More than 70 million Americans endured the muggiest start to summer ever recorded during the first two months, as climate change has significantly increased humidity levels in the Eastern United States over recent decades, according to an analysis of data by the Associated Press.

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

Regions in 27 states along with Washington, D.C., experienced an unprecedented number of days that meteorologists describe as ‘uncomfortable’ — characterized by average daily dew points reaching 65 degrees Fahrenheit or greater — throughout June and July, as per data from the Copernicus Climate Service.

This uncomfortable condition represents only the daily average. In many eastern areas, the humidity surged to nearly tropical levels during peak hours. Cities like Philadelphia recorded 29 days, Washington had 27 days, and Baltimore had 24 days where maximum dew points climbed to at least 75 degrees, which the weather office in Tampa even considers oppressive, according to weather data records.

Dew point is a metric for quantifying moisture in the air in degrees, widely regarded by meteorologists as the best descriptor of humidity. For the summer of 2025, dew points in places such as Washington, Baltimore, Pittsburgh, Richmond, Columbus, and St. Louis averaged at least 6 degrees higher than the normal range from 1951-2020, according to the AP’s calculations. Moreover, the June and July average humidity across the eastern U.S., east of the Rockies, surpassed 66 degrees, the highest recorded since measurements began in 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, climate scientist and humidity specialist Cameron Lee from Kent State University recorded dew points around 82 degrees at his personal weather station in Ohio. This reading exceeds the various charts the weather service uses to interpret how dew points affect human comfort.

“Various regions in the United States are encountering not just increased average humidity, particularly during spring and summer, but also a spike in extremely humid days,” Lee noted. He mentioned that persistently sticky days are now extending across more days and covering larger 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.”