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Wet bulb thermometer readings are significantly lower than the more familiar dry bulb readings, which do not take humidity into account. Researchers say that at a wet bulb reading of 35C, the body can no longer cool itself by sweating and such a temperature can be fatal in a few hours, even to the fittest people.

“It approximates how warm it feels to humans because we cool via sweating,” Mr Matthews says. “We rely on that exclusively. When you use that measure, the wet bulb temperature, the two regions that stand out on earth are the shores of the Gulf and the Indus Valley in Pakistan. They are truly exceptional.”