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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
One of the applications of satellite image interpretation
that has saved thousands of lives is tropical storm forecasting. Hurricanes,
cyclones, and tropical storms affect coastal regions in the tropical and
mid-latitude regions of the world, often bringing winds up to 200 mph, heavy
rain, coastal flooding, and large scale destruction. Inland areas are affected
by hurricanes as they move on land with heavy rains and high winds, often
resulting in flooding, tornadoes and other wind damage. With all of this
dangerous weather, hurricanes can be very deadly. For example, in 1900 over
6000 people were drowned in one unexpected storm in Galveston, Texas. Other
areas of the world, such as Asia, have suffered single storms with the death
toll in the tens of thousands.
Since the 1960's, satellites have allowed us to observe tropical systems from their stages of initial development, through the maturation of the storm, to the dissipation over land or cool ocean waters. This allows us to improve warning systems and try to understand how these monstrous storms form. Satellites aid forecasters in tracking the motion of hurricanes and in estimating the intensity of a storm. Using satellite imagery, researchers have discovered that the size of the storm is not as important as the shape of the storm. For example, one generalization that has provided some useful forecasting clues is that the greater the degree of spiraling in a hurricane, the more mature and intense it is.
During this activity, you will read more about how scientists attempt to estimate the intensity of a tropical storm or a hurricane using satellite imagery. You will then have access to several hurricane images with which you can practice your ability to estimate hurricane intensity.
Once you are familiar with the material and information in this section, browse the images of various recent hurricanes that affected the east coast of North America. In each image, observe the patterns in the clouds surrounding the center of the storm and estimate the wind speed and central low pressure.
Diagram of hurricane intensity estimation [hurr_dev.gif]
Hurricane Andrew [andrew1.gif]
Hurricane Andrew [andrew2.gif]
Hurricane Andrew[andrew3.gif]
Hurricane Andrew[andrew4.gif]
Hurricane Emily [emily.gif]
Hurricane Emily (contrast enhanced) [emily2.gif]
Hurricane Hugo [hugo.gif]
Close-up of Emily's eye, rain bands, eyewall [huricane.gif]
Time sequence of Roxanne (false color enhanced) [roxanne.jpg]
Time sequence of Roxanne (Grayscale, high contrast ) roxan_bw.gif]
"Conveyer Belt" of 4 tropical systems in Atlantic [tropical.gif]
Tropical cyclones are storms that originate in tropical latitudes; they include tropical depressions, tropical storms, hurricanes, typhoons, and cyclones. These various types of storms are similar; their main difference is where they form. Hurricanes are tropical cyclones that occur in the Atlantic Ocean and the eastern and central Pacific Ocean, while typhoons are tropical cyclones that originate in the western Pacific Ocean. Cyclone is a more specific term that is often used to describe tropical cyclones that form in the Indian Ocean and near Australia. Though much of this activity describes hurricanes (tropical cyclones with sustained winds of at least 120 km/hr, or 74 mph), the concepts can be equally applied to typhoons and cyclones.
In satellite imagery, tropical cyclone development is analyzed by studying the cloud patterns and determining how they change with time. Repeated observations of a tropical cyclone provide information on the intensity and the rate of growth or decay of the storm. This diagram shows a modeled depiction of the developmental pattern of a tropical cyclone. This method of intensity analysis is based on the degree of spiraling in the cloud bands. The diagrams at the top of the graph illustrate the day-by-day changes in the shape of the cloud bands for a typical storm. The vertical axis of the graph is the tropical number (T-number) of the tropical cyclone. This number rates the intensity of the storm. Normally, a cyclone will exhibit a growth rate of 1 T-number per day. The straight line represents the intensity change and typical growth rate of a hurricane. The wavy line superimposed on the graph represents the degree of expected variability of intensity on a day-to-day basis.
This graph can be used as a conceptual model for estimating
hurricane intensity and rate of development. When an observed cyclone shows
the same daily increase in spiral banding as the diagram shows, the storm
is developing at a typical rate. If the curved band spiral develops more
quickly or more slowly, the rate of growth is considered to be fast or slow,
respectively. 
The associated central pressure and wind speeds are found below each T-number. The initial stage of tropical cyclone development is first recognized when curved cloud lines and bands define a cloud system center near or within a deep cloud layer. This stage is referred to as the T1 stage. The T2 stage should appear about 24 hours later. When the curved band spirals halfway around the center of the disturbance, the weak tropical storm (T2.5) stage has been reached. The minimal hurricane stage (T4) is attained when the cloud band completely encircles the center. Once the eye is observed (T4.5), continued intensification is indicated by an increase in eye definition, increasing smoothness of the dense overcast, or embedding of the eye in the dense overcast.
As long as conditions remain favorable, a tropical cyclone should reach its maximum intensity four to six days after the T1 stage is seen. This time period varies according to the direction the hurricane is traveling. For storms that travel northward, a maximum intensity is expected in four days. A storm moving northwest is expected to reach a maximum intensity in five days, while a westward-moving storm is expected to reach a maximum in six days. The hurricane will reach its decaying stage when it moves out of the region where ocean temperatures are warm enough or when it moves across land.
