What are ‘wet-bulb temperatures’ – and why are they so important for climate change?

Cows are walking through the road alongside of the famous John Jacob Christian cemetery of Jacobabad. The graveyard has a meaningful association to the city since it is named after the Brig. John Jacob who was buried here in 1858.

Jacobabad is a city in Sindh, Pakistan, serving as both the capital city of Jacobabad District and the administrative centre of Jacobabad Taluka, an administrative subdivision of the district. The town was founded near the village of Khangarh in 1847 by Brigadier General John Jacob.
Jacobabad in Pakistan recorded 'wet bulb' temperatures of near 35C. (Getty)

"Wet-bulb temperature" is a little known scientific concept – but in the near future, it could be crucial for determining which areas of our warming planet remain habitable.

The term refers to temperatures taken with a thermometer covered in a wet cloth, which are normally slightly cooler than "dry-bulb" temperatures.

They are an important measure for survivability.

Human beings can survive very high temperatures (well over 50C) when humidity is low, but in high humidity, humans cannot survive temperatures of even 35C for long periods, because there is no way to cool down by sweating.

Even healthy people will die rapidly in such conditions.

Wet-bulb thermometers allow researchers to work out whether humans can sweat.

If the water evaporates, the thermometer cools down, so that the wet-bulb temperature will be lower than the dry bulb temperature.

In high humidity, the water will not evaporate, and the wet-bulb temperature will be the same as the dry-bulb temperature.

This makes wet-bulb temperatures an important measurement of survivability.

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Previously, wet-bulb temperatures of 35C or higher were thought impossible, but last year scientists reported that places in the Persian Gulf had reached the threshold.

This happened only briefly, and only in small areas, but it could be a warning of things to come.

“It approximates how warm it feels to humans because we cool via sweating,” Tom Matthews, a lecturer in climate science at Loughborough University told the Telegraph.

“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.”

“The Indus Valley is arguably close to being the number one spot worldwide.”

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Colin Raymond, lead author of a 2020 study into humidity, said: "Physiologically, there's a point when heat and humidity will become not just uncomfortable, but actually impossible to acclimate to."

He warned that this is “already happening and only getting worse”, and said climate change will exacerbate the effect.

Raymond said: “Previous studies projected that this would happen several decades from now, but this shows it's happening right now.

“The times these events last will increase, and the areas they affect will grow in direct correlation with global warming.”

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