As India faces rising temperatures touching 42 to 45°C this summer, meteorologists and climate scientists are drawing attention to a lesser-known but critical measure of heat stress known as wet-bulb temperature. The India Meteorological Department (IMD) has already warned of an intense heatwave season through June, raising concerns about the growing risk posed by extreme heat combined with high humidity across many regions.

Wet-bulb temperature is not the reading seen on a standard thermometer. Instead, it is measured using a thermometer wrapped in a wet cloth exposed to airflow, where evaporation cools the instrument. This reading reflects how effectively the human body can cool itself through sweating. While dry heat allows sweat to evaporate and cool the body, humid conditions prevent evaporation, making it harder for the body to regulate internal temperature.

Scientists say this difference can be life-threatening. The commonly cited danger threshold is 35°C wet-bulb temperature, beyond which the human body is unable to survive prolonged exposure even in shade with water access. However, recent studies suggest that risk begins much earlier, with heat stress affecting the body’s ability to regulate temperature at around 31°C. Between 28°C and 33°C wet-bulb levels, prolonged exposure can quickly lead to heat exhaustion or heatstroke.

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India is considered especially vulnerable due to a combination of geography, climate patterns, and population exposure. Coastal and eastern cities such as Mumbai, Kolkata, Chennai, and Bhubaneswar experience both high heat and high humidity, creating conditions where wet-bulb temperatures can become dangerously elevated even when actual air temperatures appear moderate. Monsoon moisture further intensifies this effect in many regions.

Experts also point out that millions of outdoor workers in India—such as construction labourers, farmers, and street vendors—face prolonged exposure without adequate cooling or air-conditioned environments. Urban heat islands in densely populated cities worsen the situation by trapping heat in concrete structures and reducing airflow, further increasing heat stress risks during peak summer months.

Long-term climate projections indicate an even more concerning trend. Studies suggest that a large portion of India’s population could be exposed to dangerously high wet-bulb temperatures in the coming decades if global warming continues. Scientists warn that rising humidity will make heat feel far more intense than actual temperature readings suggest, making wet-bulb temperature a crucial but often overlooked indicator of climate-related health risks.

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