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‘Corn sweat’ is making the air in the Midwest oppressively muggy

Corn contributes substantial moisture to the atmosphere during the heart of summer each year -- some call it “corn sweat.”

Fog lingers on fields of corn and soybean in the community of Lyles Station in Princeton, Ind., in 2016. (Bonnie Jo Mount/The Washington Post)

During summer, the Midwest can experience some of the most oppressive humidity in the country. Fields in Iowa can be muggier than beaches in Miami. The culprit? Billions of stalks of corn.

Akin to a person breathing, plants exhale water into the atmosphere through a process called evapotranspiration. Some call it "corn sweat."

In the Midwest and northern Plains, corn and soybean crops draw moisture from the ground through their roots into their leaves, stems and fruits. The water evaporates into the surrounding air through their leaves, joining forces with neighboring water molecules to humidify the air.

This extra humidity is making the heat wave centered over the middle of the Lower 48 states even more oppressive.

Densely planted across millions of acres, corn can bring a-maizing levels of humidity during the middle of summer. One acre of corn can release 4,000 gallons of water per day, enough to fill a residential swimming pool in less than a week.

The additional moisture from corn causes higher heat indexes — a measure of how hot it feels taking humidity into account. It can turn an oppressive day into a dangerous one. The effects are strongest in the heart of the planted fields, but a person doesn’t have to be standing in a field to feel the heat. The moisture follows the winds, mixing around to blanket the region.

What in the world is ‘corn sweat’?

Temperatures on Tuesday climbed from 95 to 105 in the Plains and Midwest, but high humidity levels pushed heat indexes to 100 to 110 degrees from Texas to southern Minnesota.

On Wednesday, heat indexes are forecast to soar that high again over much of the Southern Plains while expanding as far northeast as southern Michigan.

While hotter conditions favor higher rates of evapotranspiration, the process peaks when corn reaches its “tasseling” phase, or when it hits maximum height — with a crown of thin spikes — and begins to sprout. Tasseling generally occurs around mid-July to August, about 80 to 90 days after planting. Humidity levels can increase in the span of a week or two once the plant hits the tasseling phase.

The moisture from corn evapotranspiration may not only make it intolerably muggy during the day, it can also slow cooling at night, leaving little respite from the heat. Lows on Wednesday from Texas to Illinois only dipped to 75 to 80, about 5 to 15 degrees above normal. The low temperature in Des Moines was just 82, its warmest minimum temperature since July 1936.

When ingredients are in place for showers and storms, the added juice can make them more intense; this could be the case in parts of the Midwest on Wednesday where the National Weather Service has declared an elevated risk of severe storms.

Corn is not the only culprit in summer humidity, though. Soybeans also contribute substantial moisture through evapotranspiration. In other words, soybeans sweat, too. Moisture also evaporates from bodies of water and even from the wet soils of areas with recent rains. Not to mention a good deal of the moisture that reaches the Corn Belt during summer is sourced from the Gulf of Mexico and sometimes even from the Pacific Ocean.

Major heat wave to swell over Lower 48 after records fall in Northwest

In other words, corn does not act alone. But it can be that one more thing that pushes the summer heat from muggy to miserable.

If it’s muggy in Mid-America in midsummer, go ahead and blame the corn. Just don’t forget its friends: soybeans, soil and waterways, just to name a few.

Barb Mayes Boustead is a meteorologist and climatologist living in the heart of the Great Plains. Her interests include the overlap between weather and climate, especially in weather extremes, as well as historical weather events like the one at the heart of Laura Ingalls Wilder’s “The Long Winter.” She is a Dissertation Award winner from the American Association of State Climatologists and past president of the Laura Ingalls Wilder Legacy and Research Association.

Jason Samenow contributed to this report.