Geoscience Reference
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observed over 180 degrees or more of the circle (not
necessarily in continuous sectors) is the prevailing vis-
ibility. Thus, half the area around an observation point
may have visibility worse than the prevailing visibility,
which is important because most prevailing visibility
observations are made at airports. If the visual range
in a sector is significantly different from the prevailing
visibility, the observer at an airport usually denotes this
information in the observation record.
5
10
3
Mie regime
4
10
1
Q
s
10
-1
3
10
-3
2
Q
f
10
-5
1
10
-7
Q
a
10
-9
0
0.01
0.1
1
10
100
1000
Particle diameter (µm)
Example 7.4
If the visual range around 25 percent (90 degrees)
of the horizon circle is 5 km, that around another
25 percent is 10 km, that around another 25
percent is 15 km, and that around the remain-
der of the circle is 20 km, what is the prevailing
visibility?
Figure 7.21.
Single-particle absorption (
Q
a
), total
scattering (
Q
s
), and forward scattering (
Q
f
)
efficiencies of liquid water drops of different sizes at
=
0.50
m(
n
=
1.335,
=
1.0
×
10
−9
).
regions, 20 to 50 percent of the accumulation mode
mass is sulfate.
Thus, sulfate is correlated with particle
scattering more closely than is any other particulate
species, aside from liquid water
.
Figure 7.21 shows that liquid water hardly absorbs
visible light until particles are the size of raindrops
(
Solution
The prevailing visibility is 15 km because the
viewer can see 15 km or more around at least
50 percent of the horizon circle, but not neces-
sarily in consecutive sectors around the circle.
mindiameter). The absorptivity of raindrops
causes the bottoms of precipitating clouds to appear
gray or black.
>
1,000
Aless subjective and, now, a regulatory definition
of visibility is the
meteorological range
,which can be
explained in terms of the following example. Suppose
a perfectly absorbing dark object lies against a white
background at a point
x
0
,asshowninFigure 7.22.
Because the object is perfectly absorbing, it reflects
7.2. Visibility
Visibility
is a measure of how far we can see through the
air. Even in the cleanest air, our ability to see along the
Earth's horizon is limited to a few hundred kilometers
by background gases and aerosol particles. However,
if we look up through the sky at night, we can discern
light from stars that are millions of kilometers away.
The reason that our visibility is much lower horizontally
than vertically is that more gas molecules and aerosol
particles lie in front of us in the horizontal than in the
vertical.
Several terms describe maximum visibility. Two sub-
jective terms are visual range and prevailing visibil-
ity.
Visual range
is the actual distance away from an
ideal dark object at which a person can discern the
object against the horizon sky.
Prevailing visibility
is
the greatest visual range a person can see along 50
percent or more of the horizon circle (360 degrees),
but not necessarily in continuous sectors around the
circle. It is determined by a person who identifies land-
marks known distances away in a full 360-degree circle
around an observation point. The greatest visual range
Scattering into path
I
=0
Scattering out of path
d
x
x
0
x
Figure 7.22.
Change of radiation intensity along a
beam. A radiation beam originating from a dark
object has intensity
I
0atpoint
x
0
.Overadistance
d
x
,thebeam'sintensity increases due to scattering
of background light into the beam. This added
intensity is diminished somewhat by absorption along
the beam and scattering out of the beam. At point
x
,
the net intensity of the beam has increased close to
that of the background intensity.
=
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