Smoke from Canada wildfires has colored the skies. Why the orange hue?

Millions of horrified Americans have watched clear blue skies morph into thick orange fogs in recent days as wildfires in Canada and a stalled low-pressure weather system have sent thick plumes of smoke into the United States.

Tens of millions of Americans were under air quality alerts Thursday, and, in parts of the Northeast, officials urged residents to stay inside and limit or avoid outdoor activities for the third-straight day.

While familiar to many Americans living in the western part of the country, the eerie orange hues have frightened and confounded some in regions of the nation less accustomed to the effects of wildfires.