Visibility, Atmospheric

the possibility of visually distinguishing remote objects, separated from the observer by a layer of air of varying degrees of turbidity. The distinction is made between daylight, twilight, and night atmospheric visibility. Night atmospheric visibility is further divided into visibility of luminous and non-self-luminous objects. Atmospheric visibility is characterized by range of visibility, that is, the distance at which the object under observation (building, mountain, and so forth) can no longer be seen by the naked eye (on the condition that the object is large enough and is not below the horizon, so that its visibility depends only on the turbidity of the atmospheric air). Atmospheric visibility is affected primarily by the particles of dust and tiny droplets of water and ice crystals in the air which disperse the light and reduce the range of visibility to several km and sometimes to only a few dozen meters (in fog, precipitation, blizzards, dust storms).

The principal role in the atmospheric visibility of non-self-luminous objects is played by contrast (C) between the visible brightnesses of the object and the background. This is expressed by the formula C = ΔV/V, where ΔV is the difference between the visible brightnesses of the object and the background and V is the visible brightness of the background. The smallest magnitude of C at which the eye can distinguish an object is called the threshold of the eye’s contrast sensitivity ε. When an object is seen from an angle α of more than 1° in daylight, then ε has little relation to the angular dimensions of the object and the lighting.

The visibility of light sources, which can usually be regarded as a point source, is determined by the illumination they provide at the observation site.

The study of atmospheric visibility is of great practical importance for all kinds of transportation (visibility of road signs, landing fields, and so forth), for topography (visibility of triangulation signs), for aerial photography, and in military affairs. Fluctuations in atmospheric visibility are closely related to the weather. For this reason meteorological stations conduct systematic observations of the atmospheric visibility and give its magnitude in weather forecasts.

Better atmospheric visibility of objects of small angular dimensions can be attained through the use of binoculars. When there is no fog or haze, atmospheric visibility can be improved by looking through red tinted glass, which does not permit the passage of the violet rays of the air haze, or by photographing the distance in infrared rays. It is possible to discover and photograph objects in complete darkness and in thick fog and clouds by means of radar.