Trapped heat: Why parts of Chennai can be 64°C even on a 38°C day

CHENNAI: It was a normal Chennai summer day. The Meteorological Department said the temperature was 38°C. But out there on the road, it was anything but a normal, 38°C day. It was hot as hell, and the normal readings and data did not capture how blistering the heat was. That is when we at DT Next decided to find out what the hell was happening.

To understand reality, we first bought a thermometer. Not the usual one, but an infrared thermometer that can be used to measure surface temperature, like for instance, when we bake in the oven. As suspected, we were the turkey, and we were getting baked hours every single day in this open air oven called Chennai.

Sample this: On the day when they said the temperature was 38°C, the concrete pavement was about 54°C, a stretch of tar road on Anna Salai was 60°C, while the sand at Marina beach reached 64°C. Little wonder, then, that we are getting air-fried. There, perhaps, is no better exercise to understand the perils of rapid, mindless concretisation of the city.

Even that does not convey the full story. Under the shade of a large tree next to a traffic signal, the pavement under the shade measured just 29°C, while the part that was exposed direct sunlight hovered over 50°C.

These contrasts, recorded across the same city in the same afternoon, offer a glimpse into the Urban Heat Island (UHI) effect, where the way cities are built can make some neighbourhoods significantly hotter than others.

Explaining this phenomenon, weather blogger Srikanth K, who runs the popular social media handle, ‘Chennai Rains’, says for a long time, summer heat has been explained through air temperatures recorded at a handful of observatories in a city. But those readings do not capture the fact that cities do not heat uniformly.

"In an urbanised environment, there are pockets that are hotter than the surrounding areas because of local infrastructure. Areas with narrow roads and closely built houses tend to see higher afternoon temperatures than areas with wider roads and more open spaces that allow for better wind movement,” Srikanth says.

Why some places are hotter than others

Our field measurements showed how surface temperatures varied depending on the material and exposure to sunlight. At Anna Salai, while the tar road sizzled at 60°C, a nearby stone wall was 37.9°C. The tiled footpath beside it registered 53.3°C.

Then we headed to Anna Nagar Tower Park to see how the tree cover there would help. But we were in for a surprise, an unpleasant one. There, the concrete pavement at the edge of a shaded area registered 45°C, and a patch of unpaved ground beneath the trees recorded 58.3°C because the canopy above wasn’t dense enough to block the sunlight.

The biggest factor influencing these temperature differences is the density of tree cover, says Srikanth. "Places like Guindy National Park and IIT Madras tend to be a couple of degrees cooler than their surrounding areas because of their large tree cover.”

We saw this firsthand. Outside the city police commissioner’s office in Egmore, along a narrow pavement exposed to direct sunlight, the surface temperature reading was more than 50°C. But just three steps away, beneath the shade of a dense tree located on the edge of the same pavement, it dropped to a cool 29°C.

We also found that even small measures can go a long way in reducing the heat island effect. At one traffic signal in the heart of Chennai, the road surface was 50.5°C. But at the stretch exactly opposite to it, where the Greater Chennai Corporation has fitted an overhead shade cloth, the road surface measured 41.5°C – a full 9°C cooler.

Bike helmets, car interiors

Then we checked helmets. As suspected, the white colour helmet exposed to sun for a while was quite hot – 50°C – while a black helmet right next to it was burning at 70°C.
So, the choice is rather simple: Do you want to sport a black helmet to look cool despite your brain cells getting fried, or would you save the remaining greymatter by wearing a light-coloured helmet? If the first one is your choice, well, go ahead; there is nothing much inside that skull to save.

We then checked at a car that had been parked in the sun for several hours. Its roof surface temperature reached 53.3°C. Expected, actually, considering how hot metal gets when exposed to constant heat. But it was shocking to the point of being scary when we recorded the temperature inside. Thanks to the direct exposure to sunlight and the heat trapped inside, the seat was a scorching 74.6°C and climbing.

Also, spare a thought for the workers employed at large construction sites, who stay in tin sheds close to the workplace. During our measurements, the roof of a tin shed registered 54.3°C. Inside, the wall temperature reached 45.4°C. Even at 7 pm, the ceiling still recorded 32°C.

When the city stays hot even after sunset

In Urban Heat Islands, the heat lingers even after the sun goes down. Roads, buildings, and other built-up surfaces absorb heat during the day, retain it, and slowly release it after sunset, meaning that even nights don’t bring relief. “Most of us would have observed that nights are becoming warmer than before. Your minimum temperatures are around 29-30°C almost consistently, which means very uncomfortable nights,” Srikanth says.

