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Originally published July 29, 2009 at 12:00 AM | Page modified July 29, 2009 at 11:27 AM

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Could Wednesday be Seattle's hottest day ever?

If temperatures stay at 90 degrees or higher through Saturday, that will add up to six sweltering days in a row. And if Seattle's high reaches 100 degrees today, as forecast, it will tie the city's all-time temperature record.

Seattle Times science reporter

Seattle heat records

Hottest days

100 degrees

July 16, 1941

July 20, 1994

Hot streaks

Five straight days of 90 degrees or more

July 14-18, 1941

Aug 7-11, 1981

Not-so-hot days

In past 65 years,

15 years have had no days over 90 degrees

Most recently in 2005

Breaking 100

The temperature hit 101

in Olympia on Tuesday,

a record for that day

100

90

80

Source: National Weather Service

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How much sweat does it take to set a new heat-wave record?

Sticky Seattleites may find out this week.

If temperatures stay at 90 degrees or higher through Saturday, that will add up to six sweltering days in a row. And if Seattle's high reaches 100 degrees today, as forecast, it will tie the city's all-time temperature record.

The region's previous record hot spells lasted five days, in August 1981 and July 1941.

"We're certainly in the running," said Brad Colman, local meteorologist-in-charge for the National Weather Service.

Crawling may be more like it.

The combination of high temperatures and humidity that lingers through the night has left many folks groggy and lethargic.

"It was particularly unpleasant last night," said University of Washington meteorologist Cliff Mass, who gave up on sleep at 5 a.m. Tuesday to write his daily weather blog.

"This is definitely going to be an historic heat wave," Mass said.

On Tuesday, Olympia hit 101, a record for the day. The high of 93 in Hoquiam shattered the previous record of 81 for the day. The Seattle-Tacoma International Airport airport maxed out at 97 degrees, tying the record for the day.

Severe weather always raises questions about trends, particularly with global climate change bearing down on the planet. But weather experts caution not to read too much into any single episode.

"We can't make too much of this," Mass said. "If we start getting these every other year, that will be a different story."

The current forecast calls for temperatures to peak today then slowly decline. By Saturday, the National Weather Service says temperatures could drop to the high 80s.

Normally, cool marine air quickly dispels heat waves in Western Washington. But an unusually persistent high-pressure ridge that runs all the way to the Canadian Yukon is parked over the region.

"Sometimes the atmosphere gets locked into these patterns, and it just sits and sits," Colman said.

Unusually humid air is adding to a saunalike effect. Not only does it slow the cooling evaporation of sweat, but it also holds heat in at night.

Triple-digit temperatures today could have the effect of actually reducing humidity, Mass said.

"The air might get drier."

The Seattle area averages about three days a year when temperatures reach 90 degrees or higher. This year has already brought four, with another two almost certain.

But the summer of 1941 was equally hot. The mercury hit 100 on July 16 that year. Pavement buckled in the heat, closing bridges. "Several overcome by torridity," proclaimed one headline in The Seattle Times.

The last time temperatures reached 100 degrees at Sea-Tac was July 20, 1994.

Since then, ongoing construction and a new runway have increased heat-trapping pavement at the airport, Mass pointed out. So meteorologists aren't sure how comparable current measurements will be to those from the past.

"If we get to 102 Wednesday, would that have been 100 or 99 if they hadn't mucked around with the airport?" Mass asked.

Less intense heat waves have gripped the region repeatedly. In 1977, temperatures reached 80 degrees or higher on 15 consecutive days. July 1958 brought a total of seven nonconsecutive days when the temperature hit 90 or more.

But don't forget those years when it never got hot. In 15 of the past 65 years, including 2005, not a single day hit the 90-degree mark.

Seattle Times researcher David Turim contributed to this report.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com

Copyright © 2009 The Seattle Times Company

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