US Forest Service Federal Land Managers Visibility Overview

forest scene


Congress recognized visibility, defined as "the appearance of scenic features when viewed from a distance," as a resource to be valued and preserved. Specifically, Section 169A of the 1977 amendments to the Clean Air Act required the Environmental Protection Agency (EPA) to promulgate regulations to assure reasonable progress toward the congressionally- declared National Goal of "the prevention of any future, and the remedying of any existing, impairment of visibility in mandatory Class I federal areas, which impairment results from man-made air pollution." The intent of Congress to protect visibility was further strengthened in Section 169B of the 1990 Clean Air Act Amendments. The EPA has promulgated regulations to ensure that ongoing efforts are made to protect visual air quality.

To effectively track progress toward meeting the National Goal, numerous federal, state, tribal and local visibility monitoring sites and monitoring programs have been established.


In 1978, the Forest Service Air Monitoring Program was established to protect all Forest Service managed lands from the adverse effects of air pollution. In 1988, the Forest Service became a primary participant in the national visibility monitoring program: the Interagency Monitoring of Protected Visual Environments (IMPROVE). The objectives of the Program are to:

Establish current visibility and aerosol conditions in mandatory Class I areas.
Identify chemical species and emission sources responsible for existing human-made visibility impairment.
Document long-term trends for assessing progress towards the national visibility goals.
With the enactment of the Regional Haze Rule, to provide regional haze monitoring representing all visibility-protected federal Class I areas where practical.

Data collected at these sites are used by land managers, industry planners and air quality regulators to understand and protect the visual air quality resources in Class I areas. Most importantly, the IMPROVE Program scientifically documents for American citizens the visual air quality of their wilderness areas and national parks.


A variety of monitoring techniques exist to document visibility conditions and to make quantitative measurements of the atmospheric properties that effect visibility. The IMPROVE Program has partitioned visibility-related characteristics and measurements into three groups:

Aerosol: the physical properties of the ambient atmospheric aerosols (chemical composition, size, shape, concentration, temporal and spatial distribution, and other physical properties) through which a scene is viewed. Fine particle measurements are commonly made to quantify aerosol characteristics.

Optical: the ability of the atmosphere to scatter or absorb light passing through it. Extinction, scattering and absorption coefficients, as well as angular dependence of the scattering (known as the scattering phase function) describe the physical properties of the atmosphere. Optical characteristics integrate the effects of atmospheric aerosols and gases. Commonly applied optical monitoring instruments include transmissometers and nephelometers.

Scene: the appearance of a scene viewed through the atmosphere. Scene characteristics include observer visual range, scene contrast, color, texture, clarity and other descriptive terms. Scene characteristics change with illumination and atmospheric composition. Photographs, video and digital images are effective ways to document scene characteristics.