It can be described as the maximum distance that an object can be perceived against a background sky.
Visibility also can refer to the clarity of objects in the distance, middle or foreground.
Visibility is unique among air pollution effects because it involves human perception and judgement. Particles this size either scatter or absorb light coming from a object. Sulfates, nitrates, and elemental and organic carbon are most effective at scattering or absorbing light. Human-caused sources of these particles include woodburning, emissions from cars, trucks, and buses, soot from burning fields, and electric power generation. The visual quality of the air affects human welfare.