Having a sweaty summer? It could be the corn.
Climatologists are building evidence that crops, particularly corn, are driving up dew points as they put water into the atmosphere through evaporation. They also may make corn-growing areas cooler and alter rain patterns.
Some say the extra moisture could even add energy to thunderstorms, with one study arguing that a 2001 tornado in Benson got a power boost from corn evaporation.
"I think there's a new realization that there is a two-way interaction between weather and agriculture," said Richard Raddatz, a climatologist at the University of Winnipeg, who has studied the transformation of the Canadian prairies from grassland to cropland.
In some ways, researchers are taking a second look at a 19th century adage - "rain follows the plow." Popularized by Charles Dana Wilber in an 1881 book touting the agricultural promise of Nebraska, the phrase supported a grand notion that the western Great Plains, which in the early 19th century had been labeled the "Great American Desert," could be transformed into a garden if people would expose its moist soil to the atmosphere.
Rainy years added credibility to the idea, but it was discredited as pseudo-science after homesteaders who flooded the plains were trapped by drought and bankruptcy.
Raddatz, however, said there is a growing body of research indicating that contemporary crops do indeed change the way water, heat and energy interact with the atmosphere.
By "transpiring" more heavily than the prairie grasses that preceded them, and in relatively short periods, crops can generate air movements that can lead to storms, and intensify the season during which water is cycled through the atmosphere.
Raddatz published a summary of studies of cropping and weather in February in the journal Agricultural and Forest Meteorology. They add some oomph to a 2002 study of dew points by Northern Illinois University climatologist David Changnon, which pinned a 40-year trend toward higher dew points in the Midwest, and record-high dew points during recent heat waves, on changes in farming.
Other experts are skeptical.
Assistant Minnesota state climatologist Pete Boulay points out that in the Twin Cities, average dew points - a measure of water saturation in the air - during three of the past four summers have been below average. And much of the corn-rowed state is now in its second consecutive season of very dry conditions.
But Boulay does believe that a broadly irrigated landscape on the University of Minnesota St. Paul campus has contributed to dew points there that are higher than those at broadly paved Minneapolis St. Paul International Airport.
Peter Robinson, director of the Southeast Regional Climate Center in North Carolina, has studied dew-point trends nationally and found mixed results, including an upward nudge in corn-growing areas. But he said he is only "suspicious the two are related."
University of Oklahoma climatologist Jeff Basara, who has spent most of his summers in Minnesota, traces a link between corn evaporation and an F2 tornado that injured seven people in Benson on June 11, 2001.
"There was going to be severe weather that day. But evaporation added enough moisture to the atmosphere and turned it from a day of localized severe weather reports to a day that really was a headline-maker," said Basara, who published his research in the Bulletin of the American Meteorological Society in 2005.
Changnon, the Illinois researcher, found that since 1950, crops had replaced thousands of square miles of pastureland during the era of rising dew points. More significant, he said, was the shift in corn-planting from 40-inch rows to 30-inch rows.
"We're just pouring more water into the air," Changnon said.
Changnon said the results of his study shouldn't demonize agriculture but should prompt urban areas to be alert to the public health threat dew points in the 70s or higher can bring.