Twenty heat-related deaths have been reported in Philadelphia this summer, according to James L. Dean, medical director of the city Health Department, and more are possible. The National Weather Service has issued a heat warning for today and tomorrow for Philadelphia and other urban areas, including Camden and Norristown. Philadelphia public schools have canceled today's summer programs.

Highs are expected to hit the mid-90s both days, just shy of the records - 99 and 100 - for those dates.

The nights will be sultry, and temperatures may not drop below 70 until at least Wednesday, forecasters said. Such conditions can be hazardous, especially to elderly people who live alone in heat-trapping brick rowhouses, and can bump up heat-related fatalities.

"In many cases, we think the nights are as important, if not more important, than the days," said Laurence Kalkstein, a retired University of Delaware climatologist who is an expert on heat-wave mortality. His computer models, he said, consistently indicate that nights have a greater impact on heat deaths than daytime heating.

"You get that cumulative effect," said Dean Iovino, a meteorologist with the National Weather Service in Mount Holly. "The rowhomes don't cool off at night," making them more susceptible to heating when the sun comes up.

Studies have documented that nights are becoming warmer all over the country, evidently the result of worldwide warming, increased urban heating, and more water vapor in the atmosphere.

An Inquirer analysis has found that in the last eight summers, which generally have been quite warm, the increase in overnight heating has outpaced that in the daytime by about 30 percent.

Temperate U.S. cities are reporting 10 more "hot nights" - when the temperature fails to go below 70 - than they did 40 years ago, said Arthur DeGaetano, director of the Northeast Regional Climate Center at Cornell University.

Experts agree that the urban heat-island effect is a factor. Paved surfaces and tall buildings trap heat during the day and are slow to release it at night.

Also, worldwide warming probably has increased the water vapor in the air, which keeps solar heat from escaping after dark.

Kalkstein, who helped devise Philadelphia's heat-warning system, said that "circumstantial evidence" pointed to one of the culprits - paradoxically, the preponderance of air-conditioning, which cools interiors but heats the air outside.

Whatever the causes, the next several nights are likely to be hot as a strong area of high pressure builds over the western Atlantic, said Mike Gorse, a weather service forecaster. Winds circulate clockwise around the center of pressure, so areas to the west experience warm winds from the south laden with oceanic water vapor.

It is not at all clear when the heat will break, he said. A front may try to push through next week, but the atmosphere grows lazy and more unpredictable in summer with the weakening of temperature contrasts that drive weather systems.

Philadelphia's heat warning will remain in effect until at least 8 p.m. tomorrow. A warning triggers several actions. The city alerts its 7,000 block captains and urges relatives, neighbors and volunteer groups to check on the elderly. The Fire Department beefs up its emergency medical staff, and the Philadelphia Corporation for Aging operates its Heatline, 215-765-9040.