Why we have so many ways to measure hot weather

By Andrew Freedman, CNN

(CNN) — Temperatures are skyrocketing for the third straight day across the Eastern US, with officials in cities from Boston to Washington, DC trying to keep residents cool amid outdoor July 4th celebrations. Dozens of daily records are being set as the combination of heat and humidity sends the heat index soaring.

But “how hot is it?” is not as straightforward a question as it seems. And understanding the different heat metrics can be lifesaving, since heat is the number one weather-related killer in the US.

“The problem with temperature alone is that there’s other factors that can cause heat stress,” said Andrew Grundstein, a researcher at the University of Georgia who specializes in climate and health. Relative humidity, dewpoint, the angle of the sun and wind speed can all play a role.

Over the years, researchers have come up with a variety of ways to quantify how hot it is, and how extreme heat affects the human body. Each one has its merits and drawbacks, and some have become increasingly important as the climate heats up.

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First (and probably most familiar to people) is the heat index. The heat index is a value calculated by combining the air temperature and relative humidity, to arrive at something close to a “feels like” air temperature. In short, the higher the heat index, the more oppressive it feels outside, and the more dangerous the heat is to human health.

As with any heat metric, calculating the heat index involves making some assumptions about the person and the conditions around them.

“The heat index is based on a physiological model of human thermoregulation that assumes a healthy adult walking in the shade with the ability to keep the skin wetted with sweat,” said David Romps, a climate scientist at UC Berkeley who has closely studied the heat index.

“For someone who, for whatever reason, has an impaired ability to regulate their body temperature, or is walking in the direct sun, or is exerting themselves more vigorously, that underlying model will be less representative,” he said.

Another issue with the heat index concerns the way that it’s calculated. The system implemented by the National Weather Service falters mathematically when both the temperature and humidity are particularly high. In those situations, the heat index can underestimate the actual conditions by as much as 20 degrees Fahrenheit, Romps said.

Heat indices during the ongoing heat wave across the Eastern US have been as high as 110 degrees or greater in some of the big cities in the Mid-Atlantic region as well as in parts of the Midwest earlier in the week. But those could be underestimated because, at the same time, actual air temperatures soared into the low triple digits in these same locations and humidity was high.

Two other metrics are more focused on the physiological effects of extreme heat on the human body. First, there is the wet bulb temperature, which is the lowest temperature at which the evaporation of water can cool the air. This temperature changes as the air gets hotter and with shifts in humidity as well. Higher levels of humidity will raise the wet bulb temperature, whereas drier conditions will lower it.

The wet bulb temperature measures how efficiently our bodies can cool down through sweating. The higher the wet bulb temperature is, the harder it will be to get comfortable. Typically, the wet bulb temperature will be lower than the actual air temperature, because it is a measure of evaporative cooling. There is a theoretical upper limit to the wet bulb temperature for human survivability, which has been pegged at about 95 degrees.

But recent studies have shown that human bodies aren’t always able to cool themselves as easily as that theoretical maximum assumed. In fact, people can become vulnerable to heat-related illnesses starting at about 86 degrees. Some experts peg the survivability limit as closer to 91 degrees, depending on the exact circumstances of heat exposure.

Lastly, there is the wet bulb globe temperature, which incorporates far more factors than the other metrics. This makes it especially useful for assessing occupational hazards for outdoor workers and evaluating the safety of athletes who are competing in outdoor sports.

The wet bulb globe temperature measures heat stress in direct sunlight, taking into account temperature, humidity, wind speed and solar radiation. Some experts consider it to be the most accurate measure of heat stress. However, it is rarely used on mainstream weather maps and by TV meteorologists, who favor the heat index that has long reigned supreme and is easily understood by the public.

This may be because there are many instances when the air temperature is high, but the wet bulb globe temperature is lower than the air temperature, thanks to humidity levels, cloud cover, wind speed or other factors. That could be confusing to general audiences, especially those accustomed to seeing the heat index instead.

According to Grundstein, the communication of the heat metric is almost as important as the measurement itself. “They all have their pluses and minuses,” he said. “And ultimately, what you’re trying to get from these heat metrics is a sense about how much heat stress a person is getting and whether there’s a dangerous condition.”

He noted that there is no perfect heat metric or one that he favors. Instead, “I think you need the right metric for the right audience. I think that’s really the big thing,” he said.

Climate scientists tend to favor the use of the wet bulb temperature and wet bulb globe temperature in their studies of when and where heat stress may have surpassed, or may very soon surpass, the limits of human survivability at certain times of the year.

Research has shown that this is already on the cusp of happening in parts of South Asia and the Middle East, and that continued emissions of fossil fuels will greatly expand the areas that people will have a hard time surviving in if they go outdoors during the day for hours at a time.

“The burning of fossil fuels is ratcheting up temperatures irreversibly… and we are not far from the globe temperature at which we will see conditions on Earth that are not survivable,” Romps said.

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