The average high temperature ranked 11th warmest in records that date back to 1895, but the average low was a record-warm, 2.5 degrees above average, and 0.1 degrees above the previous record in 2016.
“Every state had an above-average summer minimum temperature with five states record warm,” the report said.
As greenhouse gas concentrations in the atmosphere keep increasing, overnight low temperatures are warming “nearly twice as fast as afternoon high temperatures,” according to NOAA. “[T]he 10 warmest summer minimum temperatures have all occurred since 2002,” it wrote.
Both Burlington, Vt., and Mount Washington, N.H., registered their warmest low temperature ever recorded during the summer.
Such warm nights have important consequences. They increase heat stress on the homeless and those without air conditioning, which can lead to heat-related illness and death. At the same time, they require increased use of air-conditioning for those with access, which adds more heat-trapping gases into the atmosphere.
Day and night combined, almost the entire nation notched above average temperatures during the June to September summer months.
From the Great Plains to the East Coast, the warmth was accompanied by frequent downpours. In the Northeast, in particular, the combination of warmth and above normal rainfall led to record-challenging humidity levels.
The waterlogged air, more difficult to cool, was a key factor in the record high nighttime temperatures.