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Visibility of Lights, Continued
Terms
The following is a list of terms associated with light visibility
computations:
Horizon distance - This is the distance expressed in nautical miles from
the position above the surface of Earth along a line of sight to the
horizon; the line along which Earth and sky appear to meet. The higher
the position, the farther the horizon distance will be. Figure 4-3 shows
the relationship of height of eye to horizon distance.
Meteorological visibility - Meteorological visibility results primarily
from the amount of particulate matter and water vapor present in the
atmosphere at the location of an observer. It denotes the range at which
the unaided human eye can see an unlighted object by day in a given set
of meteorological conditions.
Luminous range - Luminous range is the maximum distance at which a
light may be seen under the existing meteorological visibility conditions.
Luminous range does not take into account the height of the light, the
observer’s height of eye, or the curvature of Earth. It depends only on
the intensity of the light itself.
Nominal range - Nominal range is the maximum distance a light can be
seen in clear weather (meteorological visibility of 10 nautical miles).
Nominal range is similar to luminous range in that it does not take into
account elevation, height of eye, or curvature of Earth, but it depends on
the intensity of the light. Nominal range is listed in column 6 of the
Light List for all lighted aids to navigation except range lights,
directional lights, and private aids to navigation.
Geographic range - Geographic range is the maximum distance at
which a light may be seen in perfect visibility by an observer whose eye
is at sea level.
Computed range - Computed range is the geographic range plus the
observer’s distance to the horizon based on the observer’s height of eye.
Computed visibility - Computed visibility is the visibility determined
for a light using the light’s height, nominal range, and height of eye of
the observer.
4-10
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