Visibility generally refers to the quality of vision through the atmosphere . Technically, this term denotes the greatest distance in a given direction that an observer can just see a prominent dark object against the sky at the horizon with the naked eye. Visibility is a measure commonly used in the observance of weather in the United States. In this context, surface visibility refers to visibility determined from a point on the ground, control tower visibility refers to visibility observed from an airport control tower, and vertical visibility refers to the distance that can be seen vertically into a ground-obscuring medium such as fog or snow.
Visibility is affected by the presence of aerosols or haze in the atmosphere. In the ideal case of a black target on a white background, the target can be seen at close range because it reflects no light to the eye, while the background reflects a great deal of light to the eye. As the distance between the observer and the target object increases, the light from the white background is scattered by the intervening particles, blurring to some degree the edge between the target and the surroundings, since light from the background is now scattered into the line of sight to the black target. In addition, the whiteness of the background is decreased by the scattering of light out of the direct line from the background to the eye, and by the absorption of some of the light by dark particles, principally carbon (soot). Finally, particles in the line between the target and the eye scatter light to the eye, decreasing the blackness of the target as seen from the distance. At any distance greater than the visibility, then, all these actions degrade the contrast between the object and its background so that the eye can no longer pick out the object.
Some light is scattered even by air molecules, so visibility through the atmosphere is never infinite. It can be great enough to require correction for the curvature of the earth, and the consequent fact that atmospheric density is less (fewer scattering molecules per unit of distance) at the end of the sight path than at the location of the observer.
Air pollutants now produce a haze that circles the globe, significantly reducing visibility even in places as remote as the Arctic. In areas of industrial concentration, haze may consistently reduce visibility by as much as 40%. In addition, gaseous molecules scatter light; and some gases, most notably nitrogen dioxide, absorb light in the visible range, thus changing the apparent color of the background. The effects of air pollution on visibility are quite well understood, unlike some other effects of this pollution on human health and the environment .
[James P. Lodge Jr. ]
Middleton, W. E. K. Vision Through the Atmosphere. Toronto: University of Toronto Press, 1952.
Lodge Jr., J. P., et al. "Non-Health Effects of Airborne Particulate Matter." Atmospheric Environment 15 (1981): 449–458.
"Visibility." Environmental Encyclopedia. . Encyclopedia.com. (August 14, 2018). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/visibility
"Visibility." Environmental Encyclopedia. . Retrieved August 14, 2018 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/visibility