Near record rainfalls in parts of Illinois this June have set the stage for what could be many muggy nights ahead, in part because of the type of crops we grow in the state.

David Changnon, a professor of meteorology at Northern Illinois University, studies how dense Illinois corn and soybean crops can raise dew point temperatures. He worries what might happen if the moisture from these crops, coupled with evaporation from this year’s wet soil, meets high summer temperatures this year.  

“We could have incredible amounts of evapotranspiration,” Changnon said. “Not just evaporation of water from the soil at the surface but our corn and soybean plants will begin to transpire a great deal of water into the lower atmosphere. In those situations it prevents the air temperature from dropping below that dew point, which limits how much cooling you can have at night.”

In his 2004 paper on this subject, Changnon noted that the greatest increases in extreme daily dew point temperatures occurred in the Midwest in the second half of the last century. This period coincided with a doubling of corn and soybean crops in the area. In the years since, local cultivation of these crops has only increased.

And according to Changnon, these factors could combine with hot temperatures to reduce the number of Midwest summer days that fade into cool nights.  

“So now you have not only hot muggy days, but you also have warm muggy evenings, which makes it very difficult if you don’t have air conditioning to sleep and get around,” he said.

Chagnon notes that high temperatures and record high dew points also prevailed during Chicago’s steamy summer of 1999 and deadly summer of 1995 when more than 700 died in the heat.

“In both of those summers we had big heat waves in July ‘95 and the end of July ‘99 where temperatures in the Chicagoland area got close to 100 degrees if not exceeded them for a couple of days,” Chagnon said. “On those days we had dew points in the upper 70s, and we even set an all-time record at Midway of a dew point of 83 degrees.

“It was those dew points that limited the ability for the atmosphere to cool down at night and that’s what really caused the problem for most people who don’t have air conditioning systems in their homes or apartments, especially for the elderly,” he said.

Still, Changnon notes that we also had heavy June rainfall in 2014.

“Luckily it was accompanied by fairly cool temperatures, so it wasn’t that much of a problem,” he said.

Monica Eng is a WBEZ producer and co-host of the Chewing The Fat podcast. Follow her at @monicaeng or write to her at meng@wbez.org