How sweating corn will intensify this week’s Midwest heat wave

July has already been more humid than average, and now another, less obvious factor will enhance humidity this week: corn sweat.

Like a person breathing, corn exhales water vapor through its leaves — a process called evapotranspiration — which exacerbates humidity in agricultural regions during the summer.

In the week ahead, some of the country’s most oppressive heat and humidity will be found in the Corn Belt, an area of the United States that stretches from the Dakotas to Ohio, sending heat index values toward a dangerous 115 degrees. Illinois, Missouri, Minnesota, Iowa, Indiana and the Dakotas are forecast to have humidity levels that rival or exceed Florida’s.

According to Iowa state climatologist Justin Glisan, this is the time of the year when humidity contributions from corn sweat are highest — around the time of tasseling and pollination, when the flower emerges from the corn stalk.

He described the phenomenon as being thick and oppressive, particularly if there is no wind — “like being fully clothed in a sauna or steam room.”

The unique pattern will add to a brutal wave of humidity and heat developing this week, one that will start in the southern and central states and then expand into the Northeast and mid-Atlantic, where temperatures could reach 100 degrees on Friday.

A heat dome over the South will expand into the Midwest by midweek and reach the East late in the week.

The combined effect of the corn sweat and the heat dome, which will draw up hot and humid air from a warmer-than-average Atlantic Ocean, will exacerbate the risk for heat-related impacts in the Plains and Midwest. Heat advisories, watches and warnings cover hundreds of counties across the country.

When and where

Wednesday: High heat and locally extreme humidity will surge in the Midwest and western Great Lakes on Wednesday, reaching Chicago, Milwaukee and Green Bay, Wisconsin.

In Chicago, there’s an extreme heat watch in effect for Wednesday and Thursday, with heat index values forecast to range from 105 to 115 degrees.

Thursday: Extreme humidity and heat will remain in place over the Midwest on Thursday, while advancing eastward into the eastern Great Lakes and Ohio Valley, affecting Chicago and the surrounding area, Detroit, Indianapolis, Cincinnati, Cleveland, Pittsburgh and Buffalo.

Friday: High heat will surge into the Northeast and mid-Atlantic on Friday, where temperatures could push toward 100-degrees in a few locations.

Meanwhile, there will be little relief from the heat in the Midwest and Great Lakes, including southern Canada.

Due to the unrelenting humidity, more than 100 overnight temperature records are forecast to be challenged this week, mostly in central and eastern states.

This is consistent with the trend of overnight temperatures increasing at a faster rate than daytime temperatures amid a warming world, which increases heat stress for people, pets and plants.

Possible impacts

For central states, the unusually hot, humid and downpour-laden pattern shows no signs of letting up into early August.

While drought coverage is currently sporadic across the Corn Belt, the combination of excessive humidity, moisture and record warm overnight temperatures introduces other concerns.

Warm nighttime temperatures can increase respiration in crops, the process by which plants convert sugar into energy. This can result in the loss of energy that could have otherwise increased crop yields, said agronomist Megahan Anderson with Iowa State University.

In Iowa, the state with some of the highest corn-producing counties in the nation, Anderson said a potpourri of diseases also affects corn. Though the overall level is low, the conditions don’t help.

“We are set up in a way that disease could easily spread regardless of temperature, as long as humidity, extended periods of dew on leaves and/or rainfall continues,” Anderson said.