Visibility Effects of Air Pollution

Visibility is one of the primary air-related attributes that people associate with national parks. Many visitors come to parks to enjoy the spectacular vistas.

Unfortunately, these vistas are sometimes obscured by air pollutants, especially fine particles in the atmosphere. The National Park Service monitors the visibility conditions in park system units, investigates the causes of visibility impairment, and works cooperatively with air regulatory agencies to remedy the impairment.

Grand Canyon NP, Arizona visual range 390 km

Grand Canyon NP, Arizona visual range 40 km

Visibility is affected by the physical interaction of light with particles and gases in the atmosphere. The physics of this interaction can be described mathematically, however, visibility involves more than specifying how light is absorbed and scattered by the atmosphere. Visibility is a psychophysical process of perceiving the environment through the use of the eye-brain system.

Important factors involved in seeing a scenic vista are outlined in the following figure. Image-forming information from an object is reduced (scattered and absorbed) as it passes through the atmosphere to the human observer. Air light is also added to the sight path by scattering processes. Sunlight, light from clouds, and ground-reflected light all impinge on, and scatter from, particulates located in the sight path. Some of this scattered light remains in the sight path, and at times it can become so bright that the image essentially disappears. A final important factor in seeing and appreciating a scenic vista are the characteristics of the human observer.

Diagram illustrating how light is scattered and reflected.
A more complete description of visibility science is available in PDF format: Introduction to Visibility (3mb)

Types of Visibility Impairment

Air pollution can manifest itself either as layered or uniform haze (see illustration below). Layered haze can be thought of as any confined layer of pollutants that results in a visible discontinuity between that layer and its background (sky or landscape). The classic example of a layered haze is a tight, vertically constrained, coherent plume (plume blight). However, as the atmosphere moves from a stable to unstable condition and a plume mixes with the surrounding atmosphere, the plume impact on visual air quality may manifest itself in an overall reduction in air clarity (uniform haze) rather than as a layer of haze. Uniform haze exhibits itself as an overall reduction in air clarity.

Types of Visibility Impairment

Importance of Visual Air Quality to Visitor Experience

The National Park Service has conducted studies examining the relationship between visibility conditions and visitor experience as well as visitor attitudes toward clean, clear air in national parks. These studies have found that:

• Visitors are aware of visibility conditions. Interviews of visitors indicated that they rated the visibility worse when the measured visibility was worse, and better when measured visibility was better.
• An undisturbed environment, including clean, clear, air, is very important to park visitors. The studies found that people visit parks to experience a natural setting and secondarily to enjoy specific unique features associated with various parks.
• Visibility influences visitor enjoyment and satisfaction. When visitors indicated that the view was very to extremely hazy, they enjoyed the view less, enjoyed the park less, and were less satisfied with visibility conditions than those who said they were not aware of haze or were aware of only slight to moderate haze.
• Visibility conditions affect the amount of time and money visitors are willing to spend at NPS units. Interviews of visitors indicated that they would be willing to spend more time and money if visibility conditions were better and less if visibility conditions were worse.

List of Visibility Impairment Photo Images

• Acadia National Park, Maine
• Badlands National Park, South Dakota
• Bandelier National Monument, New Mexico
• Big Bend National Park, Texas
• Bryce Canyon National Park, Utah
• Canyonlands National Park, Utah
• Chiricahua National Monument-Hatchett Peak, Arizona
• Chiricahua National Monument-Mica Mountain Vista, Arizona
• Crater Lake National Park-Mount Thielsen Vista, Oregon
• Crater Lake National Park-Palisade Point Park, Oregon
• Denali National Park, Alaska
• Everglades National Park, Florida
• Glacier National Park, Montana
• Grand Canyon National Park, Arizona
• Great Basin National Park, Nevada
• Great Sand Dunes National Monument, Colorado
• Great Smoky Mountains National Park, Tennessee
• Guadalupe Mountains National Park, Texas
• Haleakala National Park, Hawaii
• Hawaii Volcanoes National Park, Hawaii
• Lassen Volcanic National Park, California
• Mammoth Cave National Park, Kentucky
• Mesa Verde National Park, Colorado
• Mount Rainier National Park, Washington
• Petrified Forest National Park, Arizona
• Pinnacles National Monument, California
• Point Reyes National Seashore, California
• Redwood National Park, California
• Rocky Mountain National Park, Colorado
• Shenandoah National Park, Virginia
• Tonto National Monument, Arizona
• Voyageurs National Park, Minnesota
• Yellowstone National Park, Wyoming
• Yosemite National Park, California