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Real-time
display panel for visibility data show the air pollution and
meteorological conditions associated with each image. These
data are collected at the site of the camera or at another
location within the scene of the photograph.
As you look at the scene and check it against
the visibility data, you may wonder if the scene is a clear
as it could be or if haze or fog is affecting the view. Here
are some tips to help you interpret what you are seeing.
1. Look again; does the picture really
seem clear? On clear days, the features on the horizon
appear crisp. These days have low pollution levels and low
relative humidity. Compare todays photo to the small
photo taken on an unpolluted day (Pristine). If today's photo
is not as crisp as the "pristine" photo, then there
may be haze, black carbon or fog obscuring the view.
2. Is it a hazy day? Haze is relatively
uniform at the horizon but tends to diminish slightly at higher
elevations. Look at the levels of man-made pollutants including
fine particles, black carbon and ozone. Also, note the relative
humidity. Haze often occurs on hot, humid summer days with
medium or high levels of fine particles, ozone and sometimes
black carbon. Relative humidity tends to be medium to high.
3. Is it a brown cloud day? A brown
cloud appears to envelop the scene but quickly thins out at
higher elevations. Brown clouds tend to occur on calm winter
mornings during rush hour traffic. Look at the particle and
black carbon levels -- they are usually high. Ozone will be
low and relative humidity may vary.
4. Is it a foggy day? Look at the
relative humidity and precipitation levels. If the relative
humidity is close to 100% and there has been precipitation
in the past hour or 24 hours, then you are probably looking
at fog. Fog tends to be gray while haze is generally white.
It does not thin out at the top of the picture and is most
common in the fall and spring. Ozone levels will be low. However,
fine particles and black carbon could be low, medium or high.
Fog is a natural condition.
To learn more about the pollutants and
meteorological conditions, please read on. Each of the types
of data and its relation to visibility are described further
below. Note that most USDA Forest Service sites do not measure
all of these variables. |
Fine Particles
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Fine particles are a mix of
microscopic solids and liquids. They can be emitted directly
into the air during any process involving burning or combustion,
including activities around the home (e.g. cooking, smoking,
space heating, and open burning) and those involving motor
vehicles, various engines, power plants, and other such sources.
Fine particles also form when gasses such as nitrogen oxides
(NOx) and volatile organic compounds (VOCs) react and condense
in the atmosphere. These gasses come from the same sources
noted above. They also come from the evaporation of fuels
and household and industrial solvents.
This variety of gasses and direct particle
emissions results in a mixture of fine particles with different
sizes, chemical properties, and health and environmental impacts.
Fine particle levels are highest on warm, sunny days and on
clear, calm winter mornings.
Fine particles affect visibility in two
ways -- by absorbing light and by scattering light. Light
absorption causes a brownness or blackness in the air. This
is most obvious over urban areas and valleys during calm mornings,
especially in winter. It is primarily caused by diesel engines
in urban areas. Light scattering causes a whitish haze, which
is most obvious in the summer over widespread urban and rural
areas. Most haze is caused by coal-fired emissions from power
plants.
Medium and high levels of fine particle
concentrations are a strong indication that poor visibility
is due in large part to pollution. If fine particle concentrations
are low, then any visibility impairment is not likely due
to pollution, but to natural causes. Click here to learn more
about fine particles and the Causes
of Poor Visibility.
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Black Carbon
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Black carbon
is one of the many components of fine particles. It is similar
to soot and is emitted directly into the air from virtually
all combustion activities. It is especially prevalent in diesel
exhaust and smoke from the burning of wood and wastes. Black
carbon absorbs light and contributes substantially to the
low-altitude, brown clouds sometimes seen during the winter
over urban areas and valleys.
If black carbon concentrations are high
when visibility is poor, then the visibility impairment is
probably due to air pollution. An exception to this rule would
be during periods of fog. Under these conditions, black carbon
will tend to stagnate in local areas, hence raising their
concentrations. However, most of the poor visibility will
be due to the fog itself. When this happens, relative humidity
will be near 100 percent and precipitation may be evident
from the image or indicated in the real-time visibility information
panel. |
Ozone
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Ozone is a colorless, odorless
gas. It occurs naturally in the upper atmosphere, where it
absorbs harmful ultraviolet rays. But at ground level, it
forms as result of air pollution from cars, trucks, busses,
power plants, fuel and paint vapors, and other sources. Ozone
is good up high, but bad nearby. Concentrations may reach
unhealthy levels on warm, sunny days. During the summer, ozone
can be unhealthy for several days in a row.
