Rare foray into ‘moderate’ zone on a windy winter day in Delhi

Millions of Delhi residents received a rare reprieve from pollution on Wednesday as strong winds and relatively warm weather pushed the air quality into the “moderate” category – giving the Capital its cleanest air day in nearly two months, and cleanest air day in December in nearly two years.

With a 24-hour average air quality index (AQI) falling to 178 (“moderate”) at 4pm on Wednesday, city residents experienced a rare sight – clear skies, fresh air, and long-range visibility, if one could find a vantage point high enough.

This marked a significant improvement from Tuesday’s “poor” AQI of 268, according to Central Pollution Control Board (CPCB) data.

The last time Delhi had better air was on October 12, with an AQI of 155, also in the “moderate” category. For the month of December, the city hasn’t recorded such clean air since December 14, 2022, when the AQI was 163.

The respite is expected to continue for at least another day, with wind speeds ranging from 10-20 km/hr likely to persist around the National Capital Region (NCR).

However, a slight rise in pollution is forecast after December 7 as a western disturbance could lead to the return of fog and slower winds, according to the Early Warning System for Delhi (EWS) and the India Meteorological Department (IMD).

“Like the past few days, the AQI has been improving due to consistent winds and lack of moisture in the air. Thus, we are seeing little fog and ample sunshine during the day,” said Mahesh Palawat, vice-president at Skymet Meteorology, stating wind speeds touched 20 km/hr during the day on Wednesday.

The improvement in air quality follows three consecutive “poor” air days, with the AQI steadily dropping from 285 on Sunday to 268 on Tuesday. Just weeks ago, on November 18, Delhi recorded an AQI of 494, one of the most polluted days since record-keeping started for AQI.

The lowest AQI recorded last year’s December was 286, while the highest was 450 on December 23.

Typically, Delhi experiences a spike in pollution during the second half of December, as temperatures dip below 5°C, wind speeds drop, and dense fog settles over the region.

CPCB classifies an AQI reading between 0 and 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”.

CPCB data showed the day’s average AQI at 4pm was calculated based on 38 out of 40 ambient air quality monitoring stations. Only one station – Aurobindo Marg – reported a “very poor” AQI of 305, while most stations recorded “moderate” or “poor” levels. The lowest AQI of 131 was observed in Narela, north Delhi.

Palawat said since then, stubble burning’s contribution kept reducing and is now completely over. Meanwhile, meteorological conditions have become favourable. “We had dense fog and no winds then. Now, we have no fog and winds are fairly strong too,” he added.

A senior IMD official said strong winds ranging from 10-20km/hr have been forecasted across Delhi-NCR, with these expected to continue till December 7.

“A western disturbance is expected to then start influencing the region, which may lead to shallow to moderate fog conditions returning from December 8 onwards. The wind speed may also drop then,” said the official.

Wednesday’s weather also brought relatively mild temperatures, with the minimum at 12°C and the maximum at 28°C, both around three degrees above normal. The IMD has forecast a warmer-than-usual December for Delhi and northwest India, with average temperatures expected to remain above normal.