NEW YORK — Temperatures hit the high 90s along the Eastern Seaboard on Wednesday as a hot spell heralded the official start of summer, with people wilting at graduation ceremonies, students trying to learn in suffocating classrooms and authorities warning folks to check on elderly neighbors.

According to the National Weather Service, the temperature was 93 degrees in New York City’s Central Park, but with humidity it felt like 97. In Boston, it felt like 100 but was 93. In Washington, it felt like 101 but was 97.

The hot spell arrived right on time — on the summer solstice and longest day of the year — in a region that’s home to some of the nation’s most densely populated cities.

Health officials warned residents to drink water, stay out of the sun and in air conditioning, and to check on elderly neighbors and pets. Public cooling centers have been set up in dozens of cities for those without air conditioning.

Several relatives of high school graduates were treated for heat exhaustion at an outdoor ceremony in North Bergen, N.J., and taken to a hospital. Ambulances were on standby at the event, which was held outside to accommodate about 5,000 people, said Capt. Gerald Sanzari of the North Bergen Police Department.

A similar scene took place in New Britain, Conn., where several people were taken to a hospital after suffering heat-related symptoms while attending the New Britain High School graduation. Captain David Koscuk of the New Britain EMS told the New Britain Herald that 24 people suffered from heat exhaustion or fainting and half of them were taken to area hospitals.

In Howell, N.J., school officials made Wednesday the last day of the school year instead of Thursday, citing the heat. And at nearby Wall High School, people attending the graduation ceremony will be able to watch a remote broadcast inside the air-conditioned building.

Connie Vincent, a mail carrier, was already sweating as she began her rounds in a residential neighborhood in Manchester, Conn., Wednesday morning.

“There’s nothing you can do,” she said as she dabbed her face with wet washcloths. “Tomorrow’s my day off, thank God. I’ve just got to make it through today.”

In a rare bending of the rules, the Metro in Washington, D.C., said passengers on Wednesday and Thursday would be allowed to drink water, an exception to their no-drinks policy. The National Weather Service said the temperature at Washington National Airport was 95 degrees just before 2 p.m., though it felt like 99.

Deborah Otchere, 59, mapped out a tree-lined route to work and brought a change of clothes to her job as a secretary in a Washington law firm. Among her traveling supplies was a partially frozen bottle of water.

“You live here long enough, you know how to prepare,” she said.

Forecasts for upstate New York on Wednesday and Thursday called for temperatures to hit the 90s from Niagara Falls to the Vermont border, with highs topping out in the mid-90s in some places. Elementary and middle schools in the Hudson Valley planned to dismiss students early because of the heat.

More than 450 cooling centers were being opened around New York City, which is under a hot weather advisory with an expected high of 94 degrees. Mayor Michael Bloomberg encouraged people without air conditioning to seek out the cooler spaces or visit the city’s beaches.

The city’s 1.1 million public school students are still in session for another week, and just 64 percent of classrooms are air-conditioned. The city is leaving it up to teachers and administrators to monitor the situation in each school, Bloomberg said.

“There’s nothing unsafe about it. It may be a tiny bit uncomfortable, but these are young, strong people, and we’re not going to ask anybody to stay in a building where we think it becomes dangerous,” he said.

In downtown Providence, R.I., at the central bus terminal, a worker for the Salvation Army — red-faced and hot herself — was handing out free bottles of water, reminding people to stay hydrated. Users of public transit were enjoying free service on buses and trolleys, offered on days when health officials declare air quality to be unhealthy and driving is discouraged.

In Philadelphia, the city’s highs in the next couple of days could break decades-old records of 98 degrees, set in 1931, and 99, set in 1923. Normally, the high for Philadelphia is about 84 degrees.

“You’re talking about almost 15 degrees above normal,” said Kristin Kline, a weather service meteorologist in Mount Holly, N.J.

Every state in the Lower 48 except for North Dakota was forecast to have 90-degree weather until Saturday, according to a model by the National Oceanic and Atmospheric Administration, the agency in charge of weather, climate and oceans.

On New York’s Long Island, Roy Gross, chief of the Suffolk County SPCA, cautioned against keeping pets in vehicles, noting temperatures can reach 120 degrees within minutes.

“Your pet can quickly suffer brain damage or die from heatstroke when trapped in these high temperatures,” Gross said.

In Manhattan’s Washington Square Park, women and small children took off their shoes to wade in a fountain. But the main attraction was a promotion by Nestle to give away a free ice cream cone to anyone who would do the hula hoop.

Tiny tourist Katie Phan, visiting New York with her family from Orange, Calif., joined several dozen people who took the frozen-treat bait. The 8-year-old expertly spun three hoops — and munched on a melting cone — all at once. It made her mother Terry proud.

“I had no idea she could do that,” she said.

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