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The Combination Of Heat And Humidity - TW35 Is Here!

Lethal combination of humidity and heat reached for the first time in human history

Even the most extreme heat waves mostly kill people who are already ill or very old and fragile. Healthy people can usually survive even prolonged heat, because human bodies can get rid of extra heat by sweating. Evaporating one gram of sweat removes 2260 joules of heat from the skin.

However, when the relative humidity of air exceeds 90 per cent, sweat no longer evaporates but only drops from the skin. At this point our natural heat regulatory system essentially breaks down. Thus the combination of high humidity and heat is much more dangerous to us than dry heat.

Wet bulb temperature (TW) is a way to measure the effective heat, or the combined impact of heat and humidity. It means the temperature given by a thermometer covered in a water-soaked cloth. 

The important threshold is TW 35, the wet bulb temperature of 35 degrees Celsius. Above this wet bulb temperature humans lose their ability to cool their bodies and die within six hours.

According to scientific litterature wet bulb temperatures above 35 degrees Celsius have never been observed on Earth. Or this used to be the case. According to recent measurements Jacobabad in Pakistan and Ras al Khaimah in the United Arab Emirates have recently started to exceed TW 35 for short periods of time lasting one or two hours.

These are the first times wet bulb temperatures above 35 degrees Celsius have ever been reported in scientific litterature.

The event is highly symbolic, because it shows that unless we will take measures to halt global warming, large parts of the Earth will soon become too hot for humans to live outside air-conditioned buildings.

(Science Advances, doi.org/ggvfk3)