Fundamental Processes in Visibility

If an observer is to see an object, light from that object must reach the observer's eye. The perceived visual character of the image depends on the light emitted from or reflected by the object and on the subsequent interaction of that light with the atmosphere. When an observer views a distant object, the light reaching the observer is weakened by two processes: absorption of energy or scattering by gases or particles in the atmosphere. These two processes are referred to collectively as extinction and are depicted in Figure 4-1.

Transmitted light is not the primary factor that determines visibility. The visibility of a distant object also is affected by light from extraneous sources (e.g., sun, sky, and ground) that is scattered toward the observer by the atmosphere (Figure 4-1). This extraneous light is referred to as air light. The air light behind an object provides backlighting and causes the object to stand out in silhouette (iv in Figure 4-1); the air light between the observer and an object tends to reduce the contrast of the object and to mute its colors (v in Figure 4-1).

Air light can be an important element of a view; it can have a positive as well as a negative effect on perception. The appearance of the daytime sky is due the scattering of sunlight by gases and particles in the atmosphere. If there were no scattering (or if there were no atmosphere), the daytime sky would be black, allowing the stars to be seen during the day. Air light also provides diffuse light to the surface below; without air light, objects viewed on Earth would have the deep shadow effects seen in photographs of the Moon.

Haze affects the quality and quantity of air light because absorption and scattering are wavelength dependent. That dependence accounts for the deep blue color of the sky in pristine areas, as well as the gray color of smog. Air light is proportional to extinction and, like extinction, depends on particle concentrations. Unlike extinction, air light also depends on viewing angle; particles scatter preferentially in forward directions, so that haze tends to appear brighter in the direction of the sun.

The extinction coefficient, bext, is a key measure of atmospheric trans-

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