Because it is an invisible gas, ozone does
not directly affect visibility. But in the summer it is usually
associated with pollution episodes involving haze and participates
in chemical reactions that lead to haze-forming particles.
Medium or high concentrations of ozone are a good indicator
that poor visibility conditions are due to pollution. However,
pollution can cause poor visibility with out necessarily leading
to high ozone concentrations.
Use the following table to determine
when poor visibility may be due to pollution or to natural
conditions, such as fog. Note that "PM" and "BC" stand for
particulate matter and black carbon, respectively.
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Indications that poor visibility
is caused by pollution |
Indications
that poor visibility is not caused by pollution |
High
ozone |
X |
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Low ozone |
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High PM |
X
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Low PM |
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X
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High
BC |
X
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Low BC |
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X
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Temperature |
Wind
Speed |
Precipitation
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Temperature is directly
related to the concentrations of ozone, fine particles, and
black carbon. Temperatures at or above the mid 80s (Fahrenheit)
favor the evaporation and emission of volatile organic compounds
(VOCs) and nitrogen oxides (NOx) and increase the speed of
chemical reactions leading to ozone and fine particles. Temperatures
below 40 degrees may enhance the condensation of some fine
particulate matter. High fine particle and black carbon concentrations
may also occur at temperatures between 40 and 80 degrees,
especially in the presence of low clouds, which can limit
the dispersion of pollutants and concentrate them near the
ground.
Relative Humidity
High levels of relative humidity are often
associated with high levels of ozone and fine particles. In
the case of fine particles, high humidity can lead directly
to increases in the size and concentration of fine particles.
This occurs because certain types of fine particles, especially
sulfates, are capable of absorbing water vapor. Once hydrated
and enlarged, these particles cause light to scatter, which
results in a whitish, regional haze. Sometimes, visibility
is limited mostly by low clouds, fog, or rain, and not by
air pollution. These situations can be visually distinguished
from man-made haze after learning how they differ in grayness,
thickness and homogeneity. They are also characterized by
extremely high levels of relative humidity (in the mid to
upper 90s).
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Wind speed affects air pollution
and visibility through its dispersive effects on pollutants.
When winds are calm or light (0 to 5 mph), pollutants can
accumulate and reach unhealthy levels. Light to moderate winds
(5 to 10 mph) sometimes increase pollutants by mixing emissions
from various sources, urban centers, and transportation corridors.
These winds also transport pollutants further downwind and
may therefore raise concentrations in places that might otherwise
be clean.
High wind speeds (15 mph or greater) tend
to disperse pollutants and prevent their accumulation. At
these speeds, the amount of dispersion outweighs the transport
effects, so high concentrations are unlikely to occur anywhere.
Wind speeds reported on FSVISIMAGES are
strictly in the horizontal direction. Air, however, also travels
in vertical directions. When vertical wind speeds (or venting)
are high, pollutants are dispersed vertically and do not become
concentrated at the ground. Venting is strongest during clear
daylight hours.
Wind Direction
Wind direction determines where pollutants
are going, and where they are coming from. The wind direction
shown on the FSVISIMAGES display panel shows the direction
that the wind is coming from. For example, a wind direction
of NE would be blowing from the Northeast.
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If measured amounts of precipitation
are shown on the visibility information panel, then poor visibility
conditions are likely due to natural phenomena, especially
if the precipitation has been measured within the past hour.
Visual Range
Visual range is defined as the distance
at which a black object can be discerned from a white background
on the horizon. However, visual range is measured by instruments
that capture small amounts of air at a fixed location on the
earth's surface. This method sometimes overestimates the visual
range, especially in foggy or rainy conditions. This is the
reason the visual ranges are shown as "less than" a particular
distance on the FSVISIMAGES web site.
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