What is the wet bulb temperature? And why is it so important?
A massive heat wave is poised to bake huge swaths of the Eastern United States and set new temperature records over the Great Lakes, New England, and the mid-Atlantic regions in the next several days, according to the National Weather Service. Forecasters also expect triple-digit heat in California’s Central Valley through the weekend. As of Thursday morning, more than 100 million Americans faced some type of heat alert.
Cities and states are bracing for the health toll from the scorching weather, opening cooling centers and declaring emergencies. These dangers are likely to persist even after the sun goes down. “Record warm overnight temperatures will prevent natural cooling and allow the heat danger to build over time indoors without air conditioning,” the National Weather Service wrote in its bulletin.
Last year showed just how devastating extreme heat can be. Looking at data from the Centers for Disease Control and Prevention, the Associated Press found that roughly 2,300 people died in part due to excessive heat in 2023, the hottest year on record. Some of the deadliest heat was in the Southwestern US, a warning sign that there’s an upper limit to heat tolerance even in a region otherwise accustomed and adapted to hot weather.
And this year, deadly temperatures have already claimed dozens of lives around the world. In India, extreme heat has killed at least 60 people so far. Mexico has seen at least 61 deaths.
Amid these extraordinary, sweltering late spring temperatures, an old measure of heat risk is getting some renewed attention: the wet-bulb globe temperature (WBGT). It tracks temperature, humidity, and sunlight, and it’s shown that it can better warn against the dangers of hot weather than temperature alone. The upper limit wet-bulb temperature for human survival is considered to be 95 degrees Fahrenheit for young, healthy people, but in the recent heat wave in India, the WBGT reached 100 degrees.
• The rising threat of extreme heat.
• What the wet-bulb globe temperature is and why the military created a new way to gauge heat danger.
• Why our most powerful measurement of heat risk ends up being so dry and inaccessible.
• How heat impacts the body and how to evaluate your own risk.
Yet in the US, WBGT hasn’t caught on, despite how much danger the country faces from high temperatures. According to the National Weather Service, heat has been the deadliest weather phenomenon in the US over the past decade. The threat is growing due to climate change: The likelihood and severity of heat waves is increasing. Yet in the US, there’s no official tracking of heat-related deaths nor a federal standard to protect workers from extreme heat.
Health authorities across states and the federal government are now racing to come up with ways to save lives as the temperatures in 2024 climb to even higher peaks. Last month, the US Department of Health and Human Services launched its Health and Heat Index to help communities plan for heat-related dangers. It uses temperature records and historical emergency room visits to measure the potential heat threats in every ZIP code, with the goal of helping communities prepare for scorching weather.
But the dangers aren’t spread evenly. Unlike disasters like tornadoes or torrential downpours, which can kill indiscriminately, the specific harms from extreme heat vary from person to person.
While people in a given region can feel the same warmth, whether that leads to just damp clothes or a trip to the ER depends on someone’s underlying health conditions, age, humidity, how long they’ve been outside, and even how strong a breeze is blowing.
That’s why the scientists who study the health risks from heat warn that thermometers are not enough. If the goal is to reduce the number of deaths from extreme heat, we need more sophisticated measurements of the weather and a better grasp of our individual vulnerabilities.
And critically, the people baking under this extreme heat need the education to grasp these hazards and the tools to cope.
To understand the dangers of extreme heat, it helps to know a little about physiology. Most human bodies operate within a narrow temperature band around 98.6 degrees Fahrenheit.
“All your biochemistry and physiology, the function, is optimized for that temperature,” explained W.
