Hundreds of wildfires have churned across western Canada in recent weeks, and southbound jet streams have pushed fire particles — Professor Denning called them “little pieces of crud, soot and ash” — into the United States, causing health departments around the country to issue air quality advisories. In eastern Colorado, a dark fog spread over the typically sunny region this week, caused by a combination of wildfire smoke and unusually cloudy weather. On Monday, the state health department issued a health advisory, urging people to stay indoors or limit outdoor activity if they saw thick smoke; the advisory was not lifted until Thursday morning