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Today’s heatwaves feel much hotter than the heat index indicates

The long-used Heat Index table (top) underestimates the apparent temperature for the most extreme heat and humidity conditions encountered today (center). The corrected version (below) is accurate across the range of temperatures and humidity that people will encounter with climate change. Credit: David Romps and Yi-Chuan Lu, UC Berkeley

If you looked at the heat index during this summer’s sticky heatwaves and thought, “It sure feels hotter,” you might be right.

An analysis by climate scientists at the University of California, Berkeley, shows that the apparent temperature, or heat index, calculated by meteorologists and the National Weather Service (NWS) to indicate how warm it feels — taking humidity into account — the observed temperature for the hottest days we experience now, sometimes exceeding 20 degrees Fahrenheit.

The finding has implications for those affected by these heat waves, as the heat index is a measure of how the body handles heat when humidity is high, and sweating becomes less effective at cooling us down. Sweating and blushing — in which blood is diverted to capillaries close to the skin to dissipate heat — and shedding clothing are the main ways humans adapt to high temperatures.

A higher heat index means the human body is more stressed during these heat waves than public health officials may realize, the researchers say. The NWS currently considers a heat index above 103 as dangerous and above 125 as extremely dangerous.

“Usually the heat index that the National Weather Service gives you is just the right value. It’s only in these extreme cases that they get the wrong number,” said David Romps, a professor of Earth and Planets at UC Berkeley. science. “The point is when you map the heat index back to physiological states and you realize, oh, these people are stressed to a state of very increased blood flow to the skin, where the body is running out of tricks. has to compensate for this kind of heat and humidity. So we’re closer to that edge than we thought we were before.”

Romps and graduate student Yi-Chuan Lu detailed their analysis in a paper accepted by the magazine Letters for environmental research and posted online on August 12.

The heat index was devised in 1979 by a textile physicist, Robert Steadman, who made simple equations to calculate what he called the relative “swellness” of warm and humid, as well as hot and dry, conditions during the summer. He saw it as an addition to the wind chill factor that is often used in winter to estimate how cold it feels.

His model took into account how people regulate their internal temperature to achieve thermal comfort under different external conditions of temperature and humidity – by consciously changing the thickness of clothing or unconsciously increasing respiration, perspiration and blood flow from the core of the body to the skin. to adjust.

In his model, the apparent temperature under ideal conditions — an average-sized person in the shade with unlimited water — is how warm a person would feel if the relative humidity were at a comfortable level, which Steadman said was a vapor pressure of 1,600 pascals.

For example, at 70% relative humidity and 68 F – which is often considered average humidity and temperature – a person would feel it is 68 F. But at the same humidity and 86 F, it would feel like 94 F.

The heat index has since been widely accepted in the United States, including by the NWS, as a useful indicator of people’s comfort. But Steadman left the index undefined for many conditions that are now becoming more common. For example, for a relative humidity of 80%, the heat index is not defined for temperatures above 88 F or below 59 F. Today, temperatures in some areas, including the Midwest and Southeast, regularly rise above 90 F for weeks.

To account for these gaps in the Steadman map, meteorologists extrapolated to these areas to get numbers, Romps said, which are mostly correct, but not based on any understanding of human physiology.

“There is no scientific basis for these numbers,” Romps said.

He and Lu wanted to expand Steadman’s work so that the heat index is accurate at all temperatures and all humidity between zero and 100%.

“The original table had a very short range of temperature and humidity and then a blank area where Steadman said the human model failed,” Lu said. “Steadman had the right physics. Our goal was to extend it to all temperatures so we have a more accurate formula.”

One condition under which Steadman’s model collapses is when people perspire so much that sweat collects on the skin. At the time, his model erroneously had a skin surface relative humidity of more than 100%, which is physically impossible.

“It was at that point that this model seemed to break, but it’s just the model that told him, hey, let the sweat drip from the skin. That was all,” Romps said. “Let the sweat drip from the skin.”

That and a few other tweaks to Steadman’s equations yielded a comprehensive heat index that matches the old heat index 99.99% of the time, Romps said, but also accurately reflects the apparent temperature for regimes beyond those that Steadman originally used. had calculated. When he originally published his apparent temperature scale, he thought these regimes were too rare to worry about, but high temperatures and humidity are becoming more common due to climate change.

Romps and Lu published the revised heat index equation earlier this year. In the most recent paper, they apply the comprehensive heat index to the top 100 heat waves that occurred between 1984 and 2020. The researchers find mostly minor disagreement with what the NWS reported at the time, but also some extreme situations where the NWS heat index was far away.

One surprise was that seven of the 10 most physiologically stressful heat waves during that time period were in the Midwest — mainly Illinois, Iowa and Missouri — and not the Southeast, as meteorologists assumed. The largest discrepancies between the NWS Heat Index and the Comprehensive Heat Index were seen across a broad swath from the Great Lakes south to Louisiana.

For example, during the July 1995 Chicago heat wave that killed at least 465 people, the maximum heat index reported by the NWS was 135 F when it actually felt like 154 F. The revised Midway Airport heat index, 141 F, implies that people in the shade would have experienced a blood flow to the skin 170% higher than normal. The heat index reported at the time, 124 F, implied only a 90% increase in skin blood flow. In some places during the heat wave, the extended heat index implies that people would have experienced an 820% increase over normal skin blood flow.

“I’m not a physiologist, but a lot of things happen to the body when it gets really hot,” Romps said. “Diverting blood to the skin puts a strain on the system because you’re drawing blood that would otherwise be sent to the internal organs and sending it to the skin to try and raise the skin temperature.” The approximate calculation used by the NWS and common adopted, inadvertently downplays the health risks of severe heat waves.”

Physiologically, the body starts to get confused when the skin temperature rises to equal to the core body temperature, usually taken as 98.6 F. After that, the core temperature starts to rise. The maximum sustainable core temperature is believed to be 107 F – the threshold for heat death. For the healthiest individuals, that threshold is reached at a heat index of 200 F.

Fortunately, humidity tends to decrease as temperatures rise, so Earth is unlikely to reach those conditions in the next few decades. Less extreme – though still deadly – conditions are nevertheless becoming common around the world.

“A heat index of 200 F is an upper limit of what is survivable,” Romps said. “But now that we have this model of human thermoregulation that works under these conditions, what does it actually mean for the future habitability of the United States and the planet as a whole? There are some terrifying things we’re looking at.”


Getting Hot and Hotter: Five Essential Lectures on High Temperatures and Human Bodies


More information:
David M. Romps et al, Chronically Underrated: A Reassessment of US Heat Waves Using the Comprehensive Heat Index, Letters for environmental research (2022). DOI: 10.1088/1748-9326/ac8945

Provided by University of California – Berkeley

Quote: Today’s heatwaves feel much hotter than the heat index suggests (2022, August 15) retrieved August 15, 2022 from https://phys.org/news/2022-08-today-lot-hotter-index- implies.html

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