Heat index—how hot it feels—is rising faster than temperature: Study

Texans are used to very hot summers. However, a recent study shows that the heat index, which is the temperature we feel, has risen much

By Ground report 20 Mar 2024

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Texans are used to very hot summers. However, a recent study shows that the heat index, which is the temperature we feel, has risen much quicker in Texas compared to the actual recorded temperature. It’s rising about three times as fast.

Climate change raises temperature impact

This means on the hottest days, it can feel between 8 to 11 degrees Fahrenheit (5 to 6 degrees Celsius) warmer than what we would experience if there were no climate change effects. David Romps, a professor of earth and planetary science at the University of California, Berkeley's study was published in the journal Environmental Research Letters.

The study, which uses Texas data from June, July, and August of 2023, highlights the problem of communicating the dangers of rising temperatures to the public. The temperature does not accurately reflect the heat stress that people feel.

According to David Romps, the heat index itself, which considers the relative humidity and the ability to cool off by sweating, provides a conservative estimate of heat stress.

In 2022, Romps co-authored a paper in which he pointed out that the method most government agencies use to calculate the heat index is inaccurate when dealing with today's temperature and humidity extremes. This causes people to underestimate their risk of suffering from hyperthermia on the hottest days and their chances of dying.

Texas does not stand out as an outlier. Arizona's most populous county, which mostly encompasses Phoenix, reported last week that the deaths associated with heat last year increased by 50% compared to 2022, with the numbers rising from 425 in 2022 to 645 in 2023. The data from 2023 shows that two-thirds of the heat-related deaths in Maricopa County were people aged 50 years or older, and 71% of the deaths happened on days that the National Weather Service had issued an excessive heat warning, the Associated Press reports.

Romps said, "I mean, we should cease additional warming because this situation will not improve if we don't stop burning fossil fuels. Undoubtedly, that's message number one. We can only cause the planet's average temperature to go in one direction, and that's up. We do that through the additional burning of fossil fuels. So, we gotta stop, and stop fast."

He said that it feels much hotter than you'd expect from the increase in ambient temperature alone because global warming is affecting the interplay between humidity and temperature. In the past, relative humidity typically dropped when the temperature increased, allowing the body to sweat more and thus feel more comfortable.

The corrected heat index created by UC Berkeley researchers fixes problems encountered at humidities and temperatures that the originator of the index thought would be rarely reached and also where the model broke down.

However, climate change keeps the relative humidity relatively constant as it increases the temperature, thereby reducing the effectiveness of sweating as a cooling method for the body.

Romps said, "We already experience irreversible temperature increases and people need to deal with this by taking precautions to avoid hyperthermia. I advice those in extreme heat situations who cannot access air conditioning to use shade and water as their friends."

"You can coat yourself in water. Get a wet rag, run it under the faucet, get your skin wet, and get in front of a fan. As long as you drink enough water and keep that skin wet in front of the fan, you're doing a good thing for yourself."

It's the humidity

Several years ago, Romps, an atmospheric physicist, was interested in the human body's response to increased temperatures due to global warming. He noted that even though the heat index was defined in 1979 based on the physiological stresses induced by heat and humidity, today's extremes of heat and humidity were not included in the heat index calculations.

Romps and graduate student, now postdoctoral fellow Yi-Chuan Lu, extended the heat index calculation to include all combinations of temperature and humidity. This development enables the use of the heat index even during the most extreme heat waves, such as the ones Texas experienced in the summer of 2023.

Over the decades, the National Oceanic and Atmospheric Administration's (NOAA) National Weather Service, the nation's major weather forecaster, has handled the absence of calculated values for high heat and humidity by extrapolating from the known values. However, when dealing with extreme temperature and humidity conditions, Romps and Lu found that this commonly used extrapolation method significantly falls short.

"Although NOAA has now calculated the heat index for all conditions using the underlying physiological model, they have not yet adopted those values."

After Lu spent a sweltering summer in Texas last year, Romps decided to take the state as a case study to determine how global warming has affected the perceived heat stress represented by the corrected heat index.