With temperatures hovering in the mid-90s, people walk near a sign Thursday in Boston. Charles Krupa/Associated Press

Brutal heat is pressing down on much of the United States this weekend, with tens of millions of Americans sweltering under heat advisories and heat indexes in the Northeast soaring into the triple digits.

Officials up and down the Interstate 95 corridor urged residents to hydrate and watch for signs of heat-related illness as people flocked to pools and cooling centers for relief from the oppressive temperatures in cities stretching from Boston to Washington D.C.

More than 30 National Weather Service stations may approach or exceed record temperatures by Sunday, the NWS Weather Prediction Center said Friday. High humidity is pushing heat indexes – the temperature that the air feels like – above 100 degrees Fahrenheit.

“The ‘Dog Days of Summer,’ ” the agency said, “are unquestionably here.”

The extreme heat is another warning sign that climate change is increasingly imperiling what traditionally is a time people think of summer vacations filled with rest and relaxation. Temperatures are rising, wildfires are becoming more severe and droughts are becoming more common – a striking change from previous generations, scientists say.

In some cities, this weekend’s extreme heat caused major events to be altered due to safety concerns. The Boston Triathlon originally scheduled for Sunday was postponed to late August “due to the current Heat Emergency.” While New York City’s triathlon and duathlon is still scheduled to take place Sunday, organizers announced they had shortened the bicycling and running segments of the competition.

“The safety of our athletes and everyone in attendance is our top priority,” New York City Triathlon organizers said.

New York officials converted public spaces to cooling centers and offered spray caps for fire hydrants, which are meant to lessen the amount of water released if people open the hydrants to stay cool. The city’s Weather Service station said the next two days would be the area’s “hottest weekend of the year so far” and warned that temperatures would climb into the 90s and could feel even higher.

If New York’s heat wave lasts through Monday, it would match a similar seven-day heat wave in 2013 when heat indexes reached at least 95 degrees each day.

Boston was set to experience temperatures in the high 80s to high 90s Saturday, with “comfortable” humidity levels. But the Weather Service warned of more oppressive heat conditions for Sunday, with heat index values up to 105 degrees.

In Philadelphia, where the heat index could reach the low to mid 100s on Sunday, the fire department warned residents not to use fire hydrants to cool off – warning that opening the hydrants could damage them and nearby property and people. It encouraged residents to find public pools and spraygrounds instead.

The heat wave is causing residents throughout the Northeast to figure out how to stay cool, where air conditioning is not as ubiquitous as it is in other parts of the country to the south and west.

Lauren Kinsley said she has been working from her Manhattan home over the past few days to avoid the heat.

“I just have one air conditioner in my apartment – one window unit – but I’m trying to keep costs down,” said Kinsley, 32, who works in fundraising. “So it’s just been sweltering in my apartment basically this whole week, and right now I just went outside to get coffee. And I came back drenched.”

Kinsley said she is planning to go see “Mrs. Harris Goes to Paris” at a movie theater this weekend, in part because it means she’ll be in an air-conditioned space.

“But you have to brave the heat to get there,” she said, adding she is holding off on running errands until the temperature cools down.