Temperatures soared to more than 38 C in several cities as Americans dipped into the water, went to the movies and rode the subway just to be in air conditioning for relief from unrelenting heat that has killed at least 30 people across half the country.

Two boys jump into a pool at the Hamilton Fish Recreation Center in the Lower East Side neighborhood of New York on Saturday. Temperatures were supposed to reach 38 C. [Agencies]

The heat on Saturday sent temperatures above 38 C in cities including a record 40.5 C in Washington, St Louis 41 C, and Indianapolis 40 C, buckled highways and derailed a Washington-area train even as another round of summer storms threatened.

Temperatures soared in more than 20 states to 40.5 C in Louisville, Kentucky, 38.5 C in Philadelphia, and 35 C in New York; besides Washington, a record of 40 C was set in Sioux Falls, South Dakota, and Baltimore set a record at 39 C.

At least 30 deaths were blamed on the heat, including nine in Maryland and 10 in Chicago, mostly among the elderly. Three elderly people found dead in their houses in Ohio had heart disease, but died of high temperatures in homes lacking power because of recent outages, officials said. Heat was also cited as a factor in three deaths in Wisconsin, two in Tennessee and three in Pennsylvania.

Officials said the heat caused highways to buckle in Illinois and Wisconsin. In Maryland, investigators said heat likely caused rails to kink and led a green line train to partially derail on Friday afternoon. No one was injured, and 55 passengers were safely evacuated.

Thousands of mid-Atlantic residents remained without power more than a week after deadly summer storms and extreme heat struck the area, including 120,000 in West Virginia and some 8,000 in the suburbs around Baltimore and Washington, DC. In the Washington area, the utility company Pepco asked customers to conserve power, saying the heat was stressing the system.

"This is becoming a black swan of heat waves, in the sense that it's such a long heat wave, such a severe heat wave and encompassing such a large area," said Chris Vaccaro, spokesman for the National Oceanic and Atmospheric Administration.

At the Smithsonian's National Museum of Natural History, Abraham Lewis and his wife, Dzifa Fianoo of Lorton, Virginia, brought their 8-week-old son out for a walk in the 38 C heat.

"I really don't want to be out, but she's a new mother and was feeling cooped up," Lewis said. "Do you see how hot it is?" he said, wiping beads of sweat from his forehead more than once.

The couple's home in northern Virginia lost power for two days last week after a severe windstorm swept through. Fianoo had to haul the family's food to a cousin's house to prevent it from spoiling, and then took it home again.

In Manhattan, customers who stepped in to see Jiro Dreams of Sushi at an IFC movie theater were there for more than entertainment.

"Of course we came to cool off!" said John Villanova, a writer who was on his second sweaty T-shirt of the day.

He said that earlier, he rode a Manhattan subway back and forth for a half an hour, with no destination in mind, "because it really keeps you cool".