Heat index vs. wet bulb temperatures: In a heat wave, which should you use?

Earlier this summer, officials from the U.N. weather agency admitted they’re still figuring out how to tell 8 billion people around the world what constitutes deadly heat.

The warning, issued during a global conference call with researchers from the World Meteorological Organization, came as the Earth’s temperature shatters all previous records. July 3 was the hottest day in modern records — until temperatures hit new highs in subsequent days. Experts say the forecasts will again topple records over the coming years.

Yet officials rely on more than 300 heat stress indexes to communicate the risk this poses to people around the world. Each one, from the National Weather Service’s heat index to local warning systems to Canada’s Humidex to New York City’s scale, embraces its own standards.

For the world’s top scientists, this tower of babble is confusing.

“All indexes give you numbers,” says David Romps, a climate scientist at the University of California at Berkeley. “But it’s not clear what you should do with those numbers.”

And it has never been more important to communicate what to do amid brutal heat waves. In the United States, where heat is already the leading weather-related killer, hundreds die each year, especially the elderly, reports the Environmental Protection Agency. By the end of the century, almost half of the world’s population is likely to experience lethal heat waves every year, even under optimistic warming scenarios.

Reporting air temperature in Celsius or Fahrenheit is no longer enough. We need a better way to talk about extreme heat.

I asked several experts to help sort through what it will take to come up with a heat scale everyone can understand.

In the 1970s, textile physicist Robert Steadman relied on tests on healthy, young adults to extrapolate how the average human body responds to heat stress.

The result is perhaps the world’s best-known heat scale: the heat index. You’ve probably heard of it.

Steadman originally called it an “assessment of sultriness,” but today weather forecasters refer to it as “apparent temperature,” “feels-like temperature” or “real-feel temperature.” It’s the go-to index for warning of heat danger for the Weather Service, among others.

While it’s expressed in degrees Fahrenheit, it’s not a reading you’ll find on any thermometer. It translates how two factors — temperature and relative humidity — affect humans’ ability to regulate their internal temperature. Behind the scenes, the scale is a complex equation involving more than a dozen estimates of sunshine, vapor pressure, height, clothing, and others affecting how your body stays cool.

For example, if air temperatures and humidity levels are moderate, say 68 degrees and 70 percent, the heat index reading will be the same as the air temperature: 68. But if air temperatures reach 86 degrees, those same humidity levels mean a heat index reading of 95 because sweat no longer evaporates as efficiently.

If you’ve ever stepped into a steamy sauna, you’ll know why: The combination of heat and humidity can be overwhelming after a short time. Under typical conditions, humans can maintain their internal body temperature as long as air temperatures are below about 95F (35C). Above this point, our bodies need sweat to carry excess heat away from our core.

But high humidity renders sweating ineffective. In 70 percent humidity, humans are in extreme danger of overheating once air temperatures reach the mid-90s. That translates into a heat index reading of around 126 degrees, which the Weather Service deems “extremely dangerous” and “unsafe to humans for any amount of time.”

Yet this warning system is not perfect. First, it assumes you’re resting in the shade, not working in the sun. If you’re exerting yourself outdoors, the effective heat index could be 15 degrees higher. Secondly, the heat index was built on the idea of a young and healthy adult — a bad assumption for the elderly, very young and sick.

“The heat index is super conservative in predicting bad health outcomes,” says Romps.

These readings sound theoretical, says Romps, but Texas saw heat index values that high twice this summer: Kelly Field, an Air Force base in San Antonio, had a heat index reading of 150 degrees in June, followed by another reading near Dallas in July, according to his calculations.

These extreme conditions mean scales like the heat index are breaking down during brutal heat waves like the ones sweeping the world now, says Cascade Tuholske, a climate researcher at Montana State University. The challenge is coming up with measurements that don’t confuse people, while accurately conveying the risk under different, often unprecedented, conditions.

Luckily, the heat index is not the only game in town. In the 1950s, the military devised a second way to measure heat stress under in extreme heat.

Few things are as grueling as military training on a hot day. Even the healthiest succumb. The U.S. military discovered this as its soldiers collapsed and even died during exercises. At least 17 U.S. soldiers have perished from heat-related illness since 2008.

