Is ‘corn sweat’ to blame for heat wave?

With temperatures in the 90s, some have blamed so-called “corn sweat” from abundant Midwest cornfields for the dangerous wave of hot and humid weather.

But what actually is corn sweat, and does it have anything to do with the cause?

The technical term for corn sweat is evapotranspiration, which refers to the process by which moisture in plant leaves evaporates into the air. That process — similar to how humans sweat — does not create heat and has no impact on the actual temperature. But it can cause humidity to rise, especially during heat waves, and make it feel hotter than it actually is. In other words, it can feel more uncomfortable to be outside.

Yet in the case of this week’s heat wave, the evapotranspiration of corn won’t be the leading cause for the spike in humidity, said Mark Frazier, a meteorologist for the National Weather Service’s northern Indiana office.

“It will have some impact, but I don’t think it’s as significant as the change in air mass. We’ll have increased dew point levels primarily because of a high-pressure air mass coming through,” Frazier said.

Even so, he said, the abundance of corn in some regions, such as Indiana, does indeed cause humidity to be higher in the summer than it would be otherwise.

When corn plants grow taller, he said, they have a greater impact on humidity. “The cornstalks are taller now than in June, and there is a difference. The vegetation releases additional moisture in the boundary layer that can cause an increase in the relative humidity,” Frazier said.

Tony Vyn, a professor of agronomy for Purdue University, said corn transpires the most moisture during its pollination cycle in July, contributing more to humidity.

“That’s because corn plants are growing the fastest at that point and are accumulating more weight per day than any other growth time,” Vyn said, adding that high temperatures accelerate evapotranspiration. “An abundance of sunshine drives transpiration and photosynthesis … And when more green leaf material is there, more water is being transpired.”

It is difficult to gauge the impact of cornfields on humidity during heat waves, Vyn said.

When humidity climbs sharply, it can feel much hotter to people than it actually is.

Frazier cautioned that when the heat index hovers around 100 degrees, people should ensure they stay hydrated and restrict outdoor activities. “If you can avoid going outside from noon to 5 p.m.,” he said, “that’s ideal because you can avoid the stress on your body.”