Skip to main content

Visibility

Visibility


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 .

See also Arctic haze; Photochemical smog; Smog

[James P. Lodge Jr. ]


RESOURCES

BOOKS

Middleton, W. E. K. Vision Through the Atmosphere. Toronto: University of Toronto Press, 1952.

PERIODICALS

Lodge Jr., J. P., et al. "Non-Health Effects of Airborne Particulate Matter." Atmospheric Environment 15 (1981): 449458.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Visibility." Environmental Encyclopedia. . Encyclopedia.com. 14 Aug. 2018 <http://www.encyclopedia.com>.

"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

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.