How Hot Is It, Really? Temperature Indexes Disagree

June and July were the hottest months on record, and if you’ve been following coverage, you might have noticed a bewildering spectrum of temperature indexes used to describe the milestones.

There’s the heat index, the wet-bulb globe temperature, the real-feel temperature, the universal thermal climate index and so on. You notice them not only in the news, but on our cellphone weather apps that include the feels-like temperature, relative humidity or UV index.

This has led to some truly eye-watering (or perhaps eye-evaporating) reports, such as the heat index climbing to 120 degrees Fahrenheit in parts of the Southern U.S. and even higher internationally, with Persian Gulf International Airport in Iran registering a heat index of 152 degrees at one point this summer—which sounds high enough to sous vide cook a chicken.

Despite the growing prevalence of these measures, “most people have no clue what it means other than it’s kind of meant to be what the temperature feels like,” said Andrew Grundstein, a geography professor at the University of Georgia who studies heat vulnerability.

For example, it’s possible under exactly the same weather conditions for one index to read 90, another to read 100 and a third to read 110.

It isn’t just the heat. It’s the humidity, wind and sunshine, too.

The different measurements recognize that the experience of heat goes beyond what the thermometer reads. Humidity, solar exposure, wind speed and so on affect people’s ability to regulate temperature. At some point, these conditions prevent the human body from cooling itself down through sweating. This can be extremely dangerous. The National Weather Service finds that heat is the leading weather-related cause of death, surpassing hurricanes, floods, tornadoes, cold, lightning and others.

The two most common measurements are the heat index and the wet-bulb globe temperature, but both have limitations. 

The heat index, used by the National Weather Service since 1979, has just two variables: temperature and humidity. High humidity makes a given temperature more stressful because it makes sweating less effective. 

But there are two ways the heat index can mislead. The first is that the weather service calculates the heat index once humidity is above 40%, which means it is always higher than the temperature. At 90 degrees and 40% humidity, the heat index reads 91. With the temperature the same, but humidity at a more dangerous 75%, the heat index reads 109. 

Second, the index is calculated for someone who is 5 feet, 7 inches tall, weighs 147 pounds, is wearing long trousers and a short-sleeved shirt, and is walking 3.1 miles an hour in the shade with the wind blowing at roughly 5.75 miles an hour. It thus ignores factors such as cloud cover, wind and radiation, which can dilute or amplify the effects of heat and humidity. 

Temperatures can feel 15 degrees hotter in direct sun than in shade, Grundstein said; the heat index won’t account for this. Conversely, it won’t account for how tolerable heat can be under a beach umbrella, with a steady breeze.

The other main measure—the wet-bulb globe temperature—is meant to compensate for those shortcomings. It was developed by the U.S. military in the 1950s to monitor heat stress in training. It’s still used by the military today, as well as in sports and occupational-safety guidelines. 

Dry bulb, wet bulb, black bulb

Wet-bulb globe temperature is actually the average of three indicators: the regular dry-bulb temperature; the wet-bulb temperature, which incorporates humidity; and the black-bulb temperature, which accounts for sunlight and wind.

But unlike the heat index, this measure can be misleadingly low. The wet-bulb temperature is arrived at by covering a thermometer with a room-temperature water-soaked cloth. When humidity is (an extremely high) 100%, the wet-bulb temperature will equal the dry-bulb temperature. As humidity drops to more common but still uncomfortable levels the wet-bulb temperature will be lower. So while a dry-bulb temperature of 85 sounds manageable, a wet-bulb globe temperature of 85 represents “extreme threat.”

Owing to some of these limitations, other indexes have emerged that might show up on apps and are often powered by proprietary algorithms.

“A lot of private companies have been getting into the heat-hazard business,” Grundstein said. “I get a little nervous about proprietary heat measures when you don’t know what goes into it. The heat index and wet-bulb globe temperatures have been studied a lot. We know their strengths and weaknesses.”

Some of the more complex measures pose an additional limitation—the data is simply not always available at the world’s weather stations, said Lucas Vargas Zeppetello,

Still, we might need to get used to a proliferation of ways to measure the heat, given climate trends. In research published last year, Vargas Zeppetello found that dangerous heat-index readings (above 103 degrees) occurred less than once a year in many places in the midlatitudes—the global region between the tropics and polar circles that includes the U.S.—in the 1980s and 1990s. For this century, those regions are on course to face three to 10 times as many dangerously hot days.

“We have four or five different measurements trying to capture the same thing,” said Vargas Zeppetello, but, he emphasized, it’s hot by any of these measures. Whether it’s 90 degrees on the wet-bulb globe temperature or 120 on the heat index, it’s blazing out there this summer.

Write to Josh Zumbrun at [email protected]