Grab-N-Go, a drive-through and walk-up convenience store in New Iberia, La., has a central air-conditioning system, a window air-conditioning unit and two small, portable air-conditioners. On a recent afternoon, all of them were running. Cool air swirled through the devil-red metal box of a building.

Still, Don Vitto, the shopkeeper, was sweating anyway.

Lately, brisk business could almost be considered a curse: Every time a customer arrived, Mr. Vitto had to slide open the window to take their order. His stockpile of chilled air spilled out. The outside air — nearly 100 degrees but feeling even warmer — forced its way inside.

“It’s a sticky, heavy heat,” Mr. Vitto said, disgust dripping from every drawn out syllable. “You can feel it in your breathing — I know I can. I can feel the thickness in the air.”

In Louisiana, and along much of the Gulf Coast, the misery of summer has never been reflected simply by a temperature reading alone. It’s not just the heat, as Southerners have explained for generations. It’s the moist, soupy, suffocating humidity that swallows up everything and conspires with the heat to make any activity without air-conditioning draining and even deadly.

And this summer — goodness gracious — it has been absolutely abysmal.

The air has felt swampier and more suffocating. Yet, confoundingly, as moist as the air has been, a scarcity of rain and clouds has made the sun all the more blistering, leaving the earth as dry and cracked as peanut brittle.

But what has made recent months so punishing is the relentlessness of it all, as the conditions have dragged on for days on end and the volume of excessive heat warnings has broken records.

“It’s been an incredibly aggressive summer,” said Barry D. Keim, Louisiana’s state climatologist. “We normally have spells like this most summers. But this summer has been very, very persistent. The breaks are the fleeting moments, and it’s been oppressive most of the time.”

In Louisiana, Gov. John Bel Edwards has taken the unusual step of declaring a state of emergency over the heat. New Orleans went well over a month with a heat index of 105 degrees or more, almost doubling a record that had been set in 2021. There was even a streak of nine consecutive days recently when temperatures felt like 115 degrees or higher. City officials said heat-related emergency calls were coming in at a pace of roughly triple what they were in past years.

This summer has presented the world — and not just the South — with a staggering array of heat-related horrors: Phoenix had a monthlong streak of 110-degree days. Crops have been scorched. Warm water has turned beaches into baths and raised the prospect of a volatile Atlantic hurricane season. July was the hottest month ever recorded on earth.

Usually, on the Gulf Coast, the thermometer readings do not have the same shock value as those in arid desert cities like Phoenix and Las Vegas. Officials, forecasters and residents wallowing in their discomfort have had to lean on the heat index — a measurement meant to illustrate what it “feels like” outside by taking temperature and humidity into account — to convey the severity of what they are enduring.

Sure, these conditions come every summer, and even at times in the fall and spring, too. But veterans of the heat and humidity said this summer was wearing them down. They acknowledged a fear that what was happening now was less an anomaly than a sneak preview.

“It’s going to get worse,” Latoya Wilson, 44, said as she sat with her family on the sand at Cypremort Point Beach, on the coast south of Lafayette. “It feels like the beginning of the end.”

Even so, she refused to completely surrender to her worries. Instead, her family made accommodations: Stay hydrated. Stay inside. Go to the beach, but wait until 7 p.m., when things are at least tolerable. “It’s not going to stop us from living,” she said.

Coping with oppressive heat and humidity is fried into virtually every part of the South’s history and culture, influencing daily rhythms and the design of homes. The Southern author Roy Blount Jr., writing about humidity for Garden & Gun magazine, noted that the word itself almost never appears in the region’s literature precisely because it is omnipresent. “If fish had a literature, you wouldn’t find the word water there,” he wrote. Yet there were plenty of references to its chief symptom, namely sweat.

That sweatiness is the result of a dome of pressure known as the Bermuda high, which parks itself in the Atlantic Ocean and spins clockwise into the Gulf of Mexico and the southeastern United States. “We just bask in this really hot, moist air,” Dr. Keim said.

Its consistent return meant that human existence in the South has forever required figuring out how to tame or at least minimize that burden.

“You talk to your grandparents,” said James H. Diaz, a professor of environmental and occupational health sciences at Louisiana State University’s School of Public Health. Indeed, many younger Southerners grew up hearing their elders speak of a world before ubiquitous air-conditioning, their stories relayed in a “trip to and from school was uphill, both ways” sort of fashion.

“They would build homes with transoms over the doors and over the windows,” Dr. Diaz explained. “They would build homes with the front door and the back door in complete alignment — you could basically create a wind current through the house.”

Days were structured around the heat, waking at 4 a.m. to work outside, then taking breaks and eating a big meal in the middle of the day.

“That’s how our ancestors dealt with it,” Dr. Diaz said.

Still, he conceded, “They didn’t deal with the kind of heat indexes that we’re dealing with now.”

The ascendance of air-conditioning was a transformative force in the South, facilitating the rise of suburbs and making the region less forbidding for companies that wanted to move in. Now, transoms and rising early have been replaced with drive-throughs and attached garages and cranking up the A.C. — for people with the means and jobs that allow it.

“People are so completely dependent on air-conditioning,” said Craig E. Colten, a professor of geography at Louisiana State University. “You run around on our campus and you see people going between classes in shorts and sweatshirts because the air-conditioning is so ramped up.”

Then, in New Iberia, a city situated in the swamps west of New Orleans, there’s Herman Marshall sitting under his carport, a fan a few feet from his face, blowing out hot air.

“It’s hell,” Mr. Marshall, 72, said. “It’s all I can say.”

He’d had the same central air-conditioning unit for 20-plus years. Recently, it pooped out on him, and he was waiting for the parts to come in to fix it. “I guess anything like that will burn out if you run it 24/7,” he said.

Mr. Marshall was no stranger to the heat. He gave up a job as a welder long ago because he could not stand how hot the work was, becoming a heavy equipment operator, which was marginally better. “It’s hard in the morning,” he said, when the dew that forms overnight contributes to the mugginess. “You stand in one place and you sweat.”

Oddly enough, though, for all its wretchedness, there was somehow a comfort in the humidity’s familiarity. “I go somewhere else and I miss it,” said Ms. Wilson, a nurse who lives in Lafayette. She even had a theory: Humidity kept people on the Gulf Coast looking youthful, acting as a natural moisturizer.

Da’lijah Nae Ozenne, 23, wasn’t so concerned about that.

“I’m staying in the house — with my air-conditioner on 68,” she said. “Every day, all day.”

Except for this rare moment when she had to separate herself from it. Sweat beaded on her face as she stood outside the window at Grab-N-Go, asking Mr. Vitto’s wife, Linda, for a strawberry Fanta.

“Everybody be complaining how hot it is,” Ms. Vitto said as she leaned out the window. “They all want ice. They all want something cool to drink.”

Mr. Vitto declined to give his age. “I’m old enough to vote,” he said. And old enough, he admitted, to recall life before air-conditioning was everywhere. He remembered playing outside all day, coming home, taking a bath in the evening and planting himself in front of a box fan.

But that was a different time, he said.

“Sometimes,” Mr. Vitto said, “it seems unbearable.”

He slid the window closed. He just wanted to savor the hard work of his air-conditioners for as long as he could. Any minute now, the next customer would pull up, and he’d have to greet them with a blast of cool air.