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With the Clean Air Act of 1977, Congress established a national goal of remedying existing and preventing future, human-caused visibility impairment in the larger Wilderness Areas, National Parks, and National Wildlife Refuges that existed at the time. Air pollution impairs visibility to some degree on all federal lands. The visual range within the eastern U.S. is often just 15 to 30 miles, estimated at one-third of what it would be without human caused air pollution. In the West, the visual range averages between 60 and 90 miles, or about one-half of the visual range under natural conditions. Find out more about visibility in each Forest Service wilderness. Find out more about visibility in each Forest Service wilderness.



Photographs taken from a vista on a clear day compared to a hazy day.

Haze is caused by fine particles in the air that scatter and absorb light. When the amount of fine particles increases, more light is absorbed and/or scattered, resulting in a shorter visual range, less clarity and altered color. Five types of fine particles contribute to haze: sulfates, nitrates, organic carbon, elemental carbon, and crustal (soil) material. The relative impact of each type of particle varies across the U.S. and from season to season.

Plume Blight

plume blight

A photo showing plume blight effects on a vista.

Plume blight occurs when a point source such as a smoke stack emits particulate matter or nitrogen dioxide into a stable atmosphere. These pollutants can form a thin, dark, coherent plume obscuring the view. This picture captures a classic example of plume blight. Blight occurs before the plume has been dispersed so widely that it is indistinct from the background. Both contrast and coloration may vary depending upon the plume constituents, the viewing background, the viewer angle, and the angle of the sun.

Monitoring Visibility


An example of an IMPROVE site co-located with other monitoring equipment. Photo courtesy of Andrea Holland-Sears.

The Forest Service conducts visibility monitoring as one of the cooperators in Interagency Monitoring of Protected Visual Environments (IMPROVE) Program). The particulates that contribute to haze are collected on filters at each IMPROVE site. Samples are then measured to determine how visibility is impacted over time and by which pollutants. In addition, the Forest Service monitors many sites the using cameras provide online images to interested viewers.

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