Chennai, Oct 8: Chennai’s intense October heat may appear unusual for a month associated with the Northeast monsoon, but the city’s weather at this time of the year is influenced by the transition between the Southwest and Northeast monsoons, along with humidity, sunshine and the growing urban heat island effect.
October is a crucial transition period when the Southwest monsoon withdraws and conditions gradually become favourable for the Northeast monsoon. The IMD has this year indicated that the Northeast monsoon over south peninsular India is likely to set in around October 15-21, leaving Chennai and other parts of Tamil Nadu vulnerable to spells of hot and humid weather before sustained monsoon activity begins.
During such transitional periods, the absence of widespread cloud cover and rainfall allows stronger sunlight to heat the land surface. A previous assessment by weather experts noted that largely cloudless skies and dry winds can push Chennai’s daytime temperatures above normal. A weather blogger had attributed unusually warm October conditions to “cloudless” skies and sunlight penetrating the atmosphere, aided by dry winds.
He had also explained that the absence of expected rainfall and thunderstorm activity could result in maximum temperatures exceeding the normal level for October. “Usually this period would see significant thunderstorm activity,” he said, noting that dry days could keep the mercury higher.
Humidity adds another dimension to the heat. Even when the recorded temperature does not appear exceptionally high, high relative humidity reduces the body’s ability to cool through sweating, making conditions feel considerably hotter. A study of heat stress in India’s four major metropolitan cities found that Chennai faces a particularly high risk of extreme heat stress because of the combination of temperature and high relative humidity.
The city’s rapid urbanisation is also making the heat more persistent. A 2026 study published in Scientific Reports found that Chennai’s built-up area increased from 36% in 2014 to 67% in 2024, while vegetation cover declined from 25% to 10%. The study found a strong positive correlation between built-up areas and surface urban heat island intensity, while vegetation had a cooling effect.
Another recent study on Chennai’s urban heat island found that dense built-up neighbourhoods retain heat, particularly at night, while green spaces and water bodies such as Guindy National Park, the Adyar River and the Bay of Bengal provide cooling effects. It also identified areas including Anna Nagar, Koyambedu and Ambattur as persistent thermal hotspots.
Researchers say the combination of seasonal weather patterns and long-term urban changes is important in understanding why Chennai can remain uncomfortable even outside the traditional summer months.