A measurement long known to climate scientists has begun creeping into the public consciousness as extreme weather conditions force us to reflect on what conditions humans can withstand.

In a summer of record-breaking heat in many places, severe humidity has made the outdoors feel particularly unbearable. While there are numerous ways to measure climate conditions, some experts say it’s time to start talking about wet-bulb temperature.

Writer and educator Hank Green explained why it matters on TikTok during late June’s Pacific Northwest heat wave: “Temperature tells us a physical attribute of the atmosphere. Heat index is about comfort. Wet bulb is about survival.”

What is wet-bulb temperature?

Wet-bulb temperature accounts for both heat and humidity, unlike the standard temperature measurement you see on your weather app. It reflects what that combination means for the human body’s ability to cool down.

As the air around you gets more humid, your body is less able to sweat effectively, meaning you can’t cool off as successfully. That’s why dry heat feels more tolerable than extreme humidity.

“If the wet-bulb temperature reading is higher than our body temperature, that means that we cannot cool ourselves to a temperature tolerable for humans by evaporating sweat, and that basically means you can’t survive,” said Tapio Schneider, a California Institute of Technology climate scientist and professor.

The term wet bulb comes from a way the measurement can be taken, by wrapping a piece of wet cloth around the end of a thermometer to see how much evaporation can decrease the temperature.

“The idea here is that you and I are essentially wet bulbs,” Schneider said. “We cool ourselves by evaporation.”

As the globe warms and bodies of water evaporate at higher rates than before, raising humidity levels, wet bulb temperatures will continue to rise.

A study on wet-bulb temperatures published in Science Advances last year bore an alarming title: “The emergence of heat and humidity too severe for human tolerance.”

It found that some places on Earth have already experienced conditions too hot and humid for human survival.

Radley Horton, one of the study’s authors and a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory, explained that the study reported for the first time that there are places in the Persian Gulf and Pakistan that have already crossed a wet-bulb temperature of 95 degrees Fahrenheit. That kind of temperature would make it impossible to sweat enough to avoid overheating, organ failure and eventual death, “even [for] someone in the best shape, in the shade, relaxing, with an endless supply of water, not wearing heavy clothes,” Horton said.

He said these lethal combinations of temperature and humidity continue to be possible — even probable — as the climate continues to change.

“Over just the last 40 years alone, during a time when global temperatures only went up by two degrees or so, which doesn’t sound like much, we saw a doubling around the world in the frequency of dangerous combinations of heat and humidity,” Horton said.

Why am I just now hearing about wet-bulb temperature?

The general public is learning about wet-bulb temperature now because during a summer of extreme weather, air temperature isn’t the only thing that’s increasing, Horton said. The amount of moisture in the air is also climbing.

As we try to understand these novel heat conditions, Stanford University climate scientist Noah Diffenbaugh said, understanding the way temperature and humidity combine will be crucial to our ability to prepare for future extreme heat. Wet-bulb temperature is a better way to assess the risk to human health than just temperature, he said.

“Certainly over the last decade we've seen an increasing awareness that where the purpose is to understand risk to human health, including the humidity is important,” he said. “And wet-bulb temperature is one of the most direct ways to do that.”

I understand what wet-bulb temperature is. Now what?

The overlap of heat and humidity is the best predictor of human suffering, Horton said. As the climate continues to heat up, looking at the wet-bulb temperature before going outside to work or exercise could save you from heatstroke or worse.

Air conditioning is especially useful when the wet-bulb temperature gets high. So places that may soon experience unprecedented heat and humidity should consider how many of their residents have access to air conditioning and whether their electric grids can handle so many AC units, he said.

Locations that haven’t historically had major demands on their electric grids during heat waves also need to start asking themselves how they can become more resilient, Horton said.

“Are they resilient to that multiday humid heat event were nights to stay hot?” Horton asked. “Is there a risk of the power going out at precisely those times when it’s a life and death issue for a lot of people?”

It’s also critical to understand who is most at risk when the wet-bulb temperature climbs, said Cascade Tuholske, an Earth Institute postdoctoral research scientist at Columbia University’s Center for International Earth Science Information Network. Susceptible groups include the elderly, those with health conditions, people who work outside and those who do not have access to air conditioning or cooling centers.

“I think we can endure hot or humid hot heat quite well if we identify who is at risk and get them the resources they need,” Tuholske said.

He emphasized that it’s not too late to prevent future extreme heat and humidity from having such disastrous effects. After the 2003 European heat wave, which killed more than 70,000 people, the European Union adapted, he said. They’ve since had other extreme heat events without the same mass fatalities, he said.

“We do have the resources and the opportunity and the capacity again to move people out of harm’s way,” he said. “It’s just really up to us whether we want to do it.”