Midwest mugginess linked to crops working up a ‘corn sweat’

Published 10:12 am Friday, July 22, 2016

The Midwest’s first dangerous bout of heat and humidity this summer is partly to blame on the moisture piped out of the ground and into the atmosphere by the increasing acreage of corn crops reaching peak maturity, meteorologists and atmospheric researchers say.

That muggy mist, jokingly called “corn sweat,” gets blown around the country, even enveloping more distant urban areas, like Chicago and Minneapolis. The phenomenon — long familiar to weather geeks, less so to the rest of us — spawned playful banter on social media this week. As heat indexes top the 100-degree mark, here is a look at the steamy situation and the role of golden ears of corn:


Evapotranspiration, the technical term for “corn sweat.” The American Meteorological Society defines it as “the combined processes through which water is transferred to the atmosphere from open water and ice surfaces, bare soil and vegetation that make up the earth’s surface.”

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Basically, when corn crops mature in late July, they act like billions of straws drawing up soil moisture. It beads up on vast leafy canopies and is carried off by warmer air.

Much of the humidity enveloping the region this week is siphoned off the Gulf of Mexico, but a measurable contribution comes from corn and soybean crops, say researchers like Ken Kunkel, a professor of atmospheric sciences at N.C. State University.

“It’s a real thing. Calling it sweat is interesting,” he said with a laugh. “I never heard that particular term before.”

Blowin’ in the wind

Once that added moisture from the corn belt is picked up in the air, it spreads fast to places like Chicago.

Kunkel took part in a modeling study that found crop moisture most likely increased humidity levels during to a 1995 heat wave blamed for hundreds of deaths there. “Chicago is affected by it as much as if you’re in the middle of a corn field in central Illinois,” he said.

Why now?

Corn acreage has rapidly increased in the United States: More than 94 million acres of corn were planted this year, an increase of 7 percent from 2015, according to the U.S. Department of Agriculture. That’s the third-highest corn acreage since 1944. Soybeans, which also contribute to humidity levels, are at record high acreage, too.


Jokesters have taken to Twitter to grouse about “corn sweat,” with some calling it the “polar vortex” of the summer.

But outside the exchange in Chicago where corn futures and agricultural commodities are traded, analyst Sharon Piet was not breaking a sweat Thursday as she scratched away at a crossword puzzle in the midday heat.

“It could be true, yeah. Who knows, right?” she said, adding with a smile that it doesn’t change the way she feels about those delicious, sweet kernels.