The average overnight temperature for the month of June in Phoenix is normally about 75 degrees. But so far this month, overnight lows have been in the high 70s or the 80s.
When high temperatures last longer into evening or even through the night, public health officials say there’s a higher risk of heat-related illness.
“Particularly for people who don’t have access to cool space in those overnight hours, the public health community tells us it’s really important for the body to cool down and get some rest overnight, and when temperatures stay high that becomes more challenging," said David Hondula, director of Phoenix’s Office of Heat Response and Mitigation.
The city opened a 24-hour heat respite site for the first time this year and extended hours at a few other cooling centers specifically for this reason. City officials looked at 911 call data from last summer and found nearly a third of heat-related emergencies happened during hours when most heat relief sites were not open.
Nationwide, summer nights have warmed about 2.6 degrees since 1970, according to an analysis from climate research group Climate Central. Phoenix’s nights have gotten nearly 6 degrees hotter in the same time period as a result of climate change and the urban heat island effect.
The hottest night in Phoenix history occurred last year on July 19, when the overnight low temperature was 97 degrees.
"I am actually anticipating that we're probably going to hit our first 100-degree minimum temperature before we hit a temperature above 122," Arizona State University professor and rapporteur on extreme records for the United Nations and World Meteorological Organization, Randy Ceverny, told KJZZ's "The Show" in May. "I think sometime in the near future — maybe not this year, maybe in the next few years — we'll have a low temperature that does not get below 100 degrees."
