that falls in the "severe" category.

Rainfall patterns also highlighted stark differences from previous years. December 2023 and 2022 saw no rain, while 2021 recorded 9.6 mm, and 2020 recorded just 1.6 mm.

What about Delhi’s temperatures?

The overall weather in the city was quite pleasant during December with an average minimum of 8.4°C that was in line with the long-period average (LPA). The average maximum temperature was, however, warmer at 23.1°C, just 0.3°C above the LPA. It made it the coldest December since 2021 when the average maximum temperature was 22.4°C.

Why was the air quality fluctuating?

Despite improvement in overall case, on six days between December 17 and 23, Delhi faced severe air quality. Experts said this was due to a decrease in the wind speed because of a western disturbance that added moisture and accompanied stagnant air conditions.

Mahesh Palawat, vice president at Skymet, mentioned the role of winds in early December.  “The winds in early December effectively dispersed pollutants, preventing dense fog and keeping the air relatively clean,” he said.

Delhi experienced eight days of "moderate" air quality this December, unlike earlier Decembers that have witnessed a maximum of three such days. On Monday, AQI for the city stood at 173 - in the "moderate" category, while on Sunday it was at 225 - "poor".

According to the CPCB, AQI categories are as follows:

0-50: Good

51-100: Satisfactory

101-200: Moderate

201-300: Poor

301-400: Very Poor

Above 400: Severe

This December’s improvement offers a glimpse of hope for Delhi residents, showing that a combination of favorable weather patterns and efforts to reduce pollution can lead to cleaner air.