There's physiological science behind our tolerance for temperature extremes.

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Ask anyone from the Sun Belt, the Deep South or the East Coast, and they’ll tell you Northwesterners are weather wimps.

A few days above 90 degrees, and the moaning gets loud enough to drown out the Blue Angels over Lake Washington.

But science says it’s not our fault.

Humans adapt to hot weather like they adapt to high altitude, said University of Washington biologist Ray Huey.

Rushing straight from sea level to the top of Mount Rainier would leave most people dizzy and sick. A similar thing happens when a region accustomed to moderate temperature is suddenly swamped with sweltering weather like yesterday’s when the temperature at Seattle-Tacoma International Airport topped out at 96 degrees.

Today should be even hotter, with the National Weather Service projecting record-breaking highs in the upper 90s.

“If we had spent a few weeks in Phoenix and came to Seattle during this heat wave, we wouldn’t be impressed by it at all,” said Huey, who used to study lizards in two parts of the world where shade is as valuable as Seattle real estate: the Kalahari Desert and the Big Bend region of Texas.

“Big Bend in the middle of July is stinking hot,” he recalled. “The first day or two you really suffer, then after that you pretty much get used to it.”

Now that he works in an air-conditioned lab investigating how fruit flies adapt to different environments and temperatures, Huey has joined the ranks of the heat-challenged.

“I really notice when it gets this hot,” he said yesterday.

But the heat wave will begin to fizzle out by tomorrow night, denying locals the chance to build up their temperature immunity.

Hot weather rarely persists here, thanks to the cooling proximity of Puget Sound and the Pacific Ocean, said UW meteorologist Cliff Mass.

And that explains why each hot spell is a shock to our tender physiologies.

When the mercury climbs, a body that isn’t used to high heat runs like a poorly tuned car. Sweat glands and other systems are suddenly forced to work harder to accomplish the same tasks — and they don’t like it.

After a week or two of hot-weather exposure, sweat glands become more efficient and effective, said Dr. John O’Kane, sports-medicine professor and head physician for the UW Husky teams.

“You sweat more heavily and your sweat becomes more dilute, so you don’t lose as much in terms of electrolytes,” he said.

Heart rate also drops, and the body produces more plasma, effectively thinning the blood, O’Kane explained.

Part of acclimation may be in the mind as well.

“When you see the same weather day after day, you’re dressed for it, you’re expecting it, it may not feel as hot,” he said.

Spokane native Betsie Rayner, who was soaking up the sun yesterday on a bench next to Lake Union, said she credits her birthplace for endowing her with a high tolerance for heat. “My body just finds its equilibrium,” she said.

Her Seattle-born co-worker Michelle Virgin said her natural affinity is for cool, cloudy weather. “I get kind of anxious when we haven’t had a gray day in a while.”

Being out of shape when it comes to temperature can be dangerous. One study found that the death rate from comparable heat waves was higher in Northern states than in the South.

Much of that difference undoubtedly stems from the fact that homes in Northern states are less likely to have air conditioning, Huey said. But physiological differences almost certainly play a role as well.

O’Kane cautions athletes and anyone else venturing out today to remember that their bodies will be under more stress than usual.

“Nobody here is acclimated well enough to be working out heavily during this weather,” he said.

Could climate change bring the region a more steady diet of hot days in the future and transform us into a more hardy breed?

A UW analysis of the past 80 years found that the average annual temperature in Western Washington has climbed by about two degrees, Mass said.

But the number of really hot days has fluctuated up and down in recent years.

Based on more than 52 years of data, the region gets an average of 25.5 days a year where temperatures are 80 degrees or higher, said Chris Hill, meteorologist in charge for the National Weather Service in Seattle.

Last year, the total was higher than usual at 31days. But 2002, 2001 and 2000 were all below average, with respective totals of 22, six and 17 days of 80-plus temperatures.

The longest heat wave on record was 15 straight days above 80 degrees — in 1977.

Ninety-plus days are much rarer: an average of 2.7 per year. Last year’s total was four, while 2002 brought two super-hot days, and 2001 and 2002 had none.

“People might think it’s getting hotter,” Hill said, “but maybe they just got acclimated to it being cooler than normal.”