Recent muggy weather in the Midwest has more people talking about “corn sweat.” The phrase describes how corn releases moisture as it grows, which can contribute to humidity.
At the end of August, as a humid heat wave settled on much of the Midwest, there was a flurry of online searches for the term, according to Google.
Transpiration is the scientific word that describes how water moves through plants, including into the air.
“It's a cooling process, just like it is for human bodies when we sweat,” said Melissa Widhalm, the associate director and regional climatologist at the Midwestern Regional Climate Center at Purdue University.
But local climate scientists say while corn sweat does affect humidity, it probably wasn’t the dominant force last week, or even during most summers in the Midwest.
“This year, I think there was a narrative out there that the heat and humidity we experienced the last week of August was due to the corn,” said Trent Ford, Illinois State Climatologist at the University of Illinois. “And that's actually not correct for a couple of reasons.”
One big reason is that corn typically hits the peak of its sweat earlier in its growing season. By this time of year, the corn is starting to mature and is not releasing as much water.
“I was driving yesterday, and I could see lots of the corn turning yellow and brown, sort of reaching the end of its life span,” Widhalm said. “That corn is not transpiring anymore. It's not moving that water anymore.”
Instead, a more dominant force in Midwestern humidity is often weather systems bringing water in from other parts of the country.
“I'd argue this time of year, the bigger contributor is just a warm air mass with southerly winds that were bringing up some of that humidity from the Gulf of Mexico,” said Zack Leasor, Missouri’s State Climatologist at the University of Missouri.
Climate change is also increasing humidity in the middle of the country. Humidity records don’t go back as far as temperature and precipitation data, but in Missouri, Leasor said humidity has been increasing and leading to more humid heat in more recent records.
“That's really interesting over the past 30 years to see that trend,” Leasor said. “We're not getting as many dry heat waves, but more low end temperatures with higher humidity.”
Ford and Widhalm also said they have seen increasing measures of humidity over time.
All of this could have serious consequences for health, the scientists said. Humid heat can be dangerous for humans because it lessens their ability to sweat. Plants can be more susceptible to fungal infections in humidity. And humidity is keeping temperatures abnormally warm overnight, which gives humans, plants and animals less of a chance to cool off.
“Human health has been more strongly tied to nighttime temperatures than it has with daytime temperatures,” Ford said.
There are a few reasons climate change could be increasing humidity in the north-central U.S. The biggest is simple: warmer air can hold more water.
“Humidity is one of those interesting variables that's directly tied to your temperature,” Widhalm said. “And so the warmer you are, the more ability the air has to hold on to moisture.”
Because so much of the humidity in the Midwest comes from elsewhere, changes in other parts of the country are also having an effect.
“That air that is initially evaporating that water from the Gulf of Mexico is warmer itself, and therefore also can contain more water,” Ford said. “And then it moves over a warmer Midwest and increases the humidity there as well.”
The Gulf of Mexico’s surface waters have warmed at twice the rate of warming in the global ocean surface, according to the National Oceanic and Atmospheric Administration.
And the scientists said climate change could also intensify corn sweat, or more broadly evaporation from plants and soil. Leasor said more research is needed into how plants might respond to warmer temperatures, but as climate change warms the air, it will be able to take on even more water from sweating plants like corn.
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest. It reports on food systems, agriculture and rural issues.
