Feeling Sticky This Summer? ‘Corn Sweat’ Could Be Raising the Humidity

In the Upper Midwest, it’s the time of year when corn is getting high in the field and the days feel languid. As a heat wave moves out of the region and residents start to cool off from the oppressive temperatures and humidity that broke records on Tuesday, the internet is abuzz with talk of a phenomenon that might be making things even stickier: so-called “corn sweat.”

“Using the term ‘corn sweat’ is kind of funny,” Illinois State Climatologist Trenton Ford tells AGDAILY’s Braeden Coon. “It’s not perfect as with most metaphors. Humans and a few other animals will perspire when we get hot, and sweat is evaporated off our skin. What corn does is a bit of a different process.”

The technical term for that biological process is evapotranspiration, and all plants—not just corn crops—do it. As part of evapotranspiration, plants take in water via their roots, transport it through their tissues and then release water vapor into the air. The process is critical for the plant’s metabolic health and for shuttling nutrients in the water from root to leaf.

You're not the only thing sweating...look at the high blob of humidity in the Midwest. That's from corn sweat! Corn releases a lot of moisture in the summer and that means high humidity. pic.twitter.com/WJMrWlwINJ

— Payton Malone WWL-TV (@paytonmalonewx) August 26, 2024

But with the nation’s largest corn-producing states concentrated in the Midwest, that moisture can add up. According to the U.S. Geological Survey, a single acre of corn can add 3,000 to 4,000 gallons of water into the atmosphere each day during the growing season, and high temperatures increase transpiration rates.

For context, farmers planted 91.5 million acres of corn in 2024, of which only about 20 percent is grown for human consumption. Of that fraction, most goes to producing high fructose corn syrup and other processed foods. The majority of the country’s corn is used in animal feed and ethanol-based fuels.

The natural prairie ecosystems that covered the drier expanses of the Great Plains before the onset of industrial agriculture appear to have contributed far less moisture to the atmosphere, compared to the corn fields of today. Prairie contributions to humidity also peak earlier in the summer, when temperatures are likely to be lower.

“Native prairies are diverse ecosystems with a variety of plant species, each with different root depths and water needs, helping to create a balanced moisture cycle,” Bruno Basso, a crop and agriculture scientist at Michigan State University, tells Vox’s Benji Jones. “In contrast, corn and soy monocultures are uniform and can draw water from the soil more quickly.”

High humidity exacerbates the health risks of extreme heat, which is the leading cause of weather-related deaths in the United States and a growing concern in a changing climate. With more moisture in the air, it’s harder for sweat to evaporate and cool our bodies. Higher humidity also traps heat overnight, Ashton Robinson Cook, a meteorologist and forecaster at the National Weather Service’s Weather Prediction Center, says to Scientific American’s Andrea Thompson. “When there’s little relief overnight, there’s more heat stress,” he adds.

But exactly how much corn sweat contributes to overall humidity isn’t something scientists know for sure. One study found that Midwestern crops—mostly corn—increased humidity by 40 percent during a heat wave in 2011. But Ford tells AGDAILY that future research with remote sensing satellites could reveal more details about the phenomenon. “There’s a common assumption among folks who aren’t in the weather community that we can measure everything, and are measuring everything,” he adds. “That’s just not the case.”

While the sticky conditions are annoying—and even dangerous—to people who live near corn fields, the “corn sweat” is actually a sign of a healthy crop.

“A lot of transpiration and humidity is a good thing for crops, because it means there’s moisture available in the soil, the crops are successfully photosynthesizing,” Meaghan Anderson, an educator and field agronomist with Iowa State University Extension and Outreach, tells AGDAILY. “They’re doing what they need to do to produce yield at the end of the season.”

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Rudy Molinek is Smithsonian magazine's 2024 AAAS Mass Media Fellow.