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Heat and humidity are putting millions of Indians in peril

Wet-bulb temperatures in Chennai and the east coast frequently soar

In Gujarat, a state in western India, dehydrated birds fall from the sky. In Punjab, in the north, withering crops have led to farmer suicides. In much of India, temperatures exceed 40°C. But it is not just the heat that causes problems. Cooler but muggier parts of the country, such as Chennai and other cities on the east coast, face the greatest threat.

That is because human vulnerability to heat depends on humidity. Evaporating sweat cools the skin, but in humid places it does so less effectively. High levels of heat and humidity— the combination of which is measured through the wet-bulb temperature, or that which is recorded by a thermometer wrapped in a moist towel—can be deadly. Even healthy humans cannot survive a wet-bulb temperature of 35°C for more than a few hours. Being outside when it reaches 32°C is exceedingly harmful.

The Economist calculated wet-bulb temperatures for six big cities in India using daily temperature and relative humidity readings at airports. Four of them have exceeded 32°C in the past five years. On May 1st the reading in Chennai soared to 32.6°C, the highest in ten years. The temperature that day touched 38°C and was accompanied by a stifling 68% relative humidity. Delhi was even hotter at 40°C but recorded a lower wet-bulb temperature of 24°C thanks to humidity of 23%.

Research suggests that eastern coastal and north-western India register some of the highest wet-bulb temperatures in the world. In 2020 Kolkata, near the east coast, experienced several days when the wet-bulb temperature exceeded 31°C. Ahmedabad, in the west, has also breached that mark. Delhi’s climate may be dryer but it is not immune. In July 2018, the wet-bulb temperature there touched 32°C.

Chennai’s response includes both short-term measures (ensuring water and power availability) and long-term (planting new forests). Last week Prime Minister Narendra Modi asked states to beef up their response mechanisms, such as regular hospital fire safety audits. But local efforts can only go so far. Unless global greenhouse-gas emissions are cut, horrendous temperatures will become more common.

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