Once again, Philadelphia faces an especially hot summer

By now, this should feel familiar. It has been hot for days, for weeks, and for that matter, remarkably often in the last 23 years.

In the period of record dating to 1874, officially eight of the 10 warmest summers in Philadelphia have occurred since 1988.

And while it is unlikely that the summer of 2010 will match the blistering intensity of 1988 - the year that global warming became something of a national obsession - the next few days are going to be pizza-kitchen warm.

Temperatures in the mid-90s, along with copious humidity from the amply wet ground and unusually warm oceans, are going to make for "pretty nasty heat," said Alex Sosnowski, a meteorologist with AccuWeather Inc.

The Philadelphia Health Department and the National Weather Service have issued the first heat warning of the year for Wednesday, and it might well stay in effect on Thursday.

The warning touches off several emergency measures, including the "heatline," operated by the Philadelphia Corporation for Aging. That will operate from 11 a.m. to midnight.

The heat should ease Friday, but by then lovers of antique temperature records may have been sweating bullets. The record high for June 23 is 97, set in 1888, and for June 24, it's 99, set in 1923.

"We're going to be darn close," Sosnowski said.

This latest heat surge continues an extraordinarily warm period that took hold in March, right after the historic snows disappeared.

In New Jersey, the March 1-June 1 period - the meteorological spring - was the warmest statewide on record, said David Robinson, a Rutgers University professor who is the state climatologist.

The rapid spring warm-up may have been related to the general lack of snow cover well to the north of the Philadelphia region, added Robinson, a snow-cover expert. The snow would have refrigerated southward-moving air masses.

It also was the warmest spring in Philadelphia, Sosnowski said. This almost certainly will be one of the warmest Junes ever, and the Jan. 1-June 30 period is likely to finish in the top five.

For those looking for a break on those power bills, the outlooks for the rest of the summer are not encouraging. Both AccuWeather and WSI Corp., which provides forecasts to energy companies, see heat followed by heat.

"It does look like it's going to persist," Sosnowski said.

If it does, it will be in keeping with recent years. In the last 22 years - starting with the historic summer of 1988, when the temperature hit 100 five times in Philadelphia and extreme heat in the Northeast fired up the global-warming debate - Philadelphia's official summer temperatures have averaged 75.8. That compares with 74.2 in the 22-year period that ended in 1910.

Paradoxically, the century mark has been elusive. Typically, it hits 100 in Philadelphia every two or three years, and it happened on July, 2, 3, and 4 in 1966. But it has not occurred here since Aug. 9, 2001.

Still, it has been generally warmer, and the increase tracks neatly with global and national trends, said Derek Arndt, chief of the Climate Monitoring Branch of the government's National Climatic Data Center.

However, Arndt cautioned that no measuring station is perfect, not even the one at Philadelphia International Airport. The airport station was moved in 1995 and got a new thermometer, replacing one that was known to run hot.

Arndt said the Allentown station was superior, since it has been subject to less change. While the temperature trend in Allentown is similar, only two of the recent summers made it into Allentown's top 10.

The Philadelphia station may be sensitive to "heat island" effects, said J. Marshall Shepherd, an urban-heating specialist at the University of Georgia.

Paved surfaces and buildings absorb heat during the day and are slow to release it at night, and the airport thermometer could be getting an extra kick from its surroundings or even plane exhaust.

"The heat island is a fairly localized signature," Shepherd said.

That said, the heat generated by the dense core of buildings in Center City can affect neighborhoods a few miles away, said Shepherd. That could be a concern in brick rowhouse neighborhoods where homes heat rapidly after the sun rises.

Historically, heat deaths in Philadelphia have been concentrated among the elderly who live alone in those neighborhoods.

Ironically, however, the early heat this season may have been just what the doctor ordered for the frail and elderly.

The Philadelphia temperature hit 90 for the 11th time this year on Tuesday, and health experts believe that early doses of heat may mitigate the impact of later heat waves.

Said Jeff Moran, the city Health Department spokesman, "We're hoping people are pretty well acclimated."