What is wet bulb globe temperature? The weather index that’s becoming more important

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In Billings, Montana and other cities across the central and northern Plains, temperatures are forecast to soar above 100 degrees Fahrenheit this week. In England, the source of the Thames River has dried up for the first time since at least 1976. And in Spain, officials have issued extreme heat warnings as yet another heatwave swelters parts of the country following its hottest recorded month of July since at least 1961. 

And as the planet continues to get hotter, humans are at greater risk for heat-related illness and death. But an index aimed at calculating the impact of temperature on the body — called wet bulb globe temperature — could help people avoid such, which is why some meteorologists are pushing for its widespread use.

“With a warming climate and additional heat waves that are bound to happen, because of that, more people will by necessity become interested in ‘What else can we do to keep people safe?” meteorologist Jen Carfagno of the Weather Channel told CBS News. “One of those things is the wet bulb globe temperature.”

What wet bulb globe temperature tells you about the weather

In hot temperatures, the human body typically creates sweat as a way to control body temperature through evaporative cooling. But in environments with high humidity and high temperatures, it can be difficult for sweat to evaporate and cool a person down, creating the risk for heat illness to occur.

So, the wet bulb temperature takes into account temperature, humidity, wind speed, sun angle and cloud cover to assess just how much a person can be cooled by water evaporation to avoid heat-related illness, according to the National Weather Service

It uses a tiered threshold, with a score of 80 or less that’s considered to be safe to perform normal activities in like playing sports. But officials warn that working 15 minutes in a wet globe bulb temperature of 90 or above could likely stress the body out. Experts advise taking at least 45 minutes of breaks each hour if working or exercising in direct sunlight of this score.

How wet bulb globe temperature is measured

Originally measured by covering the globe of a thermometer with a piece of wet gauze, the index was invented and first used during the 1950s to address an uptick in serious outbreaks of heat illness in the U.S. armed services while they performed strenuous activity, Carfagno said. More recently, a device called a sling psychrometer can be used to calculate the wet bulb globe temperature.

Wet bulb globe temperature versus heat index

Although the wet bulb globe temperature is similar to other metrics such as the heat index, which is calculated for shady areas and takes just temperature and humidity into consideration, Carfagno says key differences in the measurements’ limitations impact their effectiveness.

In a 1979 study, researcher R.G. Steadman used temperature and dew point to determine a method of assessing how sultry the air field is. 

But Carfagno noted that Steadman “made a ton of assumptions” in his research, including factors like the weight of a person, height, type of clothing worn and gender.

“That heat index equation which everybody uses is actually not completely representative of how the air is going to affect your body,” Carfagno said. “So this wet bulb globe temperature is a really good measure of how the heat could affect someone’s body because it doesn’t even worry about assumptions – about the kind of person.”

Instead, she says the wet bulb globe temperature’s limitations include environmental conditions. For example, other variables outside of those measured in the wet bulb globe temperature’s equation can also affect a person’s sweat evaporation, according to a recent study published in the National Library of Medicine.

Still, Carfagno says she thinks more platforms should shift to using the wet bulb temperature, saying that its use could help people manage their risk of heat stress.

She says many weather service apps and companies, including the Weather Channel, still rely on the heat index model to determine features like “real feel” temperatures. But others, like schools and the U.S. armed forces, already do currently take the wet bulb globe temperature into account when deciding if it’s safe to perform activities outside. 

“I think as a society, we need to be open to continually updating information,” Carfagno said. “We can’t get one number for the day and think that our day is set. When you’re outside the weather changes.” 

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