PARIS — With temperatures nearing 100 degrees Fahrenheit, the Paris sky on Wednesday was the wrong kind of Van Gogh canvas: hazy, bluish white and shimmering with heat.

By the standards of an American summer, this may not exactly seem particularly apocalyptic: The high was 95 degrees Wednesday, with an expected high of 97 later in the week. But in a city of millions that is almost entirely devoid of air conditioning, a heat wave can feel inescapable, an invisible cloud that slows the mind and stifles the body.

Getting on the metro is horrible. So is sitting in a sweltering office. The only real respite is sleep, and even to do that you need a fan, which you had better hope you already bought before this nightmare began. If not, few will be left on the shelves, and what awaits you is a cold shower and a long, restless night.

The problem is that extreme heat is becoming the new normal in urban environments that have never before experienced these kinds of temperatures with such striking frequency. In the seemingly endless tableau of European history, the hottest summers on record were in 2002, 2003, 2010, 2016 and 2018. This year is already off to a strong start.

For more vulnerable groups — young children, the elderly and the homeless — heat waves without air conditioning are potential disasters, and Paris provides a case study for a city thrust into a new reality that no one knows quite how to manage.

The anxiety is mounting. Outside Paris, where exams were postponed for certain high school students until next week, school districts announced closures for Thursday, when temperatures were expected to reach record highs elsewhere in France.

On the top of everyone’s minds was the devastating European heat wave of 2003, when about 15,000 people died in France from heat-related causes. This time, the French government — and the city of Paris — have increased their preparations, mostly as a result of smartphone technology that has emerged in the years since.

“Many things have happened in 15 years,” Ariel Weil, the mayor of Paris’s central and crowded fourth arrondissement, or administrative district, said in an interview. “We’re on the one hand continuing what we’ve done, and we had a policy in place already. But we’ve also revised.”

He added: “Each year we progress,” noting especially the rise of volunteer programs such as “Paris en compagnie,” or “Paris with company,” a City Hall initiative to provide volunteer assistance to elderly people who live alone and who may need help with shopping or other basic errands.

These isolated seniors can now be more easily monitored during situations like this week’s heat wave. Those who wish can register themselves via SMS for a free service called “Chalex” — French for “chaleur extreme,” or “extreme heat.” Those who register will be called every 48 hours by a city official, a social worker or a medical professional for an informal check.

The city also has opened certain parks 24/7 and a network of air-conditioned rooms across town. Water is also readily available at designated stations.

There is also a broader awareness of climate change and its dangers, and French media outlets have been covering the story not as a bizarre aberration but as an inevitable example of a global phenomenon.

Taking action on climate change has been a major priority of Paris Mayor Anne Hidalgo, a Socialist, ever since she took office in 2014. During her tenure, Paris has undergone significant transformations: additional bike lanes have been installed on the city’s busy thoroughfares, and more green spaces have been added.

Weil said that three new parks have recently opened in the fourth arrondissement alone. “Since the time of Baron Haussmann, I don’t have the memory that there’s been such an effort to transform the city,” he said. At the behest of Napoleon III, Georges-Eugène Haussmann was responsible for Paris’s urban renewal in the 19th century, designing the famous grand boulevards the world knows today.

But Agnès Buzyn, France’s health minister, underscored the dangers that still exist. “There can never be zero deaths after a heat wave,” she said Wednesday morning, speaking on France’s Europe 1 radio.

In the heat of the day Wednesday — blistering, but not quite as bad as temperatures forecast for later in the week — a group of young migrants from Sudan and Afghanistan were gathered in a makeshift welcome center on the outskirts of town, behind the ring road that separates the French capital from some of its less-affluent suburbs.

They were lining up to take showers in a portable facility that lets six people bathe at any given time. It’s open from 8 a.m. until noon and then again after lunch until 7 p.m.

Abdallah Salam, 20, said he arrived in Paris last week at the tail end of a long journey from his native Hiran, Somalia. He said that he has been showering here daily but that the water has been lukewarm and not necessarily a good shield against the rising temperatures.

“Somehow, you know, this is worse than our weather. I don’t know why, but the sun, it just feels so close,” he said in English. “There, at least you get good wind — you don’t get the sunshine like this. Here you can really feel it.”

Amir Sherifi, 17, just emerged from the showers, his hair wet. He said he’d arrived from Afghanistan about a week before. “It’s too hot,” he said. Asked about the weather back home, he smiled. “It’s too hot there, too.”

In each of Paris’s 20 arrondissements, there were designated “cool rooms” where residents and passersby could benefit from a few minutes — or a few hours — of air-conditioned calm. Most of these were in the elaborate city hall that houses the local government of each arrondissement, but some were in the rare neighborhood cafe that had both the seating capacity and the air-conditioning infrastructure.

In the typically well-heeled ninth arrondissement’s city hall, Nicolas Royo, 49, was reading a magazine as his daughter played with friends indoors. About a dozen people were there. Two mothers were chatting by the air-conditioning unit as their children ran wild. One woman, alone, was sipping a coffee at a table, staring into space.

“Let’s say each year we have hot weeks, but the fact that this is in June is exceptional,” Royo said.