From Clingmans Dome, the northern view is often reduced by haze largely caused by air pollution. Although the view centuries ago is unknown, records from the last half of the 20th century show that the average visible distance decreased by about 40 percent in winter and 80 percent in summer. On a typical hazy day, distant features such as Pigeon Forge can disappear completely. In the Smoky Mountains, air pollution comes mostly from the northwest and southwest, and sometimes from the east, arriving from both local and distant sources as weather patterns change over time.