SEATTLE -- When Cliff Mass flew in from New York in early July, 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?'" said the University of Washington professor of atmospheric sciences.
The likely culprit: Smoke from dozens of wildfires in Siberia, which has climbed into the jet stream in recent weeks and found its way to the Pacific Northwest. Mass said the smoke -- which is not a health risk -- may explain spectacular recent sunsets.
"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.
But this month's smoke is the worst since 2003, he said, fed by huge fires near Lake Baikal and on the Kamchatka Peninsula on Russia's east coast. RIA Novosti, a Russian state news agency, recently reported 67 wildfires in Eastern Russia.
The wildfires have burned hot enough to lift the smoke high into the air, said Lyatt Jaeglé, a UW professor who has studied how pollutants travel long distances. The jet stream then carries the smoke across the ocean to the Pacific Northwest. The recent high pressure in Washington helped pull the smoke to the ground.
The whole trip takes a week to 10 days, said Eric Taylor, an air-quality meteorologist at British Columbia's Ministry of Environment.
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 coast, some pollutants made it across the mountains into the province's interior.
Monitors atop Whistler Mountain have detected record levels of ozone in the last two weeks. And the Ministry of Environment issued air-quality warnings for the area of the interior near Kelowna based on levels of ozone -- a respiratory irritant -- "for the first time in memory," Taylor said.
Air quality has remained normal in the Seattle area, said Phil Schwartzendruber, an air-resources specialist with the Puget Sound Clean Air Agency.
Fireworks in some Seattle neighborhoods hurt the air quality more than the smoke has, he said.
Low-pressure systems and rainy days would help combat the smoke, but it could return as long as the fires continue to rage. And Russian firefighters are not doing much to put them out.
"If it stays hot and dry in that area," Jaffe said, "it could burn all summer."