The heat is coming, and the corn is sweating. No, seriously. Corn sweat is a thing, and I’m not talking about what happens after you finish an entire box of corn puffs in one go.




You might have heard meteorologists and reporters throwing around the term “corn sweat” when speaking of the abysmal heat wave that should be enveloping all of us any day now. Corn sweat is actually just a fancy term for something that corn, like all plants, does all the time: transpire water vapor. The reason everybody’s freaking out about corn sweat this week is that for some Americans, it could make the heat wave noticeably worse.

Plants are in a bit of a bind in the heat. They need to keep their pores, called stomata, open long enough to suck in enough CO2 from the air to meet their daily energy requirements. But when the air is too hot, this means losing lots of water vapor. They start to sweat.


Image: USDA

And in the midwest, it’s corn sweat that matters, because there’s a lot of corn. According to the USDA, more than 90 million acres of land—mostly in Illinois, Iowa, Indiana, eastern South Dakota and Nebraska—are planted with this single crop.

When plants transpire, they increase the humidity of the air, which raises the dew point, or the temperature at which water vapor condenses. Our bodies cool off by evaporating water in the form of sweat, and a higher dew point makes this process less efficient. As a result, the heat index, or effective temperature we feel, starts to soar.

With a hat tip to the Washington Post, here’s a handy graphic that shows how humidity and air temperature are combined to create heat index values:


You can see from the chart that we start running into trouble when the air is both hot and sweaty. At a relative humidity of 75 percent, a 96 degree day effectively feels like a whopping 132 degrees Fahrenheit.

But back to the question of the hour: How much will the corn-driven moisture injection contribute to this week’s stifling heat? This isn’t something meteorologists routinely factor into their reports, but some research indicates that corn fields can cause the dew point to rise a handful of degrees in nearby areas.


Per Minnesota Public Radio meteorologist Paul Huttner:

Measurements by Pete Boulay at the Minnesota DNR Climate Working Group have found dew point levels from 1 to 5 degrees higher inside a relatively small corn plot at the University of Minnesota’s St. Paul Campus. Pete’s brief experiment in 2010 confirms what other sources say about extensive corn crops adding to dew low level dew points. Dew point spikes of 1 to 5+ degrees are quite likely in the Upper Midwest in summer depending on wind velocity and trajectory.


A several degree uptick in the dew-point will cause a several degree uptick in the heat index. So, while corn is not to blame for this week’s heat wave, if you live in the corn belt, it could give your air an extra-sweaty edge. As it stands, much of the corn belt is now under an excessive heat warning, with heat index values of up to 115 degrees Fahrenheit expected for the Twin Cities and Quad Cities over the next few days.

Naturally, all this talk of corn sweat has got Floridians clamoring about sugar cane sweat. Whatchu got, New York?


[The Weather Channel | Minnesota Public Radio | The Washington Post]