A heat index will tell you why it feels hotter than 90 degrees, while wet bulb temperatures will tell you when it’s time to take a break and cool down.
If you are living in one of many areas of the U.S. currently experiencing a heat wave, your outdoor thermometer may say it’s in the 90s, but your local weather station tells you it feels like triple digits.
Take Memphis, Tennessee, where meteorologists last week forecast a maximum heat index of a whopping 116 degrees from an air temperature of 93 degrees and relative humidity of 59%.
The heat index is an important distinction from air temperature when it comes to safety planning, says Joshua Merson, program director of Northeastern’s Extreme Medicine certificate program.
“The heat index is a number that tells you how hot it feels, combining air temperature and humidity,” he says.
“For example, if it is 90 degrees Fahrenheit with high humidity, it might ‘feel like’ above 100 degrees. That’s the heat index,” Merson says.
“This matters because in high humidity, sweat doesn’t evaporate as well so you feel hotter than the actual temperature,” he says.
It’s a reason why the dry heat of the desert can feel less hot than the same temperature in humidity-soaked New Orleans, and plays a role in heat safety, since evaporation of sweat is an important cooling mechanism.
The heat index increasingly is being included in local forecasts, especially during heat waves of the type that prompted heat advisories and warnings across a large swath of the U.S. this week.
But those truly in the know when it comes to protecting personnel working outdoors in the heat, including the U.S. military and Occupational Safety and Health Administration (OSHA), use an index known as the wet bulb globe temperature (WBGT) as a guide.
Unlike the heat index, which is taken in the shade, the WBGT factors in direct sunlight and cloud coverage as well as temperature and humidity.
“The wet bulb temperature is a measure of heat stress on the body and is the lowest temperature your skin can reach by sweating, taking into account air temperature and humidity, but it also considers wind and sun,” Merson says.
If the wet bulb temperature for your location is above a specific temperature, “your body can no longer cool itself by sweating, putting even healthy people at risk of heat-related illnesses or death,” Merson says.
It’s called a wet bulb temperature because it is taken by wrapping the globe of a thermometer with a wet cloth. The temperatures are specific to different regions and can be found at weather forecasts such as WeatherSTEM.
The National Weather Service says it adopted the wet bulb globe temperature forecast as of June 1, but the agency says readings are sometimes delayed by minutes.
The WBGT range of readings is different from heat index readings in that the danger zone extends from as low as 80 to above 90. A reading of 95 is considered the theoretical limit for human survivability, although even 88 degrees is now acknowledged as dangerous for even young, healthy people.
“In Boston, a wet bulb temperature above 80 degrees Fahrenheit can be considered a high threat, whereas in North Carolina it is above 86,” Merson says.
Early afternoon Tuesday, the weather service’s wet bulb reading for Tennessee showed areas near Memphis with a WBGT in the low 90s, which the agency considers extremely high.
“In a high-threat wet bulb temperature environment, your body may be under stress in as little as 20 minutes,” Merson says. He says wet bulb temperature readings should be used as a warning system during heat waves.
They are “great at telling us how often we should be taking breaks if active in direct sunlight,” he says. “It’s important for all outdoor activities, whether you are a construction worker, an athletic coach or simply walking to a store.”
The National Weather Service provides a chart showing how many breaks to take in the shade or indoors according to WBGT readings.
“Recommendations suggest taking a 40-minute break every hour in a high-threat wet bulb environment,” Merson says. “That only increases if the WBGT is higher.”