Today’s heat index is going to be bonkers. Here’s a measurement that matters more.

Hot days aren’t all created the same, and a measurement that’s becoming increasingly popular is meant to better capture how hot days pose different risks to people.

“Wet bulb globe temperature” — sometimes shortened to WBGT — uses three thermometers to show what it truly feels like in the sunlight and whether conditions allow someone who is sweating to cool down.

Experts say wet bulb globe temperature offers a more accurate representation of the risk posed by hot conditions than temperature or heat index.

Understanding that risk is important for outdoor workers, high school athletes and anyone deciding whether to spend time outdoors during heat events like the one North Carolina is experiencing right now.

How is wet bulb globe temperature different from heat index?

Wet bulb temperature is seen as a comprehensive measurement of heat and how it can affect people, Kathie Dello, North Carolina’s state climatologist, told The News & Observer.

It’s meant to measure heat stress on the human body in direct sunlight, like an athlete would experience during spring practices or a worker would experience on a construction site.

“It kind of mimics how the body would cool down from sweat and also just how you would feel out in the sunshine,” Dello said.

By comparison, heat index accounts for the temperature in the shade and humidity.

Where can I find wet bulb globe temperature?

North Carolina’s State Climate Office shows real-time temperature at 44 sites statewide at its website, econet.climate.ncsu.edu/wbgt.

There are several Triangle locations in the climate office’s network, including the Lake Wheeler Road Field Lab, Reedy Creek Field Lab, North Durham Water Reclamation Facility and the Horace Williams Airport in Chapel Hill. A pair of wet bulb globe measurement devices are located in Clayton, too.

The National Weather Service’s Raleigh office makes wet bulb globe temperature forecasts available at www.weather.gov/rah/wbgt with forecasts stretching throughout the day, as well as what to expect for the next five days.

How is wet bulb globe temperature measured?

Wet bulb globe temperature includes the air temperature, the wet bulb temperature and the black globe temperature.

That means three thermometers are needed to measure it.

A plain thermometer measures the air temperature. Another is wrapped in a wet cloth, meant to mimic how sweat cools a person down. And a third thermometer is inside a black globe, capturing what it feels like when someone is in direct sunlight.

From there, the measurements from all three thermometers are weighted to determine the wet bulb globe temperature.

The wet bulb temperature accounts for 70%, the black globe temperature 20% and the air temperature 10% of the final measurement.

Sweat is the body’s key method of regulating temperature, specifically the evaporation of sweat.

Ashley Ward, the director of Duke University’s Heat Policy Innovation Hub, told The News & Observer these measurements better simulate the dynamic that plays out when someone is sweating on a sunny, humid day. The sweat can’t evaporate in those conditions, making the body’s cooling systems less effective.

That can change if it’s hot and breezy, with wind aiding the evaporation. Wet bulb globe temperature captures those dynamics better, Ward said, than heat index or air temperature.

“Things like wind speed and solar radiation play an important role overall in how our body processes heat and cools itself. So it’s a much more accurate measure of heat strain on the body,” Ward said.

What does WBGT tell me about risk?

Communicating wet bulb globe temperature can pose a challenge, Dello said, because people are used to understanding risk through temperature or heat index.

On those scales, a 92 is hot and may require some precautions. But a 92 in wet bulb globe temperature is considered extreme and can be fatal.

“If the wet bulb globe temperature gets over 90 you really need to limit your outdoor activities and have some heat mitigation measures in place like taking breaks in the shade and drinking water,” Dello said.

To help with this, scientists from the University of Georgia created a scale showing the risk of heat stress in certain wet bulb globe conditions. This scale shifts by regions, with people in cooler regions experiencing risk at lower temperatures because their bodies aren’t as acclimated to heat.

For North Carolina, any wet bulb globe temperature above 90 degrees Fahrenheit is considered an extreme threat of heat stress. A high threat is considered anything between 86.1 and 90 degrees; 82.1 to 86 is a moderate threat; and 78.3 to 82 is an elevated threat

The scale is illustrated with colors, with anything below 78.3 shown as green, an elevated threat as yellow, a moderate threat as orange, and a high threat as red. A day with extreme risk is considered a Code Black.

“In the Southeast, the wet bulb globe temperature is a much better indicator of heat risk. In other words, a much better indicator of health outcomes associated with heat than most other measures,” Ward said.

Who uses wet bulb globe temperature?

The N.C. High School Athletic Association uses wet bulb globe temperature to determine what activities are safe for players.

On a Code Black day, games can go ahead with mandatory breaks but all outdoor practices must be suspended. When the heat threat is high (Code Red), practices can go ahead but athletes must be closely monitored, cannot wear equipment like football pads and must be given a five-minute break for every 15 minutes of practice.

At lower levels of heat risk, coaches are urged to be more cautious with athletes who are either new or not yet acclimated to high temperatures.

The U.S. military also uses wet bulb globe temperature.

Marines cease all nonessential physical activity above 90 degrees and limit exercise for newly assigned Marines at lower levels. The Army uses wet bulb globe temperature to figure out how much physical activity troops can do and how much rest they need afterward, as well as how much water soldiers should be drinking.

And the U.S. Occupational Health and Safety Administration uses the measurement to make recommendations about whether strenuous activity poses a risk to worker safety, also accounting for the clothing workers are wearing. Above 77 degrees Fahrenheit, OSHA says, workers who are used to heat could be unsafe doing strenuous work and those who aren’t familiar with the conditions face a high risk of heat illness.

“What wet bulb globe does is give different thresholds at which different activities are safe,” Ward said.

This story was produced with financial support from the Hartfield Foundation and the 1Earth Fund, in partnership with Journalism Funding Partners, as part of an independent journalism fellowship program. The N&O maintains full editorial control of the work.