In Salt Lake City, Utah, residents call winter "inversion season" for its tendency to bring on a strange weather condition: Snowstorms invert the usual trend of air getting colder as the altitude gets higher by cooling the ground, which leaves behind a layer of warm air high above the metropolis. An inversion creates a sort of atmospheric roof while the mountains surrounding the city act like walls, trapping pollution in place until another storm blows it away. The resulting "smog dome" lasts four days on average, but some have lingered almost three weeks. These periods aren't just dreary; they're dangerous. Here's a peek into the dome.