Abstract of the
Training Guide in Surface Weather Observation
U.S. National Weather Service
VISIBILITY
Because it affects
pilots approaching or departing your station, visibility is one of
the most important
elements in the METAR observation. Make the visibility report
descriptive of the actual conditions at
your point of observation.
Visibility is the greatest horizontal distance at which selected
objects (visibility markers) can be seen
and identified. At times, however, you will be called upon to
estimate visibilities for which you have
no markers. Base the estimate on the sharpness with which the most
distant markers can be seen. If the
markers can be seen clearly, with little loss of colour, and with
sharp outlines, it means that the visibility
is much greater than the distance to the markers.
3.3 Visibility Chart
As an aid for determining visibility around the station, all
stations are required to have a visibility chart.
And to be most useful, this chart should be posted near the point
from which you observe visibility. This
chart should list or otherwise indicate the location of all
visibility markers, their distance from the
station, and whether they are daytime or nighttime markers. It is
also very important that this visibility
chart be kept current. An example of the type of visibility chart
used is shown in Exhibit 3-1.
Although it is not
required, it is useful to have a set of pictures of the visibility
markers handy for ready
reference. Pictures can be helpful to new observers.
OUTLINE
DETERMINING VISIBILITY
From an outside
point where all appropriate visibility markers can be observed:
1. Determine the most distant object visible, using the station
visibility chart as a guide.
2. Estimate the visibility in the direction of this object as
follows:
a. If the
object is barely identifiable, consider the visibility to be
the same as the distance to this object.
b. If the outlines of the object are in sharp relief,
estimate the distance that can be seen
beyond this object.
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