If you’re a meteorologist, putting together a good forecast doesn’t just require knowing about clouds and storm systems. Sometimes, you need some intimate knowledge about corn.

Corn is a major influencer of summertime weather in parts of the Midwest and northern Plains, along a swath of the nation’s heartland aptly dubbed the Corn Belt. Corn, soybean and other crops release water into the atmosphere through evapotranspiration. This is a cooling mechanism for the plant akin to sweating in humans.

That moisture gets soaked up by the lower atmosphere, potentially bringing unbearable tropical humidity levels during the height of summer, along with hazardous heat index levels. The dynamic is going to be on full, uncomfortable display in the coming days as a heat wave roasts this region.

In the summertime, a single acre of corn can “sweat” about 4,000 gallons a day — enough to fill a typical residential swimming pool in less than a week. Hotter conditions favor even more evapotranspiration, the heat combining with corn-induced humidity to bring sultry conditions at times juicer than the soupiest air masses of South Florida.

Already, weather models are hinting at another episode of corn-catalyzed extreme humidity this weekend over parts of Iowa and Illinois. The added humidity could combine with high temperatures to make it feel as hot as 110 degrees.

Iowa: corn sweat hot spot

Harlan is the seat of Shelby County in Iowa. It’s home to about 5,000 people. And no matter which direction you drive, odds are you’ll find corn.

Shelby County is a mecca for corn; more than 150,000 acres of corn crops were grown in the county last year. According to the U.S. Department of Agriculture, Shelby County yielded 217 bushels per acre of corn in 2019, which amounts to about 900,000 tons of corn countywide.

But in Douglas County, Neb. — just 40 miles southwest of Harlan and home to Omaha — only between 25,000 and 50,000 acres of corn were grown, and those acres were less productive. Corn yields were a sliver of what Shelby County, Iowa reported.

Both Harlan and Omaha have similar climates and experience similar temperatures. Looking at more than a million data points, including hourly observations of dew points from both sites dating back to 2000, reveals just how extensively corn affects Harlan’s summertime weather.

Comparing the data

Averaged over the entire year, Omaha’s dew point — a measure of how much moisture is in the air — was almost exactly the same as Harlan’s. That makes them a good basis for comparison.

In fact, Omaha proved to be a touch more humid than Harlan. This may be because of the location of Omaha’s official weather station, which is at Eppley Airfield, adjacent to the Missouri River.

Looking just at averages, there is no strikingly clear corn signal. However, when analyzing percentiles, corn’s influence is revealed.

Here’s a plot of the 75th, 90th and 99th dew point percentiles observed at both Omaha and Harlan over the past two decades. Notice the similarities during the springtime, with nearly identical dew points even within the upper echelon of extreme events. But between June and August, something interesting happens.

Harlan dramatically overtakes Omaha by July, with that dew point discrepancy lasting into August. Looking at the top 1 percent of August days, they are, on average, 6 percent more humid in Harlan. It’s similar in September.

The mean, 75th and 90th percentiles converge once again heading into October as the harvest occurs, but the 99th percentile — representing the top 1 percent of humid days — are still 7 percent more humid in Harlan than Omaha.

In other words, the average humidity between the two locations isn’t that different — but you are significantly more likely to wind up with days that have extremely high humidity in Harlan, primarily thanks to the corn sweat factor.

That matches up quite well with a roughly 2-degree dew point increase estimated by Bruno Basso, an earth science professor at Michigan State University.

How corn and climate factors lead to more humid nights

The corn signal was also present at different times of day. During the morning hours, which are typically the coolest, Omaha and Harlan had roughly the same dew point.

But 10 a.m. proved to be a dew point minimum for both cities during July 2019, with dew points bottoming out each morning to similar values before daytime heating and evapotranspiration kicked in.

During the afternoons, Harlan’s dew point climbed faster and higher than Omaha’s, since the corn in Harlan was helping add moisture to the air as soon as the temperature heated up.

Harlan’s dew point also stayed elevated overnight longer than Omaha’s.

“Plant transpiration is going to continue even during the evening hours, partly because it’s a cooling mechanism,” said Mark Licht, an assistant professor and cropping systems specialist with Iowa State University, in an interview. “The corn is most affecting dew points in the early July time frame. By the time you get out to mid-August, it’s becoming less and less of a factor.”

Lesser winds at night could also help make some of that moisture linger, especially in Harlan, Licht said.

Looking over 10 years’ worth of data, it’s clear that the top 1 percent of humid days were actually more humid in Harlan than they were in Miami.

Why it matters

Corn’s effects on the Midwest’s climate has real-life impacts, since it can make heat waves more dangerous by heightening the heat index.

“It can make a difference,” said Licht. “We’re not thinking about actual air temperatures changing drastically because of it. Dew point is what it feels like to us as humans.”

He says that the corn-spurred dew point increase can put a strain on the human body during hot weather.

“When [dew points hit the] upper 70s to low 80s, it’s miserable being outside, even if [the temperatures are] only 85 or 90 degrees,” Licht said. “The key [is] that corn is pumping out a lot of water.”

The synergy between elevated temperatures and humidity can dramatically exacerbate the risk for heatstroke and other heat-related illnesses.

Corn’s effects are combining with climate shifts

The corn’s added moisture is working in tandem with longer-term, human-caused climate change to bring increasing nighttime temperatures and humidity levels to the Corn Belt.

“[Maximum temperature] is actually decreasing across the Midwest, but the average nighttime temperature is still increasing at [double] that rate,” Basso said, noting the region’s increased moisture is helping keep overnight temperatures warmer. “And if [nighttime minimum temperatures are] greater, you have a greater capacity to hold more moisture.”

Together, the effects are contributing to heat waves that are more seriously taxing on the human body, without adequate nighttime cool-down periods and with the humidity producing higher heat indexes.

A 2018 paper in the Annals of the American Association of Geographers, stated that observations “indicate an increase in regional dew point” across the Corn Belt associated with corn and soybean evapotranspiration.

The study concluded that moisture stemming from the Corn Belt’s agricultural industry brought about “intensification” as a “regional climate modifier."

Increasing moisture is bad for crops

In addition to being uncomfortable and at times downright dangerous, the increased humidity and nighttime temperatures could also be a breeding ground for “many agricultural pests and pathogens for both growing plants and stored grain,” according to the 2018 National Climate Assessment.

The report also notes that increased heat and humidity “likely will degrade [crop] market quality as well as yield by mid-century.”

The assessment found more irregular rainfall patterns, punctuated by extended drought and bursts of heavy rain, were becoming more frequent because of greenhouse gas emissions from burning fossil fuels such as coal and oil.

The same report and other studies also found an increase in the occurrence of more humid heat waves, which pose greater health risks.

“Precipitation is actually increasing in the Midwest, and the variability is increasing as well,” said Basso. “Most of the farmers may be more averse to thinking climate change, but they’re certainly thinking climate variability.”

Andrew Freedman contributed to this report.