A sprawling blanket of hot and humid air smothering the nation’s midsection this week has challenged temperature records, forced schools to cancel classes and left residents sweating from the Northwoods to the Gulf Coast.

Even in a season filled with other climate shocks, this blast of late-summer heat in the Central United States stands out for its breadth and its combination of high temperatures and suffocating humidity — including in areas more associated with frigid winters than unbearable summers.

Across hundreds of miles, from Mississippi to Missouri to Minnesota, cooling centers have opened, schools without air conditioning have dismissed early or outright closed and residents have tried to limit time outdoors.

“I’ve never seen humidity like this,” said Eric L. Harris, who lives in Lincoln, Neb., where temperatures surpassed 100 degrees on Tuesday and were expected to do so again on Wednesday and Thursday.

Mr. Harris, who has been collecting signatures for a proposed ballot question on paid sick leave, said the weather made that work especially difficult.

“My biggest fear isn’t trying to get people with opposing views to listen to me,” he said. “It’s that without measures to protect me, I would likely get heat stroke just from standing out in this heat.”

Across the continent, this summer has already been defined by weather extremes: deadly wildfires in Hawaii, blankets of smoke from Canadian wildfires, floods caused by a rare tropical storm in California, weeks of stifling heat in Arizona. Though linking individual events to climate change often takes time, researchers have warned that dangerous weather events will become more common as the planet warms.

For many Midwestern states, the high temperatures this week have brought added misery in a summer already made difficult by drought. In a cruel meteorological turn, that drought has exacerbated the heat.

“The ground is already really dry — it doesn’t take much for the heat to kind of just build up over there,” said Paul Pastelok, a senior meteorologist at AccuWeather. “And that’s what makes it a bigger heat dome that we’re seeing right now.”

In the Minneapolis area, better known for its foreboding winter conditions, forecasters said daily temperature records could fall on both Tuesday and Wednesday, with readings of 99 or 100 degrees possible.

Tyler Hasenstein, a National Weather Service meteorologist in Minnesota, said a heat dome of this scale might settle over the state every five years or so, “but those typically happen in June, July — and not August, which is kind of the weird thing in this case.” The temperatures were also not expected to drop much after sunset, he said.

Meteorologists said high temperatures were forecast to reach up to 20 degrees above average throughout Iowa and neighboring states over the next few days. The humidity will make it feel even more oppressive, with heat indexes that could approach 120 degrees. Forecasters have issued heat alerts, ranging from advisories to excessive heat warnings, for roughly 100 million people across 22 states.

In Adel, Iowa, where it was cloudless and in the 90s by midday on Tuesday, the high temperatures meant Amy Heinz was not able to let the dogs play outside at the animal rescue center she runs.

“Some of them are going crazy in their kennels,” Ms. Heinz said, but the alternative was simply not safe.

Even in a place like Iowa, immune from hurricanes and coastal disasters, Ms. Heinz said extreme conditions were a growing concern.

“Every year the weather seems to get worse and worse,” she said. “Between the flooding and the heat, and then we have the extreme cold that we deal with in the winter, it seems it’s worse than it used to be.”

With more of the country expected to suffer from extreme heat in the days ahead, contingency plans were already being put in place.

Kevin Russell, the superintendent of Downers Grove Grade School District 58 in suburban Chicago, said he started keeping an eye on the forecast late last week. Most buildings in his district are not fully air-conditioned, meaning the 100-degree temperatures that forecasters have discussed are untenable.

“We do have limited air-conditioned spaces, so what we’ll do on those hotter days — in the high 80s or even the low 90s — we will rotate students and staff through cooling stations,” Dr. Russell said. “However, when you start talking about 100-plus, you really need to be in that cooling station all the time.”

The first day of class in Downers Grove had been set for Wednesday, but Dr. Russell made the difficult choice to push that back to Friday, when cooler weather is expected.

“We have thousands of kids in Downers Grove with their backpacks all ready to go and super excited to start the school year,” said Dr. Russell. “Having to delay that, no one takes any joy in that.”

But there is good news ahead. Plans are in place to install air-conditioning at all the district’s schools.

Judson Jones contributed reporting.

Mitch Smith covers the Midwest and the Great Plains. Since joining The Times in 2014, he has written extensively about gun violence, oil pipelines, state-level politics and the national debate over police tactics. He is based in Chicago.  More about Mitch Smith

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