As temperatures across India touch 42 to 45°C this summer and the India Meteorological Department warns of an intense heatwave season through June, scientists and climate experts are pointing to a metric most people have never heard of — wet-bulb temperature — as the real measure of danger. Understanding it could be the difference between life and death.
What Is Wet-Bulb Temperature?
Wet-bulb temperature is not the reading on an ordinary thermometer. It is measured by wrapping a thermometer in a wet cloth and allowing air to flow over it. As the water evaporates, it cools the thermometer. The final reading captures the combined effect of heat and humidity on the human body — and is a far more accurate indicator of heat danger than air temperature alone.
The science behind it is straightforward. The human body cools itself by sweating. But sweating only works when it evaporates. In dry heat, sweat evaporates freely and the body cools. In humid heat, sweat cannot evaporate as the atmosphere is already saturated with humidity— and the body overheats. This overheating of the body, potentially, leads to heat strokes.
Two cities at the same temperature can therefore carry very different risks. A 45°C day in Rajasthan, where the air is dry, allows sweat to evaporate and the body to cope. A 38°C day in Kolkata or Mumbai, where humidity is high, can be more lethal because the body's cooling system simply stops working.
The Survival Limit
The threshold that scientists watch most closely is a wet-bulb temperature of 35°C. Sustained exposure to a wet-bulb temperature above 35°C is considered almost certainly fatal, even for a healthy person sitting in shade with access to water. The body cannot shed heat fast enough to survive.
But the danger begins well below that ceiling. A recent study found that the body begins to lose its ability to control internal temperature at a wet-bulb reading of around 31°C in hot and humid conditions — far lower than earlier estimates of 35°C. At 28 to 30°C wet-bulb, prolonged outdoor exposure becomes dangerous. At 31 to 33°C, the risk of heat exhaustion and heatstroke rises sharply.
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Why India Is Especially Vulnerable
India's geography, climate, and social structure make it uniquely exposed. Research by Akshay Deoras from the University of Reading while speaking to Al Jazeera warns that moist heatwaves in India often go unnoticed compared to dry heat — yet they can be more dangerous because people may not realise the risk until it is too late.
The threat is concentrated in three ways. First, coastal and eastern cities — Mumbai, Kolkata, Chennai, Bhubaneswar — combine extreme temperatures with high ambient humidity, pushing wet-bulb readings into dangerous territory even when the thermometer reading alone looks manageable.
Second, the monsoon itself adds to the risk: when moisture levels rise sharply during active monsoon phases, heat stress intensifies rather than eases.
Third, millions of Indians — construction workers, farmers, delivery workers, street vendors — have no choice but to work outdoors for hours each day, with no access to air conditioning and, in many areas, unreliable power supply.
Urban heat islands compound the problem further. Cities trap heat in concrete and glass, raise local temperatures by several degrees, and restrict ventilation — making wet-bulb conditions in dense urban areas worse than in surrounding regions.
The Long-Term Picture
The trajectory is alarming. According to a study led by Professor Elfatih Eltahir of MIT's Department of Civil and Environmental Engineering, around 70 percent of India's population could be exposed to wet-bulb temperatures of 32°C or more by the end of this century, with roughly 2 percent — tens of millions of people — potentially crossing the 35°C survival threshold.
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IPCC (Intergovernmental Panel on Climate Change) author Anjal Prakash has also warned that rising humidity will make heat feel far worse than thermometers suggest — noting that 40°C can feel like 50°C in humid conditions, and that survival without cooling systems may become genuinely difficult in the decades ahead.
The IMD has already warned that many parts of India will experience intense heatwaves between April and June 2026, with 19 of the world's twenty hottest cities now located in India.
The danger, in short, is not just a rising mercury reading. It is the invisible, silent combination of heat and moisture that overwhelms the body before most people recognise it as a threat.
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