What happened to those delightfully cool summer nights?

August 29, 2019, 11:55 a.m.
kids running around outside in the summer
Photo: Zurijeta/Shutterstock

One of the best parts about the waning days of summer are the cool summer nights. After a humid, hot day, you look forward to the evening when the temperature dips as the sun goes down.

But those cool summer nights seem to be a victim of climate change.

This July was the hottest month ever measured. And as summer days continue to get more scorching, it's harder for evenings to cool off.

Climate change journalist Robinson Meyer writes in The Atlantic, "Across the lower 48 states, summer nights are now warmer than they have been at any other point since 1900, according to data from the climate scientist Zeke Hausfather. Daily minima—in other words, the temperature at the coolest part of the night—are regularly hotter now than they were even during the warmest years of the Dust Bowl. Daytime highs have actually yet to eclipse that mark."

Warm nights have become more common throughout the U.S. A recent climate assessment from North Carolina State University looked specifically in the Southeast and found large increases in nighttime temperatures with modest increases in daytime temperatures.

"The number of warm nights (minimum temperature above 75°F) has doubled on average compared to the first half of the 20th century and locally has increased at most [weather] stations," researchers wrote.

Record-breaking temps

The summer of 2018 was the hottest yet for nighttime temps. The nighttime temperature, averaged nationwide for June, July and August, was the hottest ever, recorded at 60.9 degrees, according to the National Oceanic and Atmospheric Administration (NOAA).

Every state had an above-average summer minimum temperature. Meteorologists define summer as June 1 through Aug. 31.

When the atmosphere can't cool off at night, it's a sign of global warming due to human-caused conditions, according to NOAA and other climate experts.

"In general, since records began in 1895, summer overnight low temperatures are warming at a rate nearly twice as fast as afternoon high temperatures for the U.S. and the 10 warmest summer minimum temperatures have all occurred since 2002," NOAA said in a statement.

And sweltering summer nights aren't just an inconvenience; they can be dangerous.

"The combination of high daytime and high nighttime temperatures can be really lethal because the body doesn’t have a chance to cool down during the nighttime hours," Lara Cushing, professor of environmental epidemiology at San Francisco State University, told The New York Times.

And those risks are higher in places like the California coast, where people aren't used to soaring temperatures.

"A hundred and five degrees in San Francisco is going to have a bigger impact probably than 105 degrees in Houston, Texas, where everybody has air conditioning and people are accustomed to dealing with high temperatures."