Curious Iowa: Is ‘corn sweat’ making it sticky outside?

How much does "corn sweat" really affect Iowa’s weather in the summer?

That is the question posed by a curious Iowan from Hiawatha, who wrote to Curious Iowa, a Gazette series that answers readers’ questions about our state and how it works.

The phenomenon has for years been blamed by some for sweltering summer conditions in Iowa, yet recent research shows that idea might not be true.

In search of clues, we talked to experts at Iowa State University and the Iowa Department of Agriculture and Land Stewardship.

What is corn sweat?

It's a catchy term for crop evapotranspiration — commonly abbreviated "ET" — in which corn plants draw water from the soil and release it into the air through their leaves.

All plants do it. They need water, carbon dioxide and sunlight to make a type of sugar through photosynthesis. Water that evaporates from plant leaves also helps to cool them.

It's estimated that an acre of corn can put up to 4,000 gallons of water vapor into the air each day.

The amount of water vapor that corn plants contribute to the atmosphere changes over the growing season as they develop, and that peaks in July and August.

Crop field soil also releases water into the atmosphere. That is more pronounced early in the growing season and abates as crops block sunlight from reaching it, somewhat counteracting the increased releases from corn plants.

A study of Minnesota land published in 2019 found that a field of native prairie grasses, overall, emits "marginally higher" water vapor than corn fields. Although, the intensity of those releases was different — prairie grasses peaked in the spring.

"It's likely the ET coming from a native prairie land surface would be very similar to what's coming from a cornfield," said Madelynn Wuestenberg, an Iowa State University Extension weather and climate specialist. "Corn is a grass. It's photosynthesizing similarly to other grasses."

Is corn responsible for humid Iowa summers?

No, according to State Climatologist Justin Glisan. It contributes in a very small way.

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"Evapotranspiration can add a few degrees to the dew point, but the overarching moisture source is either the Gulf (of Mexico) ... or flow off the Pacific (Ocean)," he said. "Overall it doesn't contribute a significant amount to those dew points.“

Dew points refer to a measurement of moisture in the air — the temperature at which the moisture might condensate on surfaces or form dew.

Glisan said Illinois research has indicated that evapotranspiration contributes little air moisture compared with the Gulf of Mexico, which President Donald Trump has renamed the Gulf of America. That research said the contribution was "orders of magnitude" smaller, meaning the local contribution is perhaps 1 percent of the Gulf's share.

"If you are standing in a cornfield in late July, it's gonna feel humid, because there is moisture coming from those plants," Wuestenberg said. "But if you're two miles away, standing on your front porch, you're probably feeling moisture from the Gulf."

She added: "It's been really muggy in recent days. That's definitely Gulf-driven."

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Comments: (319) 368-8541; jared.strong@thegazette.com