With air quality and visibility worsening over the years, scientists have developed several different ways to measure and describe visibility. These measures allow them to compare, quantitatively, the varying degrees of visibility for a location. Some of the accepted measurements of visibility that we will discuss are the extinction coefficient, visual range, deciview, and cumulative frequency.
Extinction Coefficient (bext)
Our eyes perceive objects as images of color due to the light that is reflected by objects into our line of vision. The extinction coefficient or bext value is a measurement of the amount of image forming light that is either scattered out of our view or absorbed by the aerosols present in the atmosphere.
Recall from the discussion on Scattering Light, that Rayleigh scattering is the selective scattering of the shorter wavelengths of the visible spectrum by the gases and smallest particles in the atmosphere. This creates the appearance of the blue sky and red sunsets. Also, recall that Mie scattering is the reflection of all wavelengths of visible light. Scientists have defined a Rayleigh atmosphere as an imaginary "pure" atmosphere composed only of gases. A Rayleigh atmosphere would be completely void of any Mie scattering. A Rayleigh atmosphere would have an extinction coeffecient or a bext value of 0.01/km. As visibility decreases as a result of the addition of aerosols, the bext value increases.
Visual Range (vr)
The visual range is an idealized measure of how far you can see through the atmosphere. The visual range measured may not be the actual distance you can see. It measures the distance you could see an absolutely black target given that all other atmospheric conditions were ideal, with the exception of the concentration of pollutants. The visual range is calculated by dividing 3.912 by the bext value for the same set of conditions.
The deciview is scaled such that a change of one deciview represents a constant change in visibility. A Rayleigh atmosphere has a deciview value of zero and increases one unit with each noticeable change in visibility. This measurement is analogous to the decibel which is a measurement of sound. One decibel represents a noticeable change in sound.
Cumulative Frequency
The cumulative frequency indicates how often a particular level of visual air quality occurs. On a given day, if the visibility that day is better than the average visibility for that location, the cumulative frequency is reported as a percentage of time that level or better occurs. Conversely, if the visibility a given day is worse than the average, the cumulative frequency is reported as a percentage of time that level or worse occurs.
The National Park Service Air Quality Division has 15 slides of the Grand Canyon on-line that demonstrate varying levels of air quality visibility based on these four quantitative descriptions.
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