Soupy fog and smog that permeated the Salt Lake Valley on Sunday caused cancelled flights and bad air.A storm forecast for Monday, however, should help clear the air.The National Weather Service issued a dense fog advisory for most of Northern Utah, stretching from the Nevada border through the Wasatch Mountains.Airlines reported at least 20 cancelled flights Sunday afternoon at the Salt Lake City International Airport, airport spokeswoman Barbara Gann said. More could be cancelled through the evening as colder temperatures create heavier, thicker fog.The fog started forming early Sunday, with dropping temperatures at sunrise filling the Salt Lake Valley and creating wide pockets of fog throughout northern Utah, said Peter Wilensky, lead forecaster for the National Weather Service in Salt Lake City. It spread up through Cache Valley and the Bear River Valley.People need to be cautious while driving because seeing a stopped car in front of you will be difficult,Wilensky said. Drivers should use their low beam headlights and watch for black ice that may form as temperatures drop, leading to slippery overpasses and bridges, he said.A Pacific storm should move in Monday, bringing rain and stirring the air enough to clear the fog and most of the smog, Wilensky said. The conditions should dissipate by Monday afternoon, he said.