Monsoon gone, heat remains: Why has Delhi recorded its hottest September morning in 6 years?

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Delhi residents sweltered through an unusually hot start to the week, with the capital recording its hottest September morning in six years. The minimum temperature on Monday settled at 28.2 degrees Celsius, a significant five degrees above the seasonal norm, according to a report by The Indian Express.

This marks the second consecutive day of intense night-time heat, following an identical reading on Sunday. The last time Delhi experienced a hotter September night was in 2019.

Why are Delhi's days and nights so hot? 



The discomfort was not confined to the mornings. The city also reeled under a harsh daytime sun. On Sunday, the maximum temperature soared to 38.1 degrees Celsius, which is three degrees above average.

This made it the hottest September day in two years, as detailed in the The Indian Express report.

What's causing this unseasonal September heatwave? 



The unseasonal warmth is a direct consequence of the monsoon’s early departure. The rain-bearing system withdrew from Delhi on September 24, its earliest retreat since 2002. Since then, an absence of cloud cover has allowed for intense solar heating during the day.

Compounding the problem, high humidity levels have prevented the city from cooling down after dark. Relative humidity fluctuated between 56% and 76%, causing moisture in the air to act like a blanket that traps heat near the surface.
Easterly winds have been transporting this moisture, further slowing night-time cooling.

A silver lining for air quality 



These same easterly winds, with speeds of up to 20 kmph, have provided a temporary respite for Delhi’s air quality. They are currently blocking the transport of smoke from farm fires in neighbouring states.

Data from the Indian Agricultural Research Institute shows 130 stubble-burning incidents have been reported so far this season. The majority, 90, were in Punjab. Their impact on Delhi’s air remains minimal for now. On Sunday, the city's 24-hour average Air Quality Index (AQI) was 139, categorised as "moderate".

When will Delhi finally get some relief? 



There is hope for relief on the horizon. The India Meteorological Department has forecast a change in weather patterns. Generally cloudy conditions with the possibility of light rain or drizzle are expected from Tuesday through Wednesday.

This cloudy spell is likely to persist until October 5. It will gradually bring down temperatures, with the maximum expected to drop to around 32 degrees Celsius and the minimum to a more comfortable 23 degrees Celsius.

The Indian Institute of Tropical Meteorology also predicts that air quality will remain in the "moderate" category throughout the week.