Visibility Terminology - Forest Service Air Resource Management Program

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 (b_ext/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.