PHILADELPHIA -- Americans dipped into the water, went to the movies and rode the subway just to be in air conditioning Saturday for relief from unrelenting heat that has killed 30 people across half the country.
The heat sent temperatures soaring over 100 degrees in several cities, including a record 105 in Washington, St. Louis (106), and Indianapolis (104), buckled highways and derailed a Washington-area train even as another round of summer storms threatened.
If people ventured outside to do anything, they did it early. But even then, the heat was stifling.
"It was baking on the 18th green," said golfer Zeb Rogerson, who teed off at 6 a.m. at an Alexandria, Va., golf course but was sweltering by the end of his round.
The heat sent temperatures soaring in more than 20 states to 105 in Louisville, Ky., 101 in Philadelphia, and 95 in New York; besides Washington, a record of 104 was set in Sioux Falls, S.D., and Baltimore set a record at 102.
In Western Pennsylvania and West Virginia, the temperature hit 99 and the heat index, which takes into account both temperature and humidity, rose well above 100. Government agencies warned people to guard against heat exhaustion and to take special precautions in the hours before an evening thunderstorm was expected to break the heat wave.
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 Metro train (part of the Washington, D.C., area's public transportation system) to partially derail in Prince George's County 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, D.C. In the Washington area, 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.
In the Midwest, some residents were drawing comparisons between the current heat wave and the severe heat and drought of the 1930s.
Around the region, corn and soybean crops shriveled from the heat and the lack of rain. In the hardest hit and hottest areas, some farmers said they had already given up on their cornfields for the season. Others say much is riding on whether the heat subsides and rain arrives in the next few days, a crucial period for corn pollination.
"There's vast uncertainty," said Bob Nielsen, a professor of agronomy at Purdue University. "There aren't many years, though, when I get this pessimistic."
Meteorologists said the recent hot streak, though not unprecedented, was unusual because of how early in the summer it struck and its duration.
The prolonged heat has been the result of a high pressure system that has set up over the central and Eastern parts of the country, said Katie Garrett, a meteorologist with the National Weather Service.
The system has been so strong that it has kept storm systems from moving in and has prevented cold fronts that usually provide relief from sweeping through. At the same time, moisture and heat from the Gulf of Mexico have been blowing into the Upper Midwest, Ms. Garrett said.
"It's just the way the storm track is running right now," Ms. Garrett said.
Even as relief is expected to come by this evening in the form of lower temperatures, forecasters said there was still cause for concern. Severe thunderstorms are expected to bowl through the Midwest and along the Eastern Seaboard today and into the early part of the week.
This is a particularly unsettling proposition for the Washington area, which was hit by severe storms last weekend that left millions without power and where more fragile, temporary power systems are in place.
The New York Times and Post-Gazette staff writer Marcus Schwarz contributed.