A hot Minnesota day was probably more tolerable 150 years ago when Minnesota's vast, relatively dry prairies kept the region comfy as temperatures rose.
The landscape is much different now. The original prairie is mostly gone, replaced by corn, soybeans and other crops — and those crops sweat. A lot. All of that moisture has made Minnesota noticeably more humid.
"At this time of year, over corn, what we would see is evaporation rates that are about 40 to 50 percent higher than say over restored prairie or natural prairie," said University of Minnesota biometeorologist Tim Griffis.
Griffis has studied this phenomenon for more than a decade with a group of researchers from around the world. He says it's true that other parts of the landscape like lakes and streams also pump out moisture. But because so much of the state is cropland and those plants sweat so much, his research shows more than 60 percent of that local moisture comes from farm fields.
Corn is likely the most prolific contributor. The National Weather Service in Des Moines, Iowa, recently noted that mature Iowa corn pumps out 49 to 56 billion gallons of water into the atmosphere each day, which can add 5 to 10 degrees to the dew point on a hot, summer day, pushing heat index values to 110 degrees or higher.
Griffis said much of his data comes from sensitive air sampling devices in place for several years on a broadcast tower on the southern fringe of the metro area. The equipment analyzes water vapor, and can even determine whether individual droplets likely originated from crops, lakes, or more distant sources, although it can't pin down exactly how much crops boost the dew point, a common measurement of air humidity.
Other researchers, however, are reaching conclusions similar to those of the Des Moines weather service office.
"When you start looking at the large scale atmospheric processes over the entire corn belt region, it's not unlikely that we might see 5-, maybe even 10-degree changes in dew point temperature over the corn related areas versus the non-corn related areas," said University of Oklahoma meteorologist Jeffrey Basara.
The moisture increases human misery when we're outside, but Basara said it's also important to remember that the crops producing the water vapor do a lot of good. They produce food that's needed here and around the world.
The extra humidity, however, also brings potentially more dangerous complications. Moisture laden air is a key ingredient for thunderstorms, and higher dew points are likely to increase the severity of those storms, said Griffis.
"The precipitation that we receive, about 30 percent of that comes from evaporation that occurred within the region," he said. "And so when you load the atmosphere with water vapor, you increase the chances of having more intense precipitation."
Add climate change to the mix and the circle is complete. Warming temperatures are increasing evaporation, further boosting the chances of severe storms. That means more moisture in the air, and more misery for everyone who dreads these muggy July days.
Ellen Bartyzal contributed to this report.