Delhi breathed just three days of ‘severe’ air in November

This November’s air ranked second cleanest with an average AQI reading of 321, just a few points behind November 2019’s average AQI reading of 312

Delhi breathed some of its cleanest November air this year, with the Air Quality Index (AQI) recording only three ‘severe’ air days this month, a first since the index’s launch in 2015 However, this November’s air ranked second cleanest with an average AQI reading of 321, just a few points behind November 2019’s average AQI reading of 312.That year, the drop in AQI was attributed to late rains.

To be sure, Delhi got lucky this time, largely due to an early Diwali (which was celebrated in October), a lack of western disturbances in the second half of the month, fewer farm fires compared to last year and favourable meteorological conditions. (Raj K Raj/HT Photo)

While this November may have been cleaner than usual, it has also been warmer than usual, with a higher maximum temperature, India Meteorological Department (IMD) data shows. Delhi recorded an average maximum temperature of 28.7°C this month, 0.5 degrees above normal, and the highest for Delhi since November 2016, when it was 29.6°C. On the flipside, nights were cooler than normal, with the mercury dipping below the 10-degree mark in the last 13 consecutive days in the month and recording an average minimum temperature of 12.3°C, 0.6 degrees below the monthly normal.

To be sure, Delhi got lucky this time, largely due to an early Diwali (which was celebrated in October), a lack of western disturbances in the second half of the month, fewer farm fires compared to last year and favourable meteorological conditions.

According to RK Jenamani, scientist at IMD, November was a month of two halves—with the first half seeing above normal minimum temperature that hovered around the 17-degree mark. However, the lack of western disturbances in the second half meant mercury did not rise above the 10-degree mark after November 18.

“Delhi, in the first half of November, witnessed three western disturbances. While they were feeble, they led to a change in wind direction and an incursion of slight moisture. When there is more moisture in the air, the skies are cloudy and the minimum temperature is higher as heat is trapped close to the earth’s surface at night,” he said, adding there was no western disturbance in the second half of the month, which led to cold northwesterly winds continuously blowing towards Delhi.

Jenamani attributed the warm days and cool nights in the second half of the month to these cold, dry northwesterly winds.

“These winds are cold and dry, which means there was very little moisture in the air. Due to clear skies, the sunlight heated the earth’s surface quite easily, meaning the maximum temperature was above normal. At night, in the absence of clouds or moisture, this heat escaped quickly and the night temperature dipped below normal,” he said.

According to the long-period average (LPA) for Delhi from 1981 onwards, the Capital’s average minimum temperature in November is 9.9°C and dips below 10°C only in the last four days of the month. Delhi’s mercury, however, fell below the 10-degree mark on November 18. According to the LPA, Delhi’s maximum temperature, meanwhile, dips to 26.5°C between November 22 and 26 and further to 25.6°C in the last four days of the month. Delhi’s maximum temperature is around 27°C so far, with the Capital recording a maximum of 27.1°C on Tuesday and 26.2°C on Wednesday.

Meanwhile, despite recording just three ‘severe’ air days, Delhi still recorded 16 ‘very poor’ days and 11 poor air days, indicating that the air was still fairly harmful, even for healthy people. The Central Pollution Control Board classifies an AQI reading between 0-50 as ‘good’, between 51 and 100 as ‘satisfactory’, between 101 and 200 as ‘moderate’, between 201 and 300 as ‘poor’, between 301 and 400 as ‘very poor’ and over 400 as ‘severe’.

Delhi recorded 11 ‘severe’ air days last November, the most number of such days in November since CPCB started measuring the AQI in 2015. In November 2020, Delhi recorded nine ‘severe’ air days and in November 2019, Delhi recorded seven ‘severe’ air days. Delhi recorded five such days in November 2018, seven such days in Novembe 2017, 10 such days in Novembe 2016 and seven such days in Novembe 2015.

Anumita Roychowdhury, executive director, research and advocacy at the Centre for Science and Environment (CSE) said an early Diwali (celebrated in October this year), along with fewer farm fires (56,391 fires in Punjab, Haryana and UP this year as compared to 82,484 last year and 91,591 the year before that) helped the region, but it is not cause for celebration just yet. “We have been able to stabilise the peak pollution due to these factors, but the AQI has still been in the ‘very poor’ category, which is almost as bad as severe air. This shows our background emissions are still fairly high and more efforts are needed to control these sources of pollution,” she said.