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, and the weather was suspected in at least three others.

Click here to see photos of Americans facing 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."

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 could get a break starting Tuesday night, with scattered showers and thunderstorms expected for parts of the region, 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.

Jennifer Caban, 16, of Queens, said she cranked up the air conditioning in her home until it broke. So she was sitting in the shade. "It was the worst night's sleep I ever had," she said. "I woke up and there was a pool of sweat on my pillow."

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," he said. "Yesterday, I drank 15 bottles."

The heat was blamed for the death of a Philadelphia woman found in her home on Monday, and outreach workers continued to check on homeless and elderly people. The city closed summer school at 11 a.m. again on Tuesday in buildings without air conditioning.

Utilities set records for electricity usage, and in Connecticut health departments advised people without air conditioning to cool off in senior centers, malls and movie theaters.

"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."

In cities such as Allentown and Hartford, Conn., swimming pool hours were extended and sometimes fees were waived. New York Gov. George Pataki waived admission fees to Long Island state beaches from 3 p.m. to 8 p.m. until Thursday.

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.

In suburban Philadelphia on Tuesday, a 23-year-old man with cerebral palsy died after apparently being left in a sweltering van by accident, authorities said.

The also heat killed a 76-year-old Oklahoma City man in a house where the air conditioner was broken, and the body of a 62-year-old woman was discovered in another home that had air conditioning that was not turned on, officials said Tuesday. Three other deaths in Oklahoma were believed to be linked to the heat.