Buoyed by rain, Delhi clocks 22 consecutive days of ‘satisfactory’ air

New Delhi
Propelled by incessant rainfall over the past month, Delhi recorded 22 consecutive days of “satisfactory” air quality till Sunday, according to Central Pollution Control Board (CPCB) data, marking the cleanest spell of air quality in the Capital since 2020, when a combination of Covid-19 lockdown and rainfall set a record streak of 51 consecutive days of good or satisfactory air quality.
Experts said good wind speed and a combination of rain throughout August has helped wash out pollutants, not allowing the air quality to spike drastically. So far this month, Delhi has recorded rain on all days, except August 3. This also means Sunday was the 15th consecutive day of rain Delhi clocked, which is the longest such streak in at least 13 years, according to India Meteorological Department (IMD) data available since 2011.
“What we are seeing since end of July and now through August is the role of good consistent rain and spells of strong wind speed in between. If there is sufficient rain, particulate matter settles down and only if there are dry spells in between that we see AQI gradually returning to moderate. As rain has been consistent, we are not seeing that spike; even gases like O3, CO and NO have been kept in check by good wind speed,” said Anumita Roychowdhury, executive director, research and advocacy at the Centre and Science and Environment (CSE).
To be sure, the streaks are based on consecutive days where the air quality index (AQI), as calculated by CPCB, has been below 100. An AQI of 0-50 is classified “good”, 51-100 “satisfactory”, 101-200 “moderate”, 201-300 “poor”, 301-400 “very poor” and over 400 “severe”, according to CPCB.
If 2020 is excluded, data shows this is the longest such spell of consecutive “good” or “satisfactory” air days in Delhi since CPCB launched AQI in 2015. The 51-day streak in 2020 was recorded between July 16 and September 4, a period where monsoon rain and limited outdoor activity due to the pandemic helped Delhi. This streak included four “good” air days too.
On Sunday, Delhi recorded an AQI of 82, a satisfactory air day, marking the streak that started on July 28, when the AQI was 83.
Parts of the Capital received a drizzle on Sunday, with overcast skies dominating the skyline. The Safdarjung weather station, representative of Delhi’s weather, recorded “trace” rainfall till 5.30pm on Sunday.
Delhi’s maximum temperature was 34.6°C, a degree above normal. The minimum was 27°C, which was also a degree above the normal. The average wind speed was 5-10 km/hr, officials said. For Monday, drizzle to very light rain was predicted, with similar temperatures.
The AQI streak of 22 days this year also eclipses the 20-day streak recorded from September 7 to 26 in 2021. Other long relatively cleaner air spells include a 15-day period in August 2019 (August 6 to 20) and a 14-day period in 2017 (July 20 to August 3).
Meteorology at play
Delhi generally breathes its cleanest air in July and August, when it receives the highest amount of rain. Historically, August is Delhi’s wettest month, when it averages 233.1mm of rainfall, followed by July, which clocks an average of 209.7mm of rainfall. This July, Delhi recorded 203.7mm and has already received 240.2mm in August.
“This latest streak shows air pollution is seasonal in nature and is linked, to a large extent, with meteorological conditions. In the monsoon period, settling down of pollutants is linked to the intensity and duration of rain. Fortunately, these factors have been favourable this year and with more rain likely, this should continue,” said Dipankar Saha, former head of CPCB’s air laboratory.
During this monsoon, which includes the rain recorded in June, Delhi has received 687.3mm of rainfall, which is already higher than the monsoon average of 640.4mm that is recorded from June to September. Forecasts show Delhi is likely to add to this streak, as well as monsoon total with drizzle to light rain expected till August 24.
Mahesh Palawat, vice president, Skymet meteorology, a private forecaster, said that until Thursday, the monsoon trough was hovering near Delhi-NCR, but has moved away, which will reduce rain intensity. “The trough was also away for most of July, so we only saw sporadic spells. In August, it has remained close to Delhi-NCR and has brought rain on almost all days this month,” he said, citing rain and good wind speeds for the improved air quality.