Heat is on - vacation

On a gentle afternoon that typified this benign summer of 2009, Gary Lindstrom fully appreciated that he was a very lucky man.

"It's gorgeous," Lindstrom said, savoring fountain droplets that sprayed at LOVE Park in Center City, under a tolerable sun, a milky blue sky, and temperatures that were almost shockingly pleasant for the first day of August.

Lindstrom and his family have a particularly advantageous perspective on what an extraordinary summer this has been across the nation - one that might well pass without a single significant heat wave in Philadelphia and the entire Northeast.

They have been visiting the last two weeks, fortuitously missing the historic hot spell that charbroiled the folks back home in Tacoma, Wash.

On Wednesday, the temperature hit 103 at the government's Seattle-Tacoma airport measuring station. That wasn't just a high for the date; it was the hottest day, period, in records dating to 1891.

And, as Lindstrom noted, somewhat sympathetically, "Very few people have air-conditioning."

Meanwhile, on the East Coast, where the hum of compressors often rivals the roar of the Atlantic, Philadelphians, New Yorkers, and Bostonians await their first heat wave of the summer. Right now it's looking more and more as though they'll be waiting until 2010.

What's going on?

Much of the northern United States has been under the cooling influence of an upper-level trough, or area of lower pressure, said Todd Miner, a Pennsylvania State University meteorologist. The trough, borne on upper-air jet-stream winds, has been draped across the nation as though suspended from spires of higher pressure and warmer air over the Atlantic and Pacific Oceans.

The result has been steep temperature anomalies, with a broad band of chill bordering an arc of warmth from the Pacific Northwest to the Southeast.

Out in the land of coffee bars, Seattle finished July a full 4.2 degrees above normal. "The iced coffees were the way to go," Dennis D'Amico, a transplanted Easterner who is a meteorologist at the National Weather Service office in Seattle, said last week. "They were selling like hotcakes out here."

By contrast, St. Louis, usually a summer pressure cooker, was 4.6 degrees below normal in July. Boston was 3.4 below in July and New York 3.8. In fact, Fairbanks, Alaska, had more 90-plus readings in June and July (one) than New York City (zero).

Philadelphia was a more modest 1.7 degrees below normal for July, but taken together the last two months were notable outliers in a run of very warm summers that took hold in 1986. This was the coolest June-July since 2000 and the second-coolest since '86.

Right now the odds are stacked firmly against any major heat surge, Miner said. July's weather is a decent indicator of August.

That's because the human mind isn't the only thing that tends to slow down in summer. The languid atmosphere is prone to fall into ruts in late summer as weather systems shift into neutral. With the sun's energy spread more evenly across the Northern Hemisphere, the temperature contrasts that drive the movement of fronts weaken.

In short, what happens tends to keep happening. "That's why July and August are so well correlated," Miner said.

The longer-range outlooks buttress that analysis. "It doesn't look as though there'll be any sustained heat in the Northeast in the next few weeks," Miner said. From then on, time becomes the enemy of sustained heat as days grow even shorter and nights longer.

WSI Corp., a private service in Massachusetts that provides forecasts to energy interests, is calling for below-normal temperatures in August throughout the Northeast.

Not that it won't be warm. It probably will hit 90 or better once or twice this week, and the nights will be less pleasant than in June.

In the last several days, the air has been heavy with water vapor more often than not as the high-pressure area over the Atlantic has tried to nudge westward, and that is helping to keep the nights warm. Air circulates clockwise around the centers of high pressure, so areas to the west experience warm, moist winds from the south.

Out over the Atlantic, it's been steamy. "If you could take the whole Eastern seaboard and shift it 500 miles it would be ungodly hot and humid," Miner said.

We know the feeling. The 1990s was the decade of the 90s. On average, the temperature hit 90 or above 29 times a year. This year, it has happened only eight times - and three were back in April.

All that said, attuned weather observers shouldn't find this summer's weather all that surprising, Miner said. And people in Lindstrom's part of the country probably would agree that it doesn't necessarily spell the end of global warming.

"I hesitate to call it unusual," Miner said. "It is usual for the atmosphere to be cyclical. What would be unusual would be if every year were normal."