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What is "visibility"?
"Visibility" is a measure of how the air looks. 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 judgment.
What causes poor visibility?
Along the Front Range, visibility impairment is caused primarily by fine
particles (0.1 - 2.5 microns in diameter). Particles this size either scatter or
absorb light coming from an 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. Visibility is
also degraded by secondary aerosols, which are tiny gas and/or liquid droplets
that are formed by chemical reactions between sulfate or nitrate and ammonia.
Ground level ozone, which contributes to haziness in high concentrations, is
formed when nitrogen oxides and volatile organic carbons (from motor vehicles
and industry) combine with sunlight.
How does visibility affect humans?
The visual quality of the air affects human welfare. Consequently, loss in
visual quality may result in economic loss as the area becomes less attractive
to residents, potential newcomers and industries. There is increasing
information that shows a link between respiratory illness and fine particles,
which also contribute to visibility impairment.
Are there standards for visibility?
Yes, the State of Colorado has enacted a visibility standard based upon the
visual preferences of Denver residents. However, the standard applies to all
communities along the Front Range that are part of the AIR (Automobile
Inspection and Readjustment) area. The visibility standard is .076 per kilometer
of atmospheric extinction, which means that 7.6% of light in a kilometer of air
is blocked. The standard applies between the hours of 8:00 a.m. and 4:00 p.m.,
and only when the relative humidity is less than 70 percent.
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