Logo of the US Forest Service, Caring for the Land and Serving the People Logo of the US Forest Service, Caring for the Land and Serving the People

Air Resource Management Home
Program Spotlight
Wilderness Air Quality
Who We Are
Our Partners
Monitoring and Data
Air Pollution
Law and Policy
Manager's Portal

USDA Forest Service
1400 Independence Avenue SW
Washington, D.C.
(202) 205-8333

Egov: My Government. My Terms -- The President's E-government Intitatives.

USA dot Gov: The U.S. Government's Official Web Portal.


Visibility Terminology

Visibility-Related Terms
Extinction The attenuation of light due to scattering and absorption as it passes through a medium. In the case of visibility, attenuation or extinction refers to the loss of image-forming light as it passes from an object to the observer.
Deciview The unit of measurement of haze, or "haze index". Deciview is a measure of visibility derived from light extinction that is designed so that incremental changes in the haze index correspond to uniform incremental changes in visual perception, across the entire range of conditions from pristine to highly impaired. The haze index [in units of deciviews (dv)] is calculated directly from the total light extinction [bext expressed in inverse megameters (Mm-1)] as follows:

HI = 10 ln (bext/10)

The haze index will be less than 0 for bext values below 10.
Natural conditions Prehistoric and pristine atmospheric states, i. e., atmospheric conditions that are not affected by human activities.
Rayleigh scattering Light scattering by the natural gases in the atmosphere. At an elevation of 1500 meters (5000'), the light extinction from Rayleigh scattering is approximately 10 inverse megameters (Mm-1).
Current Conditions Refers to contemporary, or modern, atmospheric conditions that are affected by human activity.
Standard visual range Reciprocal of the extinction coefficient. The distance under daylight and uniform lighting conditions at which the apparent contrast between a specified target and its background becomes just equal to the threshold contrast of an observer, assumed to be 0.02.