Joy Leafgreen knows Iowa’s brand of humidity well, and it doesn’t really faze her. As the lifelong Iowan’s daughters played around the splash pad at Greenwood Park in Des Moines in mid-May, she described it:
“It feels very hot and sticky, almost oppressively hot,” she said. “You feel like you are wearing a lot of stickiness.”
This year, areas of Iowa, Missouri and Illinois got an early dose of high humidity, part of a larger weather system that swathed the eastern half of the country in uncomfortable moisture starting the second week of May. For some regions of the country, the muggy conditions will persist until after Memorial Day.
Climate scientist Chris Gloninger, who used to work as a television meteorologist in Iowa, said humidity is getting worse.
“As the air warms with climate change, it holds more moisture, and that doesn’t necessarily mean just heavy rain,” Gloninger said. “It can mean higher relative humidity. Higher dew points.”
Gloninger said the higher dew points, or the amount of water in the air, are the culprit when humidity starts to creep higher. A 60-degree dew point feels muggy. A 75-degree dew point feels extremely uncomfortable.
So, if the temperature on a given day is 85 degrees with a dew point of 70 degrees, Gloninger said:
“That would be awful. That’d be a really pretty disgusting day.”
Corn and the Caribbean Sea
Gloninger pointed to two factors that account for increasingly higher humidity in the region.
“You need to look at that bigger picture of what’s happening off to the south and ocean temperature,” he said.
The Caribbean Sea is experiencing a marine heat wave, higher than normal surface temperatures for a prolonged period, according to the National Oceanic and Atmospheric Administration.
Higher sea surface temperatures generally lead to increased humidity. Warmer water evaporates faster, adding more moisture to the atmosphere. The humid air moves northward.
Gloninger draws a direct line between high dew points earlier in the year to sea surface temperatures.
“The Gulf of Mexico right now is slightly above average. The Caribbean is well above average. So when you connect all of those points, you really begin to see the problem,” Gloninger said.
Far from tropical seas, the cornfields of the Midwest play a role in rising dew points — albeit later in the year.
“Corn gives off a lot of moisture,” Gloninger said. “It needs a lot of moisture, but it also transpires, sweats out a lot of moisture.”
According to a report by Harvest Public Media, corn sweat is a summertime phenomenon that happens when the crop releases water into the air.
While many dispute that corn sweat is a leading factor in rising dew points in the region, Gloninger disagrees.
“Corn sweat is real,” he said.
Gloninger said that, while he was working as a Des Moines television meteorologist between 2021 and 2023, there were summers when the dew points were in the 80s in towns like Creston, Lenox and Atlantic — all high producers of corn.
“That’s absurd levels of humidity and extremely high dew points.”
That humidity affected areas of neighboring Wisconsin and Illinois, Gloninger said.
Health effects
The storms that ripped through St. Louis on May 16, combined with humidity, could put some people with lung conditions at risk when combined with high humidity, said Dr. Ryan Freedle, a physician at St. Louis University Hospital.
“It seems reasonable to think that severe thunderstorms and tornadoes might disturb dust and other materials for some time in their wake,” he said. “Also the physical and water damage to homes and structures where people might work may lead to greater exposure to mold, pollen, bacteria in the air and other sorts of things that might lead to worse health outcomes.”
Even before the storms, in late April, the American Lung Association gave St. Louis a grade of “D” for particle pollution in its annual State of the Air report.
“Humidity makes certain chemicals more likely to be present and more difficult to leave the air,” he said. “It makes it more likely for soot and heavy particulates to kind of hang around for a bit. Those things also might have a negative impact on folks’ breathing.”
Even in places with good air quality, Freedle said people with respiratory conditions can experience exacerbated breathing problems when the air is saturated with moisture. Those with asthma and chronic obstructive pulmonary disease (COPD) are especially vulnerable, he said.
“While the water itself is not very harmful for folks who have asthma and COPD and other related lung diseases, all of the mold spores and pollen and other sorts of things that come along with the humidity certainly are, and they can trigger folks to have kind of an allergic reaction that narrows their airways and makes it more difficult to breathe,” he said.
Freedle said another hazard of high humidity is heat stroke, which can be fatal. This happens when the body is exposed to high heat or physical activity in high temperatures for too long and the body temperature gets close to 105 degrees.
When the air is saturated with moisture, a person might not be able to sweat enough to cool their body temperature.
“What people might experience is unusual nervous system symptoms like confusion, weakness, unusual numbness that might mirror a stroke,” Freedle said.
Keeping cool
Watching her daughters frolic at the park, Leafgreen said she hasn’t really noticed any changes in Iowa’s humidity patterns.
When it comes to living with the inevitable Iowa humidity, she has a few simple routines: She and the children frequent their local swimming pool — at the end of the day rather than when it’s hottest. She says keeping hydrated is a must, as is wearing loose clothing.
Finally, her family stays indoors when the heat and humidity are too much to bear.
“You can take a shower, go outside and get hot and sticky all over again,” Leafgreen said.
The Midwest Newsroom is an investigative and enterprise journalism collaboration that includes Iowa Public Radio, KCUR, Nebraska Public Media, St. Louis Public Radio and NPR.
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METHODOLOGY
To write this article, Holly Edgell interviewed a climate scientist to gain insights into the causes of higher humidity earlier in the year. She also interviewed a physician to learn about the health effects. Nicole Grundmeier interviewed Iowans to get their opinions and experiences with humidity. In addition, Daniel Wheaton analyzed data to track changes in humidity over time.
REFERENCES
Where it will soon get much more humid in the United States (Washington Post | May 12, 2025)
The Caribbean has been unusually warm. That’s not a good thing. (Washington Post | Jan. 4, 2025)
Most Humid States: 2025 (World Population Review)
State of the Air: 2025 (American Lung Association)
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