SEATTLE — Smoke from wildfires clogged the sky across the U.S. 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 American West 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.