A "strong inversion layer" produced some odd temperatures at higher elevations on this week. "It was like spring," said hiker David Reid as he climbed to Mason Lake, at 4,300 feet.

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When David Reid started up the path to Mason Lake from the trailhead at Snoqualmie Pass on Sunday, he was wearing ski gloves, long underwear and a parka.

“It was very cold, below freezing, and with a biting wind,” said the skier and avid outdoorsman. There was enough ice that he and his hiking companion stopped at one point to put on their microspikes for added traction, he said.

But as they continued up the trail to Mason Lake, which lies at 4,300 feet elevation, “it all of a sudden started getting much warmer,” Reid said.

“It was brilliant sunshine and I started sweating; I took off my parka, I took off gloves and by the time we got to the lake,  it was a good 20 degrees warmer and balmy,” he said. “It was like spring.”

What Reid experienced was what meteorologists with the National Weather Service in Seattle described as a temperature inversion, which is pretty rare here.

The inversion is caused by a layer of warm air higher in the atmosphere that traps cooler air close to the surface, meteorologist Johnny Berg explained.

“It was a pretty strong inversion layer, too,” he said about the mass of air that kept temperatures close to freezing in the lowlands of the Seattle region while letting the air near the mountaintops warm from the sun.


At Camp Muir, 10,000 feet up Mount Rainier, the temperature Monday morning was 41 degrees, warmer than the 33 degrees recorded at the same time at Seattle-Tacoma International Airport. At Hurricane Ridge in the Olympics, it was 55 degrees on Monday morning, the weather service reported.

A front that blew into the coast on Monday night weakened the inversion factor — bringing Hurricane Ridge’s temperature back down to 39 degrees, said Dustin Guy, also of the weather service.

“There’s still an inversion but it’s weaker than it was yesterday,” Guy said.

The inversion could strengthen slightly on Wednesday with a chance of fog and a partly cloudy night sky, which could provide good viewing for the Geminid Meteor Showers, Burg said.

By Thursday, Guy said, “the inversion will break down and things will start to get unsettled again” with a chance of rain that evening and rain likely by Friday night and Saturday.