Over 70 million people across America endured the stickiest start to summer in history. Climate change is increasingly impacting humidity levels in the Eastern United States, as revealed by an Associated Press data review.

And that meant uncomfortably warm and potentially dangerous nights in many cities the last several weeks, the National Weather Service said.

In June and July, parts of 27 states plus Washington, D.C., experienced a record high number of uncomfortable days, defined by meteorologists as having average daily dew points of 65 degrees Fahrenheit or above, according to the Copernicus Climate Service data.

And that’s not just the average. Across much of the East, humidity surged to almost tropical levels at times. Philadelphia experienced 29 days, Washington recorded 27 days, and Baltimore had 24 days where dew points hit at least 75 degrees, which even the Tampa weather service office considers oppressive, based on weather service data.

Dew point indicates air moisture levels and is considered by many meteorologists as the best description of humidity. This summer, cities like Washington, Baltimore, Pittsburgh, Richmond, Columbus, and St. Louis recorded average dew points 6 degrees higher than the 1951-2020 normals, the AP found. Across the nation east of the Rockies, June and July humidity averaged over 66 degrees, the highest since 1950.

“This has been a very muggy summer. The humid heat has been way up,” said Bernadette Woods Placky, chief meteorologist at Climate Central.

Twice this summer, Cameron Lee, a climate scientist and humidity expert at Kent State University, recorded dew points around 82 degrees at his Ohio weather station, levels that surpass the usual weather service charts describing dew point effects.

“Certain areas in the United States are witnessing not just higher average humidity, especially during spring and summer, but also more intensely humid days,” Lee noted. He added that these extremely sticky conditions are now spreading over more days and wider areas.

High humidity doesn’t allow the air to cool at night as much as it usually does, and the stickiness contributed to multiple nighttime temperature records from the Ohio Valley through the Mid-Atlantic and up and down coastal states, said Zack Taylor, forecast operations chief at the National Weather Service’s Weather Prediction Center. Raleigh, Charlotte, Nashville, Virginia Beach, Va., and Wilmington, N.C., all reached records for the hottest overnight lows. New York City, Columbus, Atlanta, Richmond, Knoxville, Tennessee and Concord, New Hampshire came close, he said.

“What really impacts the body is that nighttime temperature,” Taylor said. “So if there’s no cooling at night or if there’s a lack of cooling it doesn’t allow your body to cool off and recover from what was probably a really hot afternoon. And so when you start seeing that over several days, that can really wear out the body, especially of course if you don’t have access to cooling centers or air conditioning.”

An extra hot and rainy summer weather pattern is combining with climate change from the burning of coal, oil and natural gas, Woods Placky said.

The area east of the Rockies has on average gained about 2.5 degrees in summer dew point since 1950, the AP analysis of Copernicus data shows. In the 1950s, 1960s, 1970s, 1980s and part of the 1990s, the eastern half of the country had an average dew point in the low 60s, what the weather service calls noticeable but OK. In four of the last six years that number has been near and even over the uncomfortable line of 65.

“It’s huge,” Lee said of the 75-year trend. “This is showing a massive increase over a relatively short period of time.”

That seemingly small increase in average dew points really means the worst ultra-sticky days that used to happen once a year, now happen several times a summer, which is what affects people, Lee said.

Higher humidity and heat feed on each other. A basic law of physics is that the atmosphere holds an extra 4% more water for every degree Fahrenheit (7% for every degree Celsius) warmer it gets, meteorologists said.

For most of the summer, the Midwest and East were stuck under either incredibly hot high pressure systems, which boosted temperatures, or getting heavy and persistent rain in amounts much higher than average, Taylor said. What was mostly missing was the occasional cool front that pushes out the most oppressive heat and humidity. That finally came in August and brought relief, he said.

Humidity varies by region. The West is much drier. The South gets more 65-degree dew points in the summer than the North. But that’s changing.

University of Georgia meteorology professor Marshall Shepherd said uncomfortable humidity is moving further north, into places where people are less used to it.

Summers now, he said, “are not your grandparents’ summers.”

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Borenstein reported from Washington and Wildeman reported from Hartford, Connecticut.

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