Overnight temperatures have climbed to record highs this summer across nearly half the planet, driven by extreme humidity that keeps cities and bodies from cooling down after dark.
Ben Noll and Scott Dance report for The Washington Post.
In short:
- Nearly half the world has experienced record or near-record nighttime temperatures since June, with much of the eastern United States, Europe, and Asia affected.
- High humidity prevents sweat from evaporating, raising body temperature and increasing health risks such as heart attacks, strokes, sleep disruption, and weakened immune systems.
- In the U.S., summer nights have been especially warm due to oceanic heat and high dew points, which limit how much air can cool overnight, especially in regions near large bodies of water.
Key quote:
“Our bodies can withstand some high temperatures but require respite periods during which it’s cooler and our bodies can recover.”
— Kathryn Conlon, associate professor at the University of California at Davis
Why this matters:
The body needs cooler conditions to recover from daytime heat stress, but high humidity traps warmth, making it harder to cool down. This has serious public health consequences, especially for people without air conditioning or those in dense urban areas where pavement radiates heat overnight. Elevated nighttime temperatures have been linked to higher death rates, disrupted sleep, and increased risks for heart disease and depression. As climate change pushes oceans to release more moisture into the air, humid nights are becoming the new normal.
Read more: Hotter heat waves are sticking around as the planet keeps warming
