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atmospheric visibility [1 record]

Record 1 2004-11-08

English

Subject field(s)
  • Atmospheric Physics
  • Air Pollution
DEF

The greatest distance in a given direction at which it is just possible to see and identify with the unaided eye: a) in the daytime a prominent dark object against the sky at the horizon, and b) at night, a known, preferably unfocused, moderately intense light source.

CONT

Atmospheric visibility is basically a measure of how clear the air is. It is often defined in terms of a visual range (how far an observer can see) and is expressed in nautical miles, miles or kilometers. Precipitation, clouds, and air pollution can reduce atmospheric visibility by increasing the number of small particles in the air that absorb and scatter light.

CONT

... pollutants in the atmosphere can degrade visibility, especially near towns and cities. Usually visibility is affected by natural [meteorological] factors such as fog, sea salt particles or low cloud, but sometimes pollution from human activities decreases visibility by causing haze and smog. Sources of this pollution include emissions from vehicles, pollution from home fires and industrial activities.

OBS

Visibility is quantified in terms of visual range and light extinction, or the maximum distance at which a black object of sufficient size can be seen and recognised in normal daylight.

French

Domaine(s)
  • Physique de l'atmosphère
  • Pollution de l'air
DEF

Plus grande distance à laquelle un objet noir suffisamment gros peut être vu et identifié contre le ciel ou un fond de brouillard.

CONT

Certains phénomènes peuvent réduire la visibilité dans l'atmosphère, par exemple le brouillard, la brume, la brume sèche, la fumée, la neige, la pluie modérée ou forte[, la pollution atmosphérique] et les embruns.

OBS

Dans le cas des observations de nuit, il s'agit de la distance à laquelle cet objet pourrait être vu et identifié si l'illumination générale atteignait l'intensité normale de la lumière du jour. On doit souligner qu'il ne suffit pas d'apercevoir l'objet, mais qu'il faut pouvoir l'identifier.

Spanish

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