Why the storm was so ferocious

Tens of thousands of the region's residents are still sweating over the endangered contents of their power-less refrigerators. And, ironically, in part they can blame a refreshing pulse of dry air that had made a brutally hot Thursday afternoon more bearable.

That dry air evidently gave an extra destructive jolt to some of the most damaging summer storms in the region's history.

In the final tallies, about 350,000 customers lost power as a line of thunderstorms flashed through the region at lightning speed, ripping apart trees from Glenmoore, Chester County, to Hammonton, N.J., and generating winds rarely experienced around here.

About 120,000 PECO customers are still without power, primarily in Chester and Delaware Counties.

Gusts reached 75 mph in Philadelphia, 60 in Camden County, and perhaps 90 in Chester County.

Delaware County was particularly hard-hit. In Lansdowne, more than 80 percent of the 5,300 homes lost power. And SEPTA's R3 Media/Elwyn line are still out of service.

Firefighters in Upper Darby had to rescue a 10-year-old in an iron lung, and at Aronimink Golf Club, in Newtown Square, Tiger Woods' people did a good deed.

The storms did put a damper on the heat - but not for long.

It's going to be hot weekend for cleaning up all that debris, with temperatures heading back into the 90s. The National Weather Service says Philadelphia might have to turn on the "excessive heat warning" yet again Sunday and Monday.

Understandably, Kelly Tagliaferro, 35, of Haverford Township, was piling her three kids into her purple minivan this morning in a desperate search for "anywhere with air." And she had another urgent need: "We haven't been able to make coffee. We need coffee."

No storms resembling Thursday's are in the forecast, nor are they likely to reoccur. It was a rare meteorological event, and it appears that most of the damage occurred in a matter of minutes as the storms approached.

Why were the winds so ferocious?

The amazing rapidity of the storms' movement - 55 m.p.h. - certainly was a factor, as was the heat, since storms are fueled by warm air from the surface rising over cooler air. But just as the heat reached a peak in the afternoon - the official thermometer at Philadelphia International Airport hit 97, the highest reading since June 10, 2008 - drier air moved in at mid-levels of the atmosphere, said Tony Gigi, a storm specialist at the National Weather Service office in Mount Holly.

As the downdraft rain fell into the drier air, some of it evaporated. When water evaporates, it gives off a cooling effect; that's basically how sweat cools as it vanishes.

In this case, the massive amount of evaporation gave an extra kick to the downdraft winds, which billowed the tarp at Citizens Bank Park, where the Phillies were hosting Cleveland.

"You have a little bit drier air, and more evaporational cooling," said Dave Dombek, at meteorologist with Accu-Weather Inc.

The Philadelphia temperature plummeted 26 degrees within an hour right as the rains were arriving, and the airport measured a 75 mph gust.

Some of the winds might have reached 90 mph in Chester County, where Glenmoore was particularly hard hit, said Gigi. "That's just as strong as the winds get around here," he said.

The 215,000 service interruptions in the PECO territory made this one of the biggest outage events ever - No. 11 on the all-time list. About 130,000 Public Service Gas & Electric customers lost power.

PECO said it hoped to have all the customers back by the end of the weekend. PSE&G said it expected to have all the power back on by Friday night.

Philadelphia's 311 line was inundated with calls, including 184 about downed trees. Southwest Philly and the Cobbs Creek area were favorite storm targets, as was a large piece of Delaware County.

Delaware County Emergency Services received more than 4,000 911 calls, quadruple that of normal.

"We got hit," said James Marino, Haverford fire marshall.

On Signal Road, a 30,000-volt power line ate a football-sized hole into the street, turning the blacktop into a molten mess.

The Bon Air fire department, on Royal Avenue, was blocked in by two trees, a transformer and a web of downed power lines.

"We couldn't get our fire trucks out yesterday it was so bad," said Tom McDonald, assistant chief at the Bon Air station.

In Upper Darby, firefighters responded to an unusual call around 9:30 p.m. The power went out in a Secane home where a 10-year-old was using an iron lung, and then the back-up generator began to fade, said Tom Jones, Upper Darby's chief administrator. The township fire department responded and hooked up a portable generator to power the lung.

The storm closed roads in Lower Merion Township, Montgomery County, including a portion of busy Montgomery Avenue, but it did produce another rarity - a storm-related road opening.

The Pennsylvania Department of Transportation had planned to close a heavily traveled portion of Route 422 between Route 23 and King of Prussia but the work has been rescheduled, due to the weather.

And over at the Aronimink golf course, where (you might have heard) Tiger Woods is due to appear next week, some members of his staff rescued a swim meet.

A power outage threatened the meet, but the Woods' people hooked up one of their private generators, and the lights - and the event - went on.