September marked by cleaner air in Delhi, warmer nights and shortest monsoon in years

The National Capital is ending September with three notable weather markers: its cleanest air of the month in five years, warmest nights in six years and the shortest monsoon stay over the city in 14 years, data showed on Tuesday.

The city’s average Air Quality Index (AQI) stood at 102 this September till Tuesday, the lowest since 2021, when the monthly average was 78.

The daily AQI ranged from 53 to 178, with 12 days recording “moderate” air quality and 16 days “satisfactory” till September 28, according to an analysis by the Centre for Research on Energy and Clean Air (CREA).

“Delhi’s cleaner monsoon air is not just about rainfall. Wind speed and direction, stronger atmospheric mixing and the broader meteorological conditions across Delhi-NCR all play a role in dispersing pollution,” CREA analyst Manoj Kumar told PTI.

That explains why the city’s air remained relatively clean even though rainfall was lower than last year’s. With the monsoon withdrawing and these meteorological conditions changing, pollution can begin to build up in October, he said.

In September 2021, Delhi had 27 “satisfactory” and three “moderate” days, with the AQI ranging between 60 and 122. The average AQI was 104 in 2022, 108 in 2023, 105 in 2024 and 105 in 2025.

The cleaner air, however, came against the backdrop of unusually warm nights. Delhi’s average minimum temperature in September stood at 25.5 degrees Celsius, making it the warmest September since 2020, when the average minimum was 25.7 degrees Celsius.

The average minimum temperature was also 25.5 degrees Celsius in 2021, while the average maximum temperature this month remained around 34.1 degrees Celsius.

“The reason for the high nighttime temperatures this year is the impact of El Niño and weak rainfall. For a major part of September, there was no rain or only very scattered rainfall. In fact, till September 25, the country had a 20 per cent deficiency in rainfall,” Mahesh Palawat, vice-president of private weather agency Skymet, told PTI.

He said the scattered rain in the last few days had brought down temperatures, but for most of the month, temperatures remained well above normal, keeping nighttime temperatures unusually high.

The southwest monsoon, meanwhile, made a relatively early exit from the Capital, withdrawing on September 23 after an 83-day stay that began on July 2. It was the shortest monsoon stay over Delhi in 14 years.

The monsoon remained over Delhi for 87 days in 2025, 96 days in 2024 and 97 days in 2023. The last time it stayed for fewer days was in 2012, when it arrived over Delhi on July 7 and withdrew on September 25 after a stay of 80 days, according to the India Meteorological Department (IMD).

This year’s withdrawal came two days ahead of the normal date of September 25, while the monsoon had arrived over Delhi five days later than its normal onset date of June 27.

Despite its shorter stay, the monsoon left the city with above-normal rainfall. Safdarjung recorded 728.6 mm of rainfall between June 1 and September 23, against the normal 629.7 mm, making the seasonal rainfall 16 per cent above normal.

The shorter monsoon stay did not translate into a dry September either. Delhi recorded four rainy days this month till September 28, with the highest rainfall of 37.3 mm recorded on September 5.

The cooler conditions in the final days of the month were also reflected in Tuesday’s readings.

Safdarjung recorded a maximum temperature of 32.4 degrees Celsius, 2.3 notches below normal, while Palam recorded 30.6 degrees Celsius, 4.5 notches below normal.

Lodi Road recorded a maximum of 32.4 degrees Celsius, 0.6 notches below normal, while the Ridge recorded 31.8 degrees Celsius, 3.1 notches below normal. Ayanagar recorded 32.4 degrees Celsius, 2.2 notches below normal.

The minimum temperature at Safdarjung was 21 degrees Celsius, 2.3 notches below normal and the lowest for September since 2008, according to the IMD.

Palam recorded a minimum of 19.7 degrees Celsius, 3.8 degrees below normal, while Lodi Road recorded 20.4 degrees Celsius, 1.6 notches below normal. The Ridge recorded 16.9 degrees Celsius, 6 degrees below normal, while Ayanagar recorded 20.6 degrees Celsius, 2 degrees below normal.

The AQI in Delhi stood at 71 on Tuesday, in the “satisfactory” category, according to the Sameer app.

According to the Central Pollution Control Board, an AQI of 0-50 is considered “good”, 51-100 “satisfactory”, 101-200 “moderate”, 201-300 “poor”, 301-400 “very poor” and 401-500 “severe”.

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