Houses sit above a smog-filled Salt Lake Valley as seen from Traverse Ridge in Draper
Houses sit above a smog filled Salt Lake Valley as seen from Traverse Ridge in Draper, Utah, U.S. December 12, 2017. Sometimes during the winter, temperature inversions form in Salt Lake Valley, when the upper air temperature is warmer than the air on the valley floor, forming a lid, trapping polluted air. REUTERS/George Frey