This summer was one of the hottest on record in Phoenix. And temperatures this June, July and August were especially warm at night.
From 1991 to 2020, the average low temperature June through August in Phoenix was 82.2 degrees. The average low temperature June through August this year was 84.7 degrees, according to National Weather Service records.
And, since the 1990s, typically the low temperature has been above 90 degrees about seven times per year. This summer, Phoenix had more than triple that, with 23 nights in the 90s.
The only summers with more 90-degree lows were all within the last five years — 2020 had 28 nights in the 90s, 2023 had 35 and 2024 had 39.
Arizona State Climatologist Erinanne Saffell said human-caused climate change and drier-than-average monsoon seasons are part of the reason why summer nights have been so much hotter in recent years.
“In the summer, when we have clear kinds of conditions — so when we don’t have an active monsoon season — the potential to get those nighttime temperatures hotter is there,” Saffell said.
And Saffell said the urbanization of the Phoenix area has accelerated the trend.
“Those parking lots and roads and cement sidewalks, they store that sunlight and they release it very slowly at night. It makes those nighttime temperatures hotter,” Saffell said.
Saffell said the urban heat island effect is warming overnight temperatures even in areas with much smaller populations than metro Phoenix.
“We’ve measured that in Sedona, it’s being measured in Yuma right now,” Saffell said. “When you’re having roads and buildings — that holds onto that sunlight.”
Saffell said future summers could bring even more nights in the 90s to the Phoenix area.
Utilities say hot summer nights drive up demand for air conditioning. And biologists say the hot nights stress vegetation.
But Saffell said her greatest concern with the increase in 90-degree lows is an increased health risk.
“We pay attention to that particularly because we want everyone to be safe,” Saffell said. “If you’re not able to cool off effectively at night, that can become problematic.”
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