'Corn sweat' will add to punishing heat, humidity in Midwest this week.

USA Today

4 days ago

The process is officially known as evapotranspiration, which is how plants, including corn, release water vapor into the atmosphere.
It's high summer in the Midwest and the corn is "sweating."
The process, which despite its nickname does not involve any actual sweating in our sense of the word, is officially known as evapotranspiration, which is how plants, including corn, release water vapor into the atmosphere, according to a statement from Taranis, a crop intelligence firm.
Here's how it works: Commonly called "corn sweat," water evaporating from plants enters the atmosphere, combines with other water molecules and humidifies the air. In the Plains and Midwest regions, where there are millions of acres of corn and soybean crops, this can worsen stifling heat by driving up the humidity levels, making hot summer days all the more miserable.
"What we're talking about is evapotranspiration, which occurs with all vegetation," Iowa state climatologist Justin Glisan said in an e-mail to USA TODAY.
How much does corn 'sweat'?
During the growing season, an acre of corn "sweats off" about 3,000 to 4,000 gallons of water a day, according to the U.S. Geological Survey.
In Iowa, corn pumps out "a staggering 49 to 56 billion gallons of water into the atmosphere each day" throughout the state, the National Weather Service said. That can add 5 to 10 degrees to the dew point, a measure of the humidity in the air, on a hot summer day.
In states such as Illinois, which boasts about 12 million acres of corn, that adds up to 48 billion gallons of water daily – enough to fill 73,000 Olympic-size swimming pools, according to Taranis.
Weather patterns still rule
However, Glisan said weather patterns contribute more to the heat and humidity in the Midwest than does corn sweat, which he said is "a more local or smaller scale effect."
"Corn transpiration is an important moisture contributor, though not the dominant factor," he said to USA TODAY.
He said larger-scale (think state to regional scale) weather patterns provide much more low-level moisture via southerly and westerly advection – moisture from the Gulf or through the lower-levels of the atmosphere from the Pacific, for example.
"With the large heat dome setting up over the Midwest this coming week, southerly moist flow will increase surface dew points and in the presence of unseasonably hot air temperatures, produce oppressive heat index values," Glisan said.
How hot will temperatures get this week?
How hot? The weather service in Chicago is warning that the heat index in Illinois could reach 115 degrees by Wednesday and Thursday, July 23-24.
Elsewhere, parts of Texas, Arkansas and Louisiana will have highs peak at near or above 100 degrees, and a much larger area of the South, Midwest and Great Lakes will see a heat index surpassing 100 degrees, Weather.com meteorologist Jonathan Erdman said.
Corn sweat will only add to the misery in the Midwest, Glisan said: "Of course, there's a local contribution from corn/bean transpiration which can add additional low-level moisture and exacerbate dew points."
Does corn sweat have any benefits?
While corn sweat might make summer days feel more oppressive, it's actually a sign of healthy, growing crops, Taranis reported. "The process of evapotranspiration is essential for plant growth, helping crops reach their full potential."
Contributing: Christopher Cann, John Bacon, USA TODAY