Why Is New York Seeing an 'Apocalyptic Skyline', 'Unprecedented Smoke'?

The thick smoke has caused hundreds of flights in the New York area and Philadelphia to be delayed. Schools across the US East Coast canceled outdoor activities, and millions of Americans were urged to stay indoors as smoke blanketed cities with a thick, yellow haze.
But what is behind this sudden spike in air pollution? And for how long is it expected to last? We explain.
Video: The Quint/Twitter
Music: BMG