Hot nights, hotter days: Why Delhi has been sweltering this September

The Capital also reeled under daytime heat, with Sunday’s maximum temperature rising to 38.1°C, three degrees above the seasonal average and the hottest September day in two years. The last time the maximum was higher was on September 5, 2023, at 38.6°C.

What’s behind the heat?

Though Northwest India saw 42% surplus rainfall this September, the Southwest monsoon withdrew from Delhi on September 24 — its earliest retreat since 2002. The absence of rain-bearing systems since then has reinforced the hot spell.

After the early withdrawal of the monsoon, the unusually high temperatures have been due to a mix of clear skies allowing more daytime heating and high humidity levels that slowed night-time cooling.

With the relative humidity oscillating between 56% and 76% through the day, moisture in the air acted like a blanket, trapping heat near the surface. Easterly winds kept moisture levels elevated, further curbing relief at night.

Over the past 24 hours, easterly winds with speeds up to 20 kmph have prevailed over the city, as per the IMD. The wind shift has also helped air quality by blocking smoke from farm fires up north.

According to Indian Agricultural Research Institute (IARI) data, 130 stubble-burning incidents have been reported so far this season, with 90 in Punjab, 27 in Uttar Pradesh, 6 each in Haryana and Madhya Pradesh, and 1 in Rajasthan. These have started to emerge since September 15, and their impact on Delhi’s air quality remains minimal.

Delhi’s 24-hour average Air Quality Index (AQI) stood at 139 (“moderate”) at 4 pm on Sunday, a marginal improvement from Saturday’s 169, which was also the city’s highest since June 13.

Will it get cooler?

Forecasts by the Indian Institute of Tropical Meteorology (IITM) suggest the air quality is likely to remain “moderate” through the week.

Meanwhile, the temperature conditions are expected to ease out, as per the weather department.

The IMD has forecast partly cloudy skies on Monday, followed by generally cloudy conditions with light rain or drizzle on Tuesday and Wednesday. Cloudy weather is likely to persist until October 5, gradually bringing down temperatures, with the minimum expected to dip to around 23°C and the maximum to about 32°C.