In Eastern US, Humidity Is Off the Charts

More than 70 million Americans sweated through the muggiest first two months of summer on record as climate change has noticeably dialed up the humidity in the eastern US in recent decades, an AP data analysis shows. And that meant uncomfortably warm and potentially dangerous nights in many cities the past several weeks, the National Weather Service said. Parts of 27 states and Washington, DC, had a record number of days that meteorologists call uncomfortable—with average daily dew points of 65 degrees Fahrenheit or higher—in June and July, according to data derived from the Copernicus Climate Service.

• Peak days: And that's just the daily average. In much of the East, the mugginess kept rising to near tropical levels for a few humid hours. Philadelphia had 29 days, Washington had 27 days, and Baltimore had 24 days when the highest dew point simmered to at least 75 degrees, which even the weather service office in Tampa calls oppressive.
• Record levels: Dew point is a measure of moisture in the air expressed in degrees that many meteorologists call the most accurate way to describe humidity. This summer has had dew points that average at least 6 degrees higher than the 1951-2020 normals in Washington, Baltimore, Pittsburgh, Richmond, Columbus, and St. Louis, the AP calculations show. The average June and July humidity for the continental US east of the Rockies rose to more than 66 degrees, higher than any year since measurements started in 1950.
• Beyond measure: Twice this summer climate scientist Cameron Lee of Kent State University measured dew points of about 82 degrees at his home weather station in Ohio. That's off the various charts that the weather service uses to describe what dew points feel like. "There are parts of the United States that are experiencing not only greater average humidity, especially in the spring and summer, but also more extreme humid days," Lee said.

• Less relief at night: 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 of the National Weather Service. "What really impacts the body is that nighttime temperature," he 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."
• Causes: An extra hot and rainy summer weather pattern is combining with climate change from the burning of coal, oil, and natural gas, said Bernadette Woods Placky, chief meteorologist at Climate Central. 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. From the 1950s into 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."

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• The feel: 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 warmer it gets, meteorologists said.
• Location: 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 farther north, into places where people are less used to it.