SALT LAKE CITY (AP) — Two dozen flights had to be diverted from the Salt Lake City airport Wednesday due to low visibility that meteorologists said was caused by a mix of fog and air pollution.

The low visibility forced the airplanes to instead go to surrounding airports in other parts of Utah and in Colorado and Idaho around 10 a.m. Wednesday, said Salt Lake City International Airport spokeswoman Nancy Volmer. By mid-afternoon, visibility had improved but flight delays of up to one hour remained, she said.

The murky air was caused by fog that became trapped under a thick layer of air pollution called an inversion that has been hovering over the Salt Lake City valley for several days, said National Weather Service meteorologist Mike Conger.

Inversions commonly hover over Salt Lake City in the winter as cold, stagnant air settles in the bowl-shaped mountain basins, trapping automotive and other emissions that have no way of escaping to create a brown, murky haze. The air is often clear and the skies blue in the mountain city of Park City, which is only about 25 miles (40 kilometers) east of Salt Lake City but sits at a much higher elevation above the inversion.