The hottest overnight temperatures in Phoenix history raise public health concerns

By Katherine Davis-Young
Published: Monday, July 31, 2023 - 5:05am
Updated: Monday, July 31, 2023 - 9:05am

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A Phoenix skyline
Getty Images
The sun sets over Phoenix.

St. Vincent de Paul’s daytime heat relief center just south of downtown Phoenix opens at 8 a.m. Zelphya Ynzunza, who works there, says during this heat wave, there’s been a line at the door every morning.

“You can just see it on their faces, like they’re overwhelmed or they just can’t wait to get in,” Ynzunza said.

Ynzunza says lately, a day’s worth of cold water bottles has been getting passed out within a few hours to people exhausted after nights spent outside in this heat.

“It’s just horrible, the heat, I can’t breathe,” says Raquel Parra, who has been coming to the cooling center every day this summer.

Climate Central U.S. summer nights
Climate Central
Summer overnight low temperatures nationwide are warming faster than daytime highs, according to Climate Central.

Parra was evicted from her apartment eight months ago and has been pitching a tent on a west Phoenix sidewalk every night since. She grew up in Arizona. She’s used to heat. But she’s never had to sleep outside before. She says the hot nights have been so bad she’s even had to call an ambulance to be treated for heat exhaustion.

“I was throwing up the whole night, I couldn’t eat nothing, I couldn’t put my head up, I was out of it. The dehydration really got to me that day,” Parra says.

That’s exactly why public health advocates and climate experts worry about warming overnight temperatures.

“It really does come down to being a big health concern as far as your body resting and recovering from continuous heat,” said Jen Brady a senior data analyst with the climate research group Climate Central.

Phoenix overnight temperatures
Climate Central
Phoenix's summer overnight low temperatures have warmed even faster than the national average, according to Climate Central.

Brady said when the human body never gets a chance to cool down, even at night, that’s when heat becomes much more dangerous.

And nighttime temperatures this summer have been the hottest Phoenix has ever seen. For the month of July, the average overnight low temperature was nearly 91 degrees, according to the National Weather Service. One night, the temperature never dropped below 97, making it the hottest night in Phoenix’s history.

Brady said that’s consistent with a nationwide trend. Climate change is driving up summer daytime highs. But summer overnight lows are warming nearly twice as fast.

“We’re seeing very few years where it’s not increasing from year to year,” Brady said.

In Phoenix, the trend is even more pronounced. Climate Central analysis shows nationwide, summer nights have warmed about 2.6 degrees since 1970. Phoenix’s nights have gotten nearly 6 degrees hotter in the same time period.

david hondula
Arizona State University
David Hondula

“That is really a consequence of urbanization,” said David Hondula, director of Phoenix’s Office of Heat Response and Mitigation.

Hondula said part of the reason nights have warmed so dramatically in Phoenix is the urban heat island effect.

“The dark, hard surfaces in the city tend to be really good at absorbing and retaining heat and slowly re-releasing it at night compared to the much brighter, surrounding, sandy desert environment,” Hondula said.

Since 1970, when summer nights were nearly 6 degrees cooler, the Phoenix metro area’s population has more than quadrupled. All that development brought machinery, cars, and bodies that warm up the city, along with a lot of pavement that traps heat overnight.

Meanwhile, as summer nights have warmed up, the region’s unsheltered population has grown and heat-related deaths have skyrocketed. Last year, Maricopa County reported a record 425 heat-related deaths. Individuals experiencing homelessness accounted for more than half of the county's heat-associated deaths in 2022 among cases where the person's living situation was known, according to the county. 

Hondula said one positive note about the urban heat island effect is, it’s an aspect of climate change municipalities could directly address.

“City government has its hands on the levers of urbanization to some extent,” Hondula said. “We regulate, to some extent, how the city will develop and grow over time and how it is redeveloped.”

Phoenix is expanding the use of cool pavement technology that reflects heat rather than trapping it. This spring, the city coated its 100th mile of pavement with the cooling seal. Hondula is also advocating for Phoenix to plant more trees to increase shade and hopefully slow warming trends in the city.

workers using squeegee type device to smooth material
City of Phoenix
Workers apply a treatment as part of a cool pavement pilot program at Esteban Park in June 2020.

For now, though, Phoenix’s hottest nights ever drag on. In spite of record spending by the state and local governments on heat relief and homelessness solutions, most Phoenix-area cooling centers aren’t open overnight, and there aren’t enough shelter beds to bring all of Phoenix’s unhoused people, like Parra, inside.

Parra says she sprays herself with water at night and sleeps with wet towels on her. But when she’s sleeping in a tent on hot pavement, it doesn’t help much.

“You’re still in the heat, whether the sun goes down or not. It’s still hot outside,” Parra says. 

She says she’s been selling her plasma for extra cash and has just enough for a motel room. For at least one night this summer, she says she’s going to try to get a break from the heat.

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