Overnight temperatures in Phoenix stay sky high

Phoenix tied the daily high overnight temperature for July 10, matching the record of 91°F Monday.

Context: The only other time the recorded temperature at Sky Harbor International Airport hit 91°F on that date was in 2020, Tom Frieders, a meteorologist with the National Weather Service in Phoenix, tells us.

• It was the first time this year the overnight low was above 90°F, he said.

Why it matters: While Phoenix and much of the rest of the country are suffering a potentially record-setting heat wave, with highs regularly exceeding 110°F during the day, extremely high nighttime temperatures pose a unique risk.

• "You're not getting that overnight relief as much as you would want to kind of recover from the daytime extreme heat," Frieders said.
• At its most extreme, that can cause heat stress, which increases the chances of heat stroke, Lisa Patel, executive director of the Medical Society Consortium on Climate and Health, told CNN last month.
• High nighttime temperatures also cause people to get less sleep.

Zoom in: Overnight heat is especially problematic for people experiencing homelessness or others without access to cooling, Frieders said.

The big picture: Phoenix has had 11 consecutive days of high temperatures of 110°F or more, and we could break the record of 18 days set in 1974.

• Climate change is causing heat waves to become more frequent, intense and longer-lasting.

Between the lines: Monday's record-tying low temperature aside, Phoenix has actually seen fewer days with excessive overnight highs this summer, Frieders said.

• As of Monday, NWS recorded nine consecutive days with low temperatures above 80°F, and only 15 for the year.
• That's slightly below the 21 80°F-plus days we experience through July 10 in an average year.
• The drier-than-average monsoon season we've had is responsible. The dry air that's driving high daytime temperatures is keeping things relatively cool overnight.

Yes, but: NWS expects increased moisture to drive up nighttime temperatures over the next few days.

• No rain is expected and the increased moisture will be subtle, Frieders said, but enough to boost overnight lows. NWS forecasts nighttime lows from 90-93°F for the rest of this week and into next week.

Of note: The highest low temperature recorded at Sky Harbor was on July 15, 2003, when the mercury hit 96°F overnight.