Across large portions of the nation, it has been an exceptionally long, steamy summer. While there have been plenty of hot afternoons, unusually warm nights have set this summer apart, piling up in record numbers.

From Southern California to northern Maine, scores of U.S. cities have racked up more warms nights than ever previously recorded, as indicated by the frequency of low temperatures of 70 degrees or higher.

Washington has observed such warm low temperatures a record 85 times this year, compared with the normal of 57. Fifteen have occurred in September alone, a record for the month. And there are probably more to come.

Many locations along the East Coast have joined Washington in accumulating these warm nights in record numbers, including:

  • Caribou, Maine
  • Boston
  • Salisbury, Md.
  • Richmond
  • Norfolk
  • Wilmington, N.C.
  • Charleston, S.C.

Especially in the Northeastern United States, the record number of warm nights is closely related to the fact that it has been so humid. Temperatures at night don’t fall as much when the air is loaded with moisture.

It’s been humid and it’s been cloudy — the two often go hand in hand. Instead of the crystal-clear skies September is typically known for, Washington just finished one of its cloudiest stretches in memory, lasting 10 days.

The record frequency of warm nights is perhaps unsurprising, considering that the National Oceanic and Atmospheric Administration recently announced low temperatures in the Lower 48 states ranked warmest on record between June and August.

An increase in the frequency and intensity of warm nights is an expectation of climate change, because of urbanization and increasing greenhouse gas concentrations in the atmosphere. Overnight low temperatures are warming “nearly twice as fast as afternoon high temperatures,” NOAA said.

For its part, Washington has seen a dramatic increase in its annual frequency of low temperatures of 70 degrees or higher, as shown in the chart below. A century ago, the District averaged half as many nights with lows at 70 or higher, compared with current times.

It is not alone. Other locations in the broader Washington region exhibit similar trends, including less urban areas toward the mountains.

All data in this post was obtained from xmACIS2 or the Southeast Regional Climate Center. An earlier version of the cities comparison graphic with an error has been corrected.