Sisters Joanne Isdahl, front right, of Plymouth and Katie Maiers, front left, of Minneapolis launch from Bohemian Flats for a day trip down the Mississippi River in Minneapolis. (Leila Navidi/The Minnesota Star Tribune)

Minnesota summers aren’t getting hotter. But they are getting more humid.

Thanks to a heat wave set to last through Sunday evening, the state is in for both this weekend. The heat index — or how hot it feels when you add in humidity — could reach as high as 110 degrees in parts of southern Minnesota, according to the National Weather Service.

That’s a rare event for this early in the summer, said Kenneth Blumenfeld, senior climatologist for the Minnesota Department of Natural Resources. Since 1970, Minnesota has only experienced a total of 24 June days when the heat index clocked in at 100 degrees Fahrenheit or higher, he said.

Yet even as the global average temperature continues to climb, data shows that Minnesota summers haven’t gotten hotter, just more balmy, Blumenfeld said.

“We see increases in the frequency of humid days, days where the dew-point temperature gets to 70 degrees or higher,” he said. “But what we’re really seeing are increases in the intensity of the humidity, so how high that dew point gets, or how high the heat index gets.”

It’s one of the ways climate change has manifested during the summer, which so far has been the season least affected in the state by rising global temperatures. By contrast, Minnesota winters have warmed significantly, data shows, causing more midwinter melts and fewer days with frozen lake ice and snow on the ground.

Hotter overall temperatures allow the air to hold more moisture, Blumenfeld said. It also means more water is evaporating from the ocean and being dumped over land, including in Minnesota, which receives air from the Gulf of Mexico.

“Minnesota is in an area where annual precipitation, on average, has increased over the past few decades, and the extremes of precipitation … have also increased on average,” Blumenfeld said.

The most recent National Climate Assessment shows that much of the Midwest, including Minnesota, has gotten wetter over the last several decades — by roughly 5-15%.

But rising temperatures aren’t the only contributor to Minnesota’s increasingly humid summers. Another culprit: corn.

Blumenfeld said corn is particularly good at absorbing water from the ground and releasing some of that moisture back into the air, a process known as evapotranspiration — or “corn sweat,” he said.

And because corn crops are irrigated, more water is available for that process compared with in nature, he said. Corn is also planted more densely than other crops, he added, meaning there’s simply more of it in a given area.

That said, corn sweat likely isn’t playing much of a role in this weekend’s balmy conditions, Blumenfeld said, because the plants aren’t mature enough at this point in the growing season.

As Minnesotans prepare for the weekend, officials are encouraging people to exercise caution and keep an eye out for updates to the forecast. Hot and humid days can be especially dangerous, health experts warn, because humid conditions slow the body’s natural cooling process.

Heat is the leading cause of death related to weather in the United States, the NWS warns, resulting in hundreds of fatalities in the country each year. In Minnesota, at least 77 people have died due to heat illness since 2000, according to the Minnesota Department of Health.

“Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun,” NWS officials wrote in their heat advisory, “and check up on relatives and neighbors.”