In the 1950s, the Marine Corps deployed an alternative method to measure heat stress: wet bulb globe temperature (WBGT).

The WBGT uses three types of sensors: a thermometer covered by a water-soaked cloth to measure evaporative cooling — a proxy for how humidity impairs our ability to sweat; a standard “dry” thermometer of air temperature; and a black globe estimating the intensity of sunlight on our skin.

Together, these offer a more effective indicator of heat stress if you’re exercising in the sun, says the Weather Service. Direct sunlight can add as much as 15 degrees to the effective heat index for athletes, soldiers and anyone working outside.

Unfortunately, the WBGT is not intuitive. The readings are measured in Fahrenheit, but they bear little resemblance to our daily experience. For example, the scale jumps from “low” risk at 80 to 85 degrees to “extreme” risk at just above 90, conditions that could lead to convulsions or even death.

Still, ignoring it can be deadly, even for the healthiest among us. Over the past 25 years, at least 50 high school football players in the United States have died of heatstroke. Today, many schools have rules restricting athletic practices to one hour when the WBGT rises above 90 and canceling workouts above 92. If you’re exercising outside in hot, humid weather, researchers recommend using the WBGT scale instead of the heat index. Check the Weather Service’s experimental forecast and guidelines.

These measures are designed for healthy adults. We’re just starting to adopt indexes of how heat will affect the most vulnerable.

Over the past decade, the Weather Service has been working on a third scale, HeatRisk, expected to roll out nationally later this year.

The five-point scale (zero to 4) aims to forecast heat-related risk for vulnerable populations using localized weather data, health indicators, and data extrapolated in places without weather stations.

The Weather Service hopes it will serve as an early warning system even when conditions aren’t at their most extreme, supplementing existing heat warning systems that overlook heat-sensitive populations.

The scientists I interviewed said there will probably never be a universal heat risk index. We must grapple with the unique interactions of geography, physics and physiology to assess risks around the world. That means interpreting the data to fit different people in different places.

“I struggle with this,” says Romps. “How do you take all this complexity and boil it down to one number? But there’s not really one number that will tell us if this is safe or this is dangerous.”

Researchers are still piecing together how extreme heat affects human populations using incomplete weather and epidemiological data. Health records are spotty. Poor tropical nations lack extensive weather coverage. Even the human body is adapting as temperatures rise.

“The problem is we’ve hardly ever reached these levels of temperature and humidity in the historical record,” says Jane Baldwin, an earth systems science professor at UC-Irvine, “yet we see people dying in that very regularly.”

Until agencies standardize their language, the best way to use heat risk indexes is to study the categories of risk, rather than temperature readings of any particular scale: 95 on a standard thermometer is safe, but the same reading on the WBGT could be deadly.

Then assess your situation. What are your risk factors? Can you find cooling if the power goes out? Think about your neighbors as well. Adjust for your — or their — personal level of risk.

Here’s a quick guide to stay safe in the heat.

Daily life: Use the heat index

Baldwin recommends using the heat index to gauge the heat stress you might encounter over the course of an average day. “In my own life,” she says, “that’s what I end up paying attention to.” But this is the most conservative estimate: It assumes you’re walking slowly in the shade. When possible, she defers to a local heat warning system to account for regional variations, such as the one New York City developed. Instead of deciphering numbers, these warnings are issued in plain language.

Outside activities: Use the WBGT

For people working outside, the WBGT offers a more precise estimate of risk. Humidity plays such an important, and dangerous role, in heat stress, that relying on heat index alone during strenuous exertion is dangerous. The WBGT measures heat stress in direct sunlight while accounting for temperature, humidity, wind speed and the intensity of solar radiation. Check the Weather Service’s experimental forecast and guidelines.

Vulnerable populations: Play it safe on every scale

Any risk factors, from age to dehydration to cardiovascular disease, elevate the chances of injury or death. If you are vulnerable, stay in a cool indoor place during the day when temperatures are high, and recover as much as possible at night, especially if temperatures stay high. The Red Cross offers steps you can take.

Cumulative exposure, even below lethal thresholds, matters. “Know yourself, where you are, and be very cautious, especially if you have some kind of heart condition,” says Lucas Vargas Zeppetello, an atmospheric climate scientist at Harvard University. “We are really underestimating the danger.”