Philadelphia ? Even in black pants, a black vest and long-sleeved shirt, Amish flower vendor David Stoltzfus had no complaints about working outdoors Tuesday as the stagnant air simmered at close to 100 degrees.

“I do it every day, work in warm weather,” said Stoltzfus, who came from his farm in Lancaster County to sell flowers near Philadelphia’s Rittenhouse Square. “We’re fine. I’m comfortable.”

But he was among the few people not complaining about the heat wave that has gripped much of the nation with temperatures in the 90s and 100s since the weekend. Cities across the country took steps to ease the health threat and conserve energy. At least six deaths have been blamed on the heat.

In New York City, the mayor ordered city offices to turn off unnecessary lights. In Pennsylvania, swimming pools stayed open late and summer school classes were dismissed early.

In Washington Square Park in Manhattan, sweaty residents cheered when a large fountain shot on and started wading in the ankle-deep water. No one worried about whether it was dirty.

“It’s too hot to care about that stuff right now,” said Alex Glowacki of Brooklyn. “And anyway, it doesn’t look all green and slimy like it usually does, so I feel OK without shoes on.”

Julianna Fierro, 11, right, and her brother Marcos Fierro, 9, both from Santa Fe, N.M., hang out at the Bicentennial Pool in Santa Fe. Pools were popular attractions across the country on TUesday as sweltering heat kept a grip on much of the U.S.

The thermometer hit 95 degrees by 2:30 p.m. in Central Park, the National Weather Service said. The heat index, measuring the combined effects of heat and humidity, was expected to reach 104.

Heat advisories and warnings were lifted for much of Pennsylvania, except for the southeastern portion that includes Philadelphia. Parts of the Midwest also got a little relief from a Canadian cool front. The 8 a.m. temperature in Milwaukee was 65, compared with 76 at the same time Monday.

The Northeast got a break Tuesday night, with scattered showers and thunderstorms, but the heat was likely to persist in the southern Plains until Friday. Forecasts in Nebraska called for temperatures around 110 degrees Wednesday.

In New York, Mayor Michael Bloomberg reminded residents to conserve electricity. He directed City Hall to do its part: Only natural light filtered into the rotunda Tuesday, and some passageways and rooms were dark. “There’s plenty of light coming through the windows – you don’t have to keep the lights on – and if everybody did some of that, that would really save some electricity,” the mayor said.

As the temperature rose to almost 100, an electrical cable failed at LaGuardia Airport, knocking out power to one terminal and part of a second terminal for several hours. American Airlines canceled 45 flights, and Delta canceled 11 flights.

Relief could not come soon enough for many people working outdoors.

In Newark, N.J., bicycle patrolman Manuel Arias wore a uniform of black pants and a helmet as temperatures climbed well into the 90s. “I’m on my fifth bottle” of water, he said. “Yesterday, I drank 15 bottles.”

“If I see sprinklers or hydrants, I just drench myself,” said New Haven, Conn., letter carrier Ceferino Roman, who walked his route in 20-minute intervals, taking breaks in between. “This is the worst.”

Amtrak travelers riding between Washington, D.C., and New York on Tuesday evening experienced delays of at least an hour that conductors blamed on the extreme heat, which requires trains to travel at reduced speeds.

Transportation officials in Connecticut monitored the overhead lines that power some commuter trains to New York City because the cables tend to sag in extreme heat, causing delays.