DEVELOPMENT AND APPLICATIONS OF A STANDARD VISUAL INDEX

A standard visual index appropriate for characterizing visibility through uniform hazes, is defined in terms of either of the traditional metrics: visual range or extinction coefficient. This index was designed to be linear with respect to perceived visual changes over its entire range in a way that is analogous to the decibel scale for sound. Neither visual range nor extinction coefficient have this useful property, which has resulted in their misuse for some applications. By assuming the availability of sensitive scenic targets at every distance, it can be demonstrated that any specific fractional change in extinction coefficient (or visual range) is equally perceptible regardless of baseline visibility conditions. The new index is defined so that its scale, which is expressed in deciview (dnu), is linear with respect to fractional changes in extinction coefficient, b(ext), as 10 ln(b(ext)/0.01 km-1), where extinction coefficient is expressed in km-1. A 1 dnu change is about a 10% change in extinction coefficient, which is a small but perceptible scenic change under many circumstances. Since the deciview scale is near zero for a pristine atmosphere (dnu = 0 for Rayleigh conditions at about 1.8 km elevation) and increases as visibility is degraded, it measures perceived haziness. Use of the deciview scale in visibility perception and benefits research will promote improved study design and more appropriate use of results. It should also allow the visibility consequences of alternative air pollution-related decisions to be more easily presented and evaluated.