‘Corn sweat’—and other weird weather phenomena—explained

As the U.S. Midwest swelters under the effects of yet another heatwave, temperatures and humidity in the region are being further fueled by “sweating” corn. If you didn’t have sweaty produce on your 2024 bingo card, you’re not alone; but “corn sweat” is a real phenomenon, well known to meteorologists and farmers alike, that adds moisture to the atmosphere and makes hot conditions even hotter and more unpleasant.

It’s just one unexpected way in which human activities, from agriculture to deforestation and pollution, can have surprising effects on regional weather patterns, sometimes enhancing or even temporarily limiting the impacts of climate change.

How does corn sweat make heatwaves worse?

Corn, like other plants, uses a process called evapotranspiration to draw water from the soil, take what it needs to function, and release the rest into the atmosphere via evaporation. The higher the ambient temperature, the more water corn needs, and the greater amount of water that evaporates.

According to AccuWeather agriculture expert and  meteorologist Dale Mohler, just one acre of corn can release 3,000 gallons of moisture in a day. “Any place you have corn fields and it’s hot, you get corn sweat,” he says; in a state like Iowa, where approximately 24 million acres of land are devoted to growing corn and soybeans, “it can bump the humidity on a hot day by up to 30 or 40 percent.”

With climate warming comes a constant feedback loop: warmer weather in the Midwest is also leading to more precipitation, meaning that there is more water in the soil for corn to absorb and evaporate, which adds more water vapor to the air and further fuels warming.

The increased humidity from corn sweat not only makes heat waves even more uncomfortable, but it can also be dangerous.

“When the air is dry, you perspire and the evaporation cools you off,” says Mohler. “But that doesn’t work as well in humid conditions, because there is so much moisture in the atmosphere already. That’s why you feel that much more miserable when it’s humid as well as hot.”

‘Warming hole’ delays a climate change trend

Even as the world warms, temperatures in a few spots around the world have held relatively steady or even slightly cooled. One of these is the southeastern U.S.; since 1958, even as average temperatures across the country have risen by 1.8 degrees Fahrenheit, the southeast has actually cooled by a degree.

In a 2018 study of the phenomenon, Trevor Partridge of the U.S. Geological Survey and colleagues argued that a major reason for this “warming hole” was a change in the jet stream over the eastern United States, which resulted in northerly winds bringing cool air to the southeast more often.

But there may also be other factors. Widespread deforestation in the late 19^th and early 20^th centuries left much of the region effectively denuded; but many of those forests have since grown back, to the extent that nearly 70 percent of Alabama, for example, is now forested.  Greater forest cover means greater shade, which reduces local warming.

As Partridge explained to National Geographic in 2022, such impacts show that “what we do to our land surface can have pretty profound impacts on local climate.”

Ultimately, however, the intensity of global warming will overwhelm even the most stubborn pockets of resistance. Temperatures are now on the increase in the southeast, and are expected to rise by 4 to 8 degrees by the end of the century.

The hurricane 'drought' of the 1970s and 1980s

Another possible contributor to the “warming hole” of the southeast might be the influence of aerosol pollution from sources like coal-fired power plants, which can reflect sunlight and cause localized cooling— ironic, given the impact of burning coal on global temperatures.

Aerosols may also have played a role in another little-known climate phenomenon: the “hurricane drought” of the 1970s and 1980s.

According to a 2022 study by Raphael Rousseau-Rizzi and Kerry Emanuel of the Massachusetts Institute of Technology, Atlantic hurricane activity experienced a “pronounced lull” during those decades, a period that coincided with peak emissions of sulfate particles from power plants—until environmental regulations reduced their impact.

Rousseau-Rizzi and Emanuel contend, however, that any aerosol-induced cooling over the Atlantic was not solely responsible for the recorded decrease in hurricane activity.

They think the cooling effect of this pollution reached as far as northern Africa, reducing the northward range of the West African monsoon, which helps drives Atlantic hurricanes.

That resulted in drier conditions over the Sahel, creating more and larger dust storms. That dust was then transported by wind currents over the Atlantic, where it exacerbated the cooling effect of the aerosols, leading to lower sea surface temperatures and depriving hurricanes of the heat energy they need to form.

Since the 1980s, however, as aerosol pollution has diminished, hurricane activity has returned to higher levels, with the 2024 season expected to be particularly active and intense.