SEATTLE — Smoke from wildfires clogged the sky across the US West, blotting out mountains and city skylines from Oregon to Colorado, delaying flights and forcing authorities to tell even healthy adults in the Seattle area to stay indoors.

As large cities dealt with unhealthy air for a second summer in a row, experts warned that it could become more common as the region faces larger and more destructive wildfires because of heat and drought blamed on climate change.

Officials also must prioritize resources during the longer firefighting season, so some blazes may be allowed to burn in unpopulated areas.

Seattle's Space Needle was swathed in haze, and it was impossible to see nearby mountains. Portland, Oregon, residents who were up early saw a blood-red sun shrouded in smoke and huffed their way through another day of polluted air. Portland Public Schools suspended all outdoor sports practices.

Thick smoke in Denver blocked the view of some of Colorado's famous mountains and prompted an air quality health advisory for the northeastern quarter of the state.

The smoky pollution, even in Idaho and Colorado, came from wildfires in British Columbia and the Northwest's Cascade Mountains, clouding a season that many spend outdoors.

Portland resident Zach Simon supervised a group of children in a summer biking camp who paused at a huge water fountain by the Willamette River, where gray, smoky haze obscured a view of Mount Hood.

Simon said he won't let the kids ride as far or take part in as many running games like tag while the air quality is bad.

"I went biking yesterday, and I really felt it in my lungs, and I was really headachy and, like, lethargic," Simon said on Monday. "Today, biking, you can see the whole city in haze and you can't see the skyline."

The latest round of prolonged smoke happened as hot temperatures and high pressure collided, said Andrew Wineke, a spokesman for the state Ecology Department's air quality program.

ASSOCIATED PRESS