URBANA, Ill. — It’s not just the heat; it’s the humidity and “corn sweat” making muggy Midwestern summers feel even hotter.
In agricultural states, such as Illinois, large-scale fields of corn releasing water through their leaves boost humidity levels, making people feel hotter. One acre of corn can add between 3,000 to 4,000 gallons of water a day to the atmosphere, according to the U.S. Geological Survey.
Illinois State Climatologist Trent Ford with the Illinois State Water Survey says scientists in major agricultural states have studied how crops affect local weather patterns since the mid-1980s.
“The amount of humidity coming off crops does matter,” Ford says. “It’s less than what’s being blown in off the Gulf of Mexico, but our growing crops do have a documented impact.”
Corn doesn’t actually sweat, says Doug Gucker, a University of Illinois Extension local foods small farms educator serving Central Illinois. It breathes.
“It is more like when humans exhale; our breath has a higher water content than the surrounding air,” Gucker says. “It is the same thing in the corn field, except plants do not have lungs, so they transpire.”
When temperatures are high, plants open pores on their leaves called stomata, bringing in oxygen and releasing carbon dioxide and excess water. The water evaporates, cooling the plant.
Corn growth peaks around the same time summer temperatures do, intensifying the effect.
“Corn is a grass,” says Russel Higgins, Extension commercial agriculture educator in Northern Illinois. “It’s taller and has more leaf area, so it transpires more water than a soybean crop. The corn crop demand for water also peaks earlier in the season than it does for the soybean crop.”
The hotter it is, the more plants transpire, as long as they have available water. Anyone who has detasseled or scouted corn knows exactly how standing in a field mid-July can feel.
“The corn is 7 to 9 feet tall, and there’s often limited wind movement in the field, and all those plants are transpiring,” Higgins says. “Under those conditions you can feel the elevated humidity in there.”
Most Illinois heat waves come with higher levels of humidity, Ford says. Some of that is from the state’s 10 million acres of corn.
“In the summer we have an actively growing crop as well as natural vegetation,” Ford says. “That means that a pretty decent amount of our humidity near the surface is coming from transpiration.”
The added humidity creates conditions for thunderstorms, but crops may also be preventing regional heat waves. While Ford says the number of extremely hot days are increasing in other regions, the Midwest has escaped the trend, and researchers are trying to find out why.
“The cooling effect that transpiration provides may be reducing the number of hot days,” Ford says. However, corn stops transpiring when moisture levels are low, which lowers humidity levels and causes a spike in daytime temperatures.
Looking ahead, Higgins says that higher humidity levels, which slows moisture loss from crops, may be beneficial since June was warmer and drier than usual. An article in the Illinois Bulletin by Dr. Emerson Nafziger notes that 20% of Illinois is currently “abnormally dry.”
— University of Illinois Extension
For more articles out of Illinois, click here.