Daytime high temperatures have only broken one daily heat record so far this summer in Phoenix. But overnight lows are a different story, with 10 heat records set since May.
Friday’s low was 97 degrees — the hottest overnight temperature ever recorded in Phoenix. The city’s low temperature has been 97 just one other time, on July 19, 2023.
From 1990 to 2020, Phoenix averaged seven nights per year with low temperatures at or above 90 degrees, according to National Weather Service data. But for each of the past three years, Phoenix has had more than 20 nights with temperatures in the 90s.
This year, Phoenix has already had 11 lows at or above 90 degrees, with more scorching overnight heat still in the forecast.
Climate scientists say summer nighttime lows are warming much faster than daytime highs because human-caused carbon pollution and urban development are trapping heat close to the earth, even after the sun sets.
According to the climate research organization Climate Central, summer overnight warming is a nationwide trend. On average, summer nights across the U.S. have warmed about 3 degrees since the 1970s.
The trend is even more pronounced in Phoenix, where summer nights have warmed 6.5 degrees since the 1970s, according to Climate Central.
Medical experts say the overnight heat raises public health concerns.
In Maricopa County, heat-related deaths tend to increase when nights stay hotter and people never get the chance to cool off, Maricopa County Department of Public Health medical director Dr. Nick Staab told KJZZ in May.
“It’s kind of that sustained heat with high overnight lows that we see as the biggest risk,” Staab said.
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