Summer minimum temperatures in Phoenix
Source: NOAA’s Global Historical Climatology Network·Temperature distributions show daily minimum temperatures for the summer months (June-August) in each decade, measured by the airport weather station. Data for the 2020s consists of the 2020 and 2021 summers. Nights are considered abnormally cold or hot based on the 5th and 95th percentile of the 1960s temperature distribution.
This summer was unusually hot, especially at night. Minimum temperatures were the hottest on record for every state on the West Coast and parts of the Northeast. Most other states neared their record highs for overnight temperatures this meteorological summer (June through August).
This is part of a trend that aligns with the predictions of climate models: Across the U.S., nights are warming faster than days. This effect is amplified in cities, which are typically warmer than their surroundings.
“At nighttime, the deserts cool off really, really fast, but our city does not," said Jennifer Vanos, a professor in the School of Sustainability at Arizona State University, referring to Phoenix.
"Not having that break from the heat is really hard on the human body — it builds up," she said. "And knowing the temperatures in Phoenix, we're going to be in the 90s overnight and we're going to be up to 110 sometimes in the day. None of those are safe for a person that doesn't have access to air-conditioning."
To see how summer nights have gotten hotter in recent decades, we charted 60 years of daily weather data from nearly 250 U.S. airports that have kept consistent weather records.
Summer minimum temperatures in Las Vegas
Summer minimum temperatures in New Orleans
Summer minimum temperatures in Philadelphia
Summer minimum temperatures in Atlanta
These charts show a clear trend: Nighttime temperatures that would have been unusually hot in the 1960s are increasingly common.
The warming trend seen in these charts reflects two factors that add together: Global warming from human activity is raising the planet’s background temperature, and many cities are becoming more urbanized.