For Phila. region, a midsummer night's chill

True heat has been scarce, actual heat waves utterly absent. But most of us have been sleeping through what might be the most remarkable aspect of the summer of 2009.

It has been amazingly free of uncomfortable nights, interrupting, at least, a national trend of rising nighttime temperatures.

Through July 15, only once did the Philadelphia official thermometer fail to fall below 70. In 136 years of record-keeping, just one other time, in 1963, did temperatures fail to drop below 70 at least twice before July 15.

The lack of overnight heat has been literally a lifesaver; health experts say hot nights pose a greater danger to the elderly and frail than blistering afternoons.

Not that we've had many of those. Philadelphia had more 90-plus days in April (three) than it has had since June 1 (two). And the most impressive number is this: zero, as in zero heat-related deaths in Philadelphia. The city has yet to sound its first heat-warning bell, and the Philadelphia Corporation for the Aging still awaits its first fan giveaway.

Meanwhile, the pleasant sleeping nights have been a boon to the restless and, of course, payers of bills for air-conditioning.

"I haven't been using it a whole lot," said Gary Szatkowski, the meteorologist in charge of the National Weather Service in Mount Holly, echoing a refrain that isn't necessarily music to the ears of your local utility.

Peco Energy will release its quarterly report this morning, and the numbers might not hold a candle to last year's. According to PJM Interconnect, the regional grid operator, Peco's power use in June was down about 15 percent from June 2008, and Public Service Electric & Gas' was down 17 percent.

And that has a whole lot to do with the overnight coolness, Peco spokesman Michael Wood said. During a sequence of hot nights, he said, "there's an exponential increase in power consumption."

So where have the hot nights gone?

In most summers, a strong area of high pressure centered near Bermuda - the so-called Bermuda high - rules. Winds circulate clockwise around high centers, so Philadelphia and areas to the west experience warming winds from the south.

This summer, that high evidently has been upstaged by a general trough, or area of low pressure, over the Northeast, and that has kept things cool. "It's just been very persistent," said Mike Halpert, deputy director of the government's Climate Prediction Center outside Washington.

The result has been a brisk traffic in cool fronts crossing the region, Szatkowski said. Not much heat, by day or night.

While that might explain what has happened, meteorologists acknowledge it doesn't address why. Halpert said it might be tied to broad air-pressure swings between the far northern and middle latitudes, the North Atlantic Oscillation, but he can't be certain.

Whatever is going on out there, the ocean certainly is acting strangely.

Since June, tides have been running above the normal predicted heights along the East Coast - as much as two feet along the Delaware River at Philadelphia - said Mike Szabados, an oceanographer with the National Oceanic and Atmospheric Administration.

NOAA is trying to figure out why. Because the ocean and the overlying atmosphere work in tandem, the tides have something to do with the weather. But just what that is, no one knows.

Higher tides would add to any storm surge if a tropical storm or nor'easter hit the region, Szabados said.

Fortunately, those also have been missing. July rainfall has been less than half of average.

Usually, lack of rain is an enabler for heat, but temperatures are averaging 3 degrees below normal, a symptom of the cooler nights that go against a well-documented trend.

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

In the short term, at least, it appears that the nights are getting a tad toastier around here. It did stay above 70 on the 16th and 17th, and Halpert said some signs point to a relenting of the summer pattern. Showers are in the forecast through Thursday, and it might hit 90 tomorrow.

In most summers, that would be worth hardly a mention.