Page 169 - Fundamentals of Air Pollution

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Effects on the Atmosphere,

Soil, and Water Bodies

I. THE PHYSICS OF VISIBILITY

Impairment of visibility involves degradation of the ability to perceive
the environment. Several factors are involved in determining visibility in
the atmosphere (Fig. 10-1): the optical characteristics of the illumination
source, the viewed targets, the intervening atmosphere, and the characteris-
tics of the observer's eyesight (1).
In order to see an object, an observer must be able to detect the contrast
between the object and its surroundings. If this contrast decreases, it is more
difficult to observe the object. In the atmosphere, visibility can decrease for
a number of reasons. For example, we may be farther away from the object
(e.g., an airplane can move away from us); the sun's angle may change
with the time of day; and if air pollution increases, the contrast may de-
crease, reducing our ability to see the object.
Objects close to us are easily perceived, but as we attempt to detect
objects farther and farther away from us, the contrast between the object
and the background decreases. The lowest limit of contrast for human
observers is called the threshold contrast and is important because this value
influences the maximum distance at which we can see various objects.

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