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Adapted from An Introduction to Satellite Image Interpretation, Eric D. Conway and the Maryland Space Grant Consortium, ©1997, Johns Hopkins University Press, Baltimore, 255 pp with Interactive CD-ROM.
For more information about this book and how to order copies go to the JHU Press On-line Catalog
Another important application of satellite imagery
is in the monitoring of air quality around the globe. Using satellite data,
it is often possible to locate source points for large amounts of pollution,
ash, or smoke entering the atmosphere. The data collected by satellites
also offer graphic display of the aerial extent of pollutants and can be
used to track and predict their movement throughout the atmosphere. The
role of pollution in regional cloud and weather modification can be assessed
from satellites, as well.
In this activity, you will read about how to identify two types of atmospheric pollutants in satellite imagery; fires and volcanoes. Both are natural processes in the atmosphere (although fires started by man through carelessness or slash and burning practices can be located with satellite imagery). You will then investigate imagery containing fires or volcanic eruptions and attempt to locate the feature in the image. At the end of this activity you should be able to identify the characteristics of airborne particulates such as smoke and ash that allow them to be detected from satellite.
New Jersey fires - infrared [fires_ir.gif]
New Jersey fires - visible [fires_nj.gif]
Yellowstone fires of 1988 [fire_yel.gif]
Mt Redoubt, Alaska [redoubt.gif]
Mt St. Helens, Washington [sthelen1.gif]
Mt St. Helens, Washington [sthelen2.gif]
Mt St. Helens, Washington [sthelen3.gif]
One of the most useful applications of satellite technology
is in the monitoring of forest and brush fires. From the vantage point of
space, fires in remote areas are easily spotted, and the proper fire-fighting
personnel can then be notified to take action. Illegal slash burning in
the Pacific Northwest, as well as in tropical and subtropical slash areas
in Central America and South America, can be monitored using satellite data.
These data also help in tracking the smoke and particulates released into
the atmosphere, which can reduce visibility and degrade air quality.
The smoke from forest and brush fires is often visible in both VIS and IR satellite imagery as a light gray plume. At the actual point where the fire is located, the smoke is seen as a dense, narrow plume that spreads out downwind. The smoke plume often has a wedge shape that points upwind, caused by the spreading of the smoke. Once it is under the influence of the prevailing winds, smoke thins and spreads out considerably. Smoke can often be seen hundreds of kilometers downwind of the original source point as a milky gray area in VIS imagery.
In the summer of 1988, extensive forest fires
in and around Yellowstone National Park destroyed over 2 million acres of
vegetation. In this image, taken on September 8, 1988, smoke plumes can
be seen extending toward the east in northern Wyoming and Idaho. The smoke
plumes are detectable in IR imagery since they are cold, but they are not
as cold as cloud tops in Kansas and Nebraska. Thus, the plumes are primarily
low level. This indicates that low-level winds are westerly in this area.
Note that at the origin of the smoke cloud the plume is thicker, and therefore
it appears brighter. As the smoke cloud is carried downwind, it thins and
spreads out, and its brightness decreases.
Volcanic eruptions add extremely large amounts of aerosols to the atmosphere in a very short period of time. The dense clouds of airborne volcanic particulate material are known as ash clouds. Ash clouds associated with volcanic eruptions can cause a variety of air quality problems. Areas close to a volcanic eruption can be covered by several inches or even several feet of volcanic ash. Further from the volcano, ash clouds can cause air quality to decline so severely that people with respiratory problems and other health conditions are in danger. Additionally, ash clouds are a hazard to aviation. On a global scale, aerosols from volcanoes that persist in the upper atmosphere are thought to spread around the entire planet and partially block solar radiation from reaching the lower atmosphere. The result is a short-term cooling of the Earth. Thus, not only do volcanoes affect the areas immediately surrounding them but they can also play a role in regulating the global climate.
Ash ejected from volcanoes can be seen under clear sky
conditions in satellite imagery. Ash clouds generally have a high albedo
and can be detected in VIS satellite imagery, especially where the plume
is very thick. The debris can form a large, light-colored cloud. As the
ash plume spreads and thins, it develops a hazy appearance. Material from
the ash cloud can often be tracked for hundreds of kilometers as it moves
under the influence of the prevailing winds. 
IR imagery can also be used to locate and monitor volcanic activity. Since volcanic ash clouds often extend very high in the atmosphere, they tend to be cold, and they appear bright in IR satellite imagery. This image is an IR image of Alaska as Mount Redoubt erupted on March 23, 1990. The brilliantly bright white spot in the image is the ash plume from the volcano. The water feature to the right of Mount Redoubt is Cook Inlet. Anchorage is located at the head of this inlet.
The Mt St Helens images listed above illustrate how VIS imagery can be used to monitor the spread of volcanic material in the atmosphere. The first image shows the ash cloud associated with the eruption of Mount St. Helen's in Washington on May 18, 1980. The ring-shaped ash cloud can be seen as it spreads away from the volcano. Several hours later, westerly winds carried the ash from the eruption several hundred kilometers. In the second image, material from the eruption can be seen spreading as far away as southwest Montana. This cloud buried some towns with several inches of ash. By the next day, the winds spread the ash cloud as far as Oklahoma and the northeast Texas panhandle. This can be seen in the third image as a wide area with a hazy appearance over the central plains states. Note that the plume extends to the northwest across Wyoming, Idaho, and northwest Oregon.
