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Heat stress is not a temperature. India needs to learn that

A 45°C day in Phoenix, US, — functionally the hottest major city in the developed world — and a 45°C day in Banda, Uttar Pradesh, are arithmetically identical. But they are not the same event.

This week, the same chart kept showing up on my feed: by late April, every single one of the world’s 50 hottest cities was in India. The India Meteorological Department recorded 47.4°C in Banda, Uttar Pradesh, with five other cities also crossing 46°C and dozens more across northern and central India recording temperatures above 45°C. The Union Health Ministry directed every state to operationalise dedicated heatstroke management units. The National Human Rights Commission urged 21 states to take advance measures, warning that intensifying heatwaves disproportionately affect outdoor workers, the elderly, and the homeless.

Two reactions followed almost immediately.

The first: this is climate apocalypse. Indian cities are becoming uninhabitable.

The second: India has always been hot. We’ve lived through 40°C summers for decades.

Both are wrong, and they are wrong for the same reason. Heat stress is not a temperature.

What heat stress actually is

Heat stress is what happens when the human body, in a specific climate, doing a specific kind of work, in a specific kind of building, can no longer dissipate the heat it produces.

Simply put, the human body cools through four channels: convection (air moving over skin), radiation (giving off heat to cooler surroundings), conduction (touching cooler surfaces), and evaporation (sweat). When ambient air is hotter than core body temperature of around 37°C, the first three reverse and the body starts absorbing heat from its surroundings. Sweat becomes the only channel left. And sweat only works when the air can absorb the moisture.

This is why scientists measure heat stress with three metrics, not raw temperature:

Wet Bulb Globe Temperature (WBGT) – It combines temperature, humidity, wind, and radiant heat. It is the standard for setting safe limits in outdoor labour, sport, and military operations.

Heat Index – It is the “feels-like” temperature, which factors humidity into perceived heat. This is what your weather app shows.

Wet Bulb Temperature (WBT) – It is the most critical. It is the lowest temperature achievable by evaporative cooling. The widely cited 35°C wet bulb threshold is the theoretical survivability limit for a healthy adult in shade with unlimited water. Beyond........

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