Sizzling at 60°C: Thermal scans reveal Chennai’s real heat

A Chennai Metro train glides into the station. Inside, commuters enjoy the relief of air-conditioning. Outside, a metal roof is baking under the afternoon sun. While the weather app is the standard source of info, a thermal camera can tell a vastly different story.Over the past few weeks, citizen-led civic platform Reclaim Chennai has been sharing thermal images of the city, capturing temperatures that aren't part of official weather readings. The scans reveal how dramatically heat can vary depending on where you’re standing.What’s happening?One of the group’s recent observations focused on Little Mount Metro station. Using thermal imaging, they found that the station’s metal roof reached nearly 50°C during daytime, while the station itself remained around 32°C thanks to ventilation and a steady breeze.The group also turned its thermal camera towards Chennai Metro coaches. Sharing footage from an off-peak weekend trip, it observed that the air-conditioning appeared to be functioning well, though it noted that weekday peak-hour observations would provide a clearer picture. Together, the findings suggest that temperatures can vary significantly within the same public space depending on materials, airflow and exposure to sunlight.Why shade mattersPerhaps the most striking comparison came from a stretch of pavement. Thermal images show a footpath exposed directly to sunlight registered a surface temperature close to 60°C. A nearby shaded section beneath a tree registered a surface temperature of around 30°C.This implies heat in Chennai is not shaped by weather alone. The built environment can significantly influence how hot a neighbourhood feels. “Areas with substantial tree cover, such as Guindy, often feel noticeably cooler than densely built-up parts of North Chennai. Trees help reduce surface temperatures through shade and improve local microclimates, making them a crucial tool in mitigating urban heat,” says Pradeep John, popularly known as the Tamil Nadu Weatherman.'Thermal camera observations should be interpreted along with meteorological data'VR Durai, director of the Regional Weather Forecasting Centre, IMD, says, "The temperature reported by the IMD is the air temperature measured in a shaded and well-ventilated enclosure about 1.5m above the ground. In contrast, thermal cameras measure surface temperatures and help identify urban heat hotspots such as roads, rooftops, parking lots, and industrial areas. Since human heat stress also depends on air temperature, humidity, wind, and solar radiation, thermal camera observations should be interpreted along with meteorological data."The real-feel factorPradeep John says Chennai’s biggest heat challenge often goes beyond the temperature displayed on weather apps. “The biggest issue is the ‘real feel’ temperature, which often does not get enough attention. Chennai’s combination of heat and humidity can make conditions much more stressful for the human body than the actual air temperature suggests,” he says.Elaborating further, he explains, "A 40°C day in Chennai is often associated with dry northwesterly winds from regions such as Rayalaseema or Vellore. These dry winds reduce humidity levels, sometimes bringing them below 40%, making the heat more tolerable despite the higher temperature. On the other hand, a 36°C day in June may be accompanied by humidity levels of 70–80% or higher. Under these conditions, sweat evaporates less efficiently, making it harder for the body to cool itself. As a result, a humid 36°C day in Chennai can feel more uncomfortable and physically exhausting than a dry 40°C day in many inland cities.”Yet, he maintains that thermal scans do not replace official weather data. Instead, they offer a different perspective on how Chennai experiences heat at street level.Urban Heat Island (UHI)What it is: A phenomenon where cities become significantly hotter than surrounding rural areas.What causes it: Dense concrete, asphalt, and metal absorb solar heat all day and radiate it back out, while a lack of trees removes natural cooling.UHI in Action: This is why a sun-exposed Chennai pavement spikes to a scorching 60°C, while a nearby tree-shaded section remains bearable at 30°C.HEAT BY THE NUMBERSChennai is getting more thermally uncomfortable. Recent studies suggest that rising temperatures, humidity and urbanisation are increasing heat stress across Chennai and Tamil Nadu.More discomfort days aheadA climate risk assessment by Anna University’s Centre for Climate Change and Disaster Management projects that thermal discomfort days in Tamil Nadu could rise from around 100–125 days a year today to 200–250 days annually by 2050 under a high-emissions scenario.Why Chennai feels hotterHeat stress is influenced not only by temperature but also by humidity. According to a Centre for Science and Environment analysis in 2023, Chennai recorded the highest summer heat index among India’s major cities at 37.4°C. The heat index combines air temperature and humidity to estimate how hot conditions feel to the human body.The urban heat effectStudies have linked increasing built-up areas and declining green cover to rising urban heat stress. Concrete, asphalt and metal surfaces absorb and retain heat during the day, while limited tree cover reduces natural cooling.Trees help reduce surface temperatures through shade and improve local microclimates, making them a crucial tool in mitigating urban heat – Pradeep John, a.k.a. Tamil Nadu WeathermanWritten by Aashna Reddy