South Dakota 'corn sweats' contribute to August humidity

Dominik Dausch
Farm Forum
A National Oceanic and Atmospheric Administration humidity map shows high levels of humidity in the eastern U.S. Friday, Aug. 5, 2022.

If you had "corn sweats" on your 2022 weird weather bingo card, well, here you go. 

It turns out the strange turn of phrase is a real thing, according to Laura Edwards, state climatologist for South Dakota State University, and it's culpable for the pulpy humidity that — combined with a heat wave — makes a walk in the park feel more like a swim in a sauna in parts of the Pacific Northwest and central and eastern U.S. in early August.

"Corn sweats" is formally known as "evapotranspiration" in the scientific community — a portmanteau of the words evaporation and transpiration. It occurs when plants release water vapor through their leaves as well as the natural evaporation of water in the soil.

A diagram showing the process of evapotranspiration, or "plant sweats."

Barb Mayes Boustead, a meteorologist and climatologist based out of Omaha, wrote about the phenomenon in a Washington Post article. When summer heat reaches its peak, the process becomes accelerated, boosting the amount of moisture that's pumped into the air, Boustead said.

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"Corn and soybean contribute to evapotranspiration. So do native prairie grasses. Anything that's growing and green will contribute to evapotranspiration in the summer," Boustead said.

But it's not like you'll find cobs of corn perspiring like a farmer driving a combine with no A/C: "You won't actually see corn sweat, but if you go out and stand in a corn field, it will certainly feel muggy," Edwards said.

However, corn tends to release more vapor because it is a taller plant and tends to use more water than most crops, Edwards added.

Heat waves and corn stalks

Another climatological bingo phrase to cross out is "heat domes" — a high-pressure, heat-trapping climate system. 

"We're just in a weather pattern right now where we have high pressure parked over us. In the summertime, that prohibits rainfall or precipitation. And that, too, could contribute to how humid it feels. In Aberdeen, we've had quite a lot of fog. It's humid and there's very little air movement, so it tends to feel muggy at times," Edwards added.

Dennis Todey, director of the U.S. Department of Agriculture Midwest Climate Hub, said there is an intersection between climate change and agriculture — or "heat domes" and "corn sweats" — that is making heat waves even more unbearable.

"Ag interacts with the atmosphere. Crops react with the atmosphere, temperature and precipitation," Todey said. "So, is climate change a part of this? Yes."

Corn acreage by county.

South Dakota agriculture adds to the humidity, too, but the sweatiness of the Mount Rushmore state's plants is important in its own unique way: Corn began to overtake oats as a major crop between 1950 and 1995, according to an SDSU report on crop systems, which Todey said contributed to a shift in the timing of peak humidity in parts of the U.S.

"South Dakota is one of the places where we've seen some changes occur more recently than some of the other parts of the Midwest, and the reason I say that is the southeast part of South Dakota has been largely corn and soybeans for some time, but as you go further west and north, there has been an expansion of corn and soybean aces over the last 20 years more readily," the USDA director said.

"When we used to grow more small grains like spring and winter wheat, rye, oats or things like that, they are mostly ending their growing season by this time of the year, so they don't transpire much moisture," Todey added.

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However, because corn tends to finish growing around mid-July and August, humidity levels also spike within one or two weeks, Boustead wrote in the Washington Post article.

Todey added heat waves have also changed a bit as a result: "Now [heat waves] come with very warm temps and high dew points. Then, in the evenings, you don't cool down as much, so you're shrinking your daytime temperature range between the high and the low."

Still, the climatologists don't recommend driving out to a nearby cornfield to give the crops an earful — the sweats are just one factor to the humidity.

"By far, our biggest source of moisture in the atmosphere in the summer is the Gulf of Mexico — that's what feeds our summer storms. The local effect of usages of water during this time of year is a possible contributor," Edwards said.

Dominik Dausch is the agriculture and environment reporter for the Argus Leader and editor of Farm Forum. Follow him on Twitter and Facebook @DomDNP and send news tips to