At 6,643 feet, Clingmans Dome is the highest mountain in Great Smokey Mountains National Park and one of the highest peaks in the eastern United States. An observation tower at the summit rises above the treetops for panoramic views, although clouds, frequent rain, cooler temperatures, wind, and thinner air can affect conditions. The paved trail to the tower is steep and extends 1/2 mile, rising 330 feet from the parking area, with a walking time of 30 minutes. Dead trees along the trail were Fraser firs, which grow only at the highest elevations in the Smokies. During the past 30 years, the balsam woolly adelgid, a tiny insect accidentally introduced from Europe, has killed more than 70 percent of the park's mature firs by feeding on tree sap and interfering with the flow of water and nutrients, sometimes killing a fir within a few years. From the tower, the mountains' natural misty blue-gray clouds can be seen, formed by great quantities of evaporating moisture, but an unnatural whitish haze from air pollution, caused chiefly by sulfate particles from coal-fired power plants scattering sunlight, may also limit the view.