Those storms: Roaring rarities

It all happened in a matter of minutes, and this morning it was clear that this one of the most damaging summer storms in the region's history.

In all, 215,000 PECO customers lost power, most of them in Delaware and Chester Counties, making it No. 11 on the all-time PECO outage list.

Why were the storms so ferocious?

The heat certainly was a factor, as was the amazing rapidity of the storms' movement. Ironically, however, the conditions that kept the heat from being unbearable yesterday probably fueled the storm.

The official thermometer hit 97 early yesterday, but just as the heat peaked, some drier air crossed the region.

As the storms approached, said Tony Gigi, the National Weather Service's storm specialist, it moved at 55 m.p.h. through that dry air.

The dry mass led to mass evaporation of the moisture falling into it. When water evaporates, it gives off a cooling effect. That's how sweat cools you off.

In this case, it accelerated the cooling and gave a kick to the winds. The temperature dropped 26 degrees within an hour, and the wind gusts reached hurricane force.

Some of the winds might have hit 90 m.p.h. in Chester County. As Gigi said, "That's just as strong as the winds get around here."

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