Chennai hottest Indian megacity, says study

CHENNAI: If the dog days of the past three days made you bemuse that Chennai is the hottest city, you are right as science also corroborates. An analysis of summer temperatures of India’s megacities since 2011 has revealed that Chennai is the hottest with an average summer heat index of 37.4 degrees Celsius.

The study conducted by the Urban Lab of the Centre for Science and Environment (CSE) said that due to the rise of relative humidity during summer, the heat index (HI) has risen among megacities. “Chennai’s summer average heat index stood at 37 degrees Celsius (°C) making it the hottest among the megacities. The impact of humidity on Chennai’s heat index is 6.9°C," the study said.

As a part of the study, summer temperatures between 2001 and 2011 were compared with summer temperatures of 2014-2023 in Delhi, Mumbai, Kolkata, Hyderabad, Bengaluru and Chennai between 2011 and 2023.

“Chennai’s summertime has registered an increase of 0.4°C in decadal average ambient air temperature while the relative humidity has increased by 5 per cent between 2001-10 and 2014-23. March-April, 2024 so far has been hotter (about 1°C) compared to the average of 2014-23,” the study said.

High humidity is responsible for adding on average 6.3°C of heat stress to the city. The heat index of the city has increased by over 5 per cent. Moreover, days with a heat index exceeding 41°C or the danger mark have tripled compared to 2001-10.

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On the other hand, both pre-monsoon and monsoon have become thermally more uncomfortable in Chennai with about a 2°C rise in the heat index. Thermal distinction between pre-monsoon and monsoon has almost disappeared with both periods being equally hot and muggy, the study said.

Making the situation worse, the cooling down of land surface temperature during the night has reduced by 5 per cent. Pointing out that urban heat island phenomena are strong in Chennai, the study found that daytime in the core city is 0.8°C warmer than its peripheries and peri-urban areas during the summer. At night, the core city is 0.9°C warmer than its peripheries and peri-urban areas.

The heat indices of Kolkata are 36.5°C and Mumbai is 34.3°C. Delhi and Hyderabad have an HI of 32.2°C and 29.3°C respectively. Bengaluru with an HI of 26.9°C is the least.

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Increase in built-up area cited as reason

Attributing the increase in heat index in Chennai, the study pointed out that the built-up area in the city has doubled over the last two decades showing the most rapid concretisation among the mega cities.

The built-up area has increased from 30.7 per cent in 2003 to 73.5 per cent in 2023. At the same time, green cover has decreased from 34 per cent to 20.3 per cent during the period, which is the highest decline percentage.

Despite having the highest percentage of its land under concrete and the lowest green cover among the megacities, Kolkata has registered only a 10 per cent increase in built-up area. Hyderabad has doubled its green cover in the last two decades and green cover has declined in Mumbai, Kolkata apart from Chennai.

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