The findings mirror recent studies that show Chennai's nights are getting hotter. Between 2012 and 2022, the city recorded four additional "very warm nights" each summer, according to the Council on Energy, Environment and Water (CEEW), reducing the body's ability to recover from daytime heat.

How to cool the city

To mitigate the Urban Heat Island effect, one solution already being tested in Chennai is the use of cool roofs. Under the UNEP Cool Coalition's cool roof initiative, a few government schools have coated their rooftops with highly reflective paint.

When we checked one such roof at the Perunthalaivar Kamarajar Government Girls Higher Secondary School in Kizh Ambattur, the surface temperature was 47°C and the adjoining parapet wall 49°C. However, inside the classroom right below the terrace, the ceiling registered just 37°C. In stark contrast, the ceiling of a classroom beneath a conventional roof reached nearly 50°C. In June 2026, the State government announced that the cool roof initiative would be expanded to 55 schools in Chennai, including 49 Corporation schools and six government schools.

Urban greening can also help. Green roofs, which are basically rooftops covered with vegetation, cool down buildings by providing shade and releasing moisture through evapotranspiration by these plants. They also absorb rainwater, improve air quality, and create habitats for wildlife. A 2023 study published in Building and Environment found that green roofs reduced roof-level daytime air temperatures by about 1.5°C.

Several cities have already demonstrated how such urban cooling measures can work at scale. Medellín in Colombia created more than 30 green corridors linking roads, parks, streams, and vertical gardens. Designed to mimic natural forests with layered vegetation and tall shade trees, the corridors have helped reduce temperatures by about 2°C since 2016 and improve air quality in a region where air pollution was linked to nearly 2,000 premature deaths in 2016.

Closer home, Singapore has emerged as a global model for tackling UHI by making urban greening a core planning strategy. It has more than 400 parks, four nature reserves, and extensive vertical greenery integrated into buildings, bridges, retaining walls, and major roads. By 2030, it aims to plant another one million trees and ensure every resident lives within a 10-minute walk of a green space. The approaches adopted by Medellín and Singapore align with the recommendations of a recent Scientific Reports study, which calls for Chennai to develop interconnected green infrastructure and preserve urban vegetation, cautioning that the city’s fragmented and disconnected green cover is no longer enough to counter the warming effects of rapid urbanisation.

The 2026 Urban Heat Island Assessment and Strategic Guidelines for Urban Cooling in Tamil Nadu estimates that major urban areas such as Chennai could experience 200 to 250 thermal discomfort days annually by 2050, underscoring the urgent need for measures such as cool roofs, urban greening and shaded streets. As the city continues to urbanise, we hope this little experiment of ours would make a compelling case for urban cooling to become a core part of city planning rather than an afterthought.

The urban heat problem: In numbers

-- Chennai's built-up area increased to 67% in 2024 from 36% in 2014, while vegetation cover shrank to 10% from 25%. (Recent study by National Institute of Technology (NIT) Calicut researchers published in Scientific Reports)

-- 200-250 thermal discomfort days a year projected for Chennai and other major Tamil Nadu cities by 2050. (Urban Heat Island Assessment and Strategic Guidelines for Urban Cooling in Tamil Nadu, 2026)

-- Chennai among 25 blocks in Tamil Nadu facing both long-term warming and severe present-day heat stress. (Decadal Assessment of Urban Growth and Thermal Stress in Tamil Nadu, 2025) 

-- Night-time land surface temperatures in Chennai, surrounding blocks have risen by 3-4°C over the past two decades. (Decadal Assessment of Urban Growth and Thermal Stress in Tamil Nadu, 2025)

-- Chennai recorded four additional very warm nights each summer between 2012 and 2022 compared with historical averages. (Council on Energy, Environment and Water)

-- Around 35% of Chennai's population lives in slums, where heat-absorbing roofs, sparse tree cover and poor ventilation increase heat exposure. (Decadal Assessment of Urban Growth and Thermal Stress in Tamil Nadu, 2025)

-- 12 heatwave-related deaths reported in Tamil Nadu in 2023, up to June (Parliament records, 2023)

-- 20,615 heatstroke deaths were recorded in India between 2000 and 2020. (NCRB)