Interpreting the Visibility Data
The display panel for the visibility data
shows the air pollution and meteorological conditions associated
with the photograph. 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 as 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.
- 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 today’s photo to the small photo taken on an unpolluted
day adjacent to today’s photo. If today's photo is not as
crisp as the "clean" photo, then there may be haze,
black carbon or fog obscuring the view.
- 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.
- 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.
- 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
some CAMNET sites do not measure all of these variables.
Fine particles, also called particulate matter (PM), 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. To learn more about fine particles and visibility, see the
Causes of Poor
Black carbon (BC) 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 photo or indicated in
the visibility information panel.
Ozone (O3) 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 without necessarily leading to high
For information on these pollutants and their effects on health, see the health effects page.
Pollution vs. Natural Conditions
Use the following table to determine when
poor visibility may be due to pollution or to natural conditions,
such as fog.
||Indications that poor visibility
is caused by pollution
|Indications that poor visibility
is not caused by pollution
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.
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
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 CAMNET 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 determines where pollutants
are going, and where they are coming from. The wind direction
shown on the CAMNET display panel shows the direction the wind
is coming from. For example, a wind direction of NE would be blowing
from the Northeast. In most Northeast and Mid-Atlantic locations,
a wind direction of W, SW, or S during the summer is often associated
with high pollution levels.
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 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.