Was the unusual spate of wildfire smoke over the eastern U.S. primarily the result of a rapid, inexorable climate trend? Or were other, more immediate factors involved? While the role of climate change can’t be ruled out, the surge of Canadian smoke appears to be primarily attributable to a unique combination of factors, a kind of perfect storm for producing an emergency to the south.

First, a rare weather event spawned an enormous number of fires almost simultaneously. On June 1, a wave of thunderstorms swept across Quebec from the northwest. A geostationary satellite operated by the National Oceanic and Atmospheric Administration (NOAA) caught the flashes of thousands of lightning strikes moving in an arc across the province. The timber was exceptionally dry, so many of these strikes started tiny blazes. Under normal conditions, most of these—many involving perhaps a single tree—would have petered out on their own. But the next day came an untimely shift in the winds: a high pressure ridge over the Atlantic pushed an air mass across Quebec from the northeast. Satellite images show dozens of smoke plumes erupting almost at once as the strong winds literally fanned the flames.