As the Midwest faces a grueling heat wave, one plant is making the conditions worse: corn.

Thousands of gallons of water, or “corn sweat,” are produced each summer day by corn crops. That moisture can increase the dew point, making it feel even more humid. In states such as Iowa and Illinois, which both have corn as far as the eye can see and topped 100 degrees in the past days, corn sweat can be devastating.

What is corn sweat?

A quick science lesson: Evapotranspiration is the process by which water moves from the earth’s surface to the atmosphere. The water can be evaporated when it moves directly from soil, or transpired when it moves from a plant’s leaves. Both of these processes occur with corn: The soil and plant both release significant amounts of water vapor into the air.

All that water vapor has been coined “corn sweat,” and it has some traceable patterns. Corn sweat reaches its peak between late July and late August, according to the Weather Channel. It’s also not new: In states including Iowa and Illinois, which had a combined 24 million acres of corn in 2023, talk of corn sweat crops up each summer.

It’s the sheer quantity of water vapor that turns corn sweat from a simple biological process to a weather nightmare. According to the Washington Post, one acre of corn can release 4,000 gallons of water per day. The state of Iowa alone planted an estimated 13.1 million acres of corn in 2023.

Corn sweat can materially affect the dew point, making the air oppressive. The dew point is a reference measurement to relative humidity, but can be more accurate in estimating how dry or humid the air will feel, according to the National Weather Service. The higher the dew point, the more humid the air. The dew point is also used to calculate the heat index. Thus, a greater quantity of corn sweat can be directly linked with a higher heat index.

How is corn sweat impacting the Midwest heat wave?

The National Weather Service has issued excessive heat warnings and advisories across counties in Missouri, Illinois, Indiana, Ohio, Kentucky, and West Virginia. This meant the heat index temperatures in these regions were above 100 degrees for an advisory, or 105 degrees for a warning, for at least two days straight. Just days ago, parts of Iowa, Illinois, and Missouri faced heat indices of up to 115 degrees.

While major Midwestern cities like Chicago may be safe from corn sweat’s humidity bump, those living in more rural areas are feeling the wrath. Iowa is a hotspot: It’s the U.S.’s largest corn producer, and about 90% of the state is farmland. On Monday, Iowa faced a cross-state heat warning with temperatures above 100 degrees, with several public schools choosing to release their students early.