SEATTLE – When Cliff Mass flew in from New York last week, he couldn’t believe how hazy the Seattle sky looked.

“As I descended down into Sea-Tac, I said, ‘What the hell is going on here?’ ” he said.

The unlikely culprit: Smoke from dozens of wildfires burning in Siberia, which has climbed into the jet stream in recent weeks and found its way to the Pacific Northwest. Mass, a professor of atmospheric sciences at the University of Washington, said the smoke — which is not a health risk — also may be the reason behind the recent series of spectacular sunsets.

The smoke’s long journey isn’t as strange as it might seem.

“We always see some level of smoke and pollution” from East Asia, said Dan Jaffe, a professor of atmospheric and environmental chemistry at UW-Bothell. It’s especially common in the springtime.

But the smoke that has drifted here in the last two weeks is the worst since 2003, Jaffe said, fed by huge fires burning near Lake Baikal and on the Kamchatka Peninsula on Russia’s east coast. According to RIA Novosti, a Russian state news agency, 67 wildfires were burning in Eastern Russia into last week.

The wildfires have burned hot enough to lift the smoke high into the air, said Lyatt Jaegle, a UW professor who has studied how pollutants travel long distances. Once it is high enough, the smoke gets caught in the jet stream, which carries it across the ocean to the Pacific Northwest. The high pressure of the past week in Washington state has helped bring the smoke to the ground.

The whole trip takes about seven to 10 days, said Eric Taylor, an air-quality meteorologist with the Ministry of Environment in British Columbia, which has also seen smoky skies recently.

The course of the jet stream makes the Pacific Northwest the front line for East Asian pollution on the West Coast. Meteorologists have observed the smoke of the last two weeks from British Columbia down to Oregon, where Jaffe has monitored it from his research station on Mount Bachelor, near Bend.

Smoke, dust and other pollutants from across the Pacific don’t usually make it too far inland. “A lot of this stuff is taken out by the Cascades and the Rockies,” Mass said.

But not all of it. While the haze in British Columbia has mostly affected the coastal area near Vancouver, some pollutants have made it across the mountains into the province’s interior.

“It seems that this smoke bloom from Siberia also carried with it a lot of ozone,” Taylor said.

Monitors on top of Whistler Mountain have detected record levels of ozone in the last two weeks. And the Ministry of Environment has issued air-quality warnings for the area of the interior near Kelowna based on levels of ozone “for the first time in memory,” Taylor said.

Low-pressure systems and rainy days would help combat the smoke, but it could return as long as the fires continue to rage.