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Lesson 03: |
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Contents
Web Resources
Introduction
CommentaryOf all the new space technologies developed and in use, methods for monitoring and investigating weather are the most widely accepted and immediately available on a daily basis in our homes. Weather telecasts in most countries are accompanied by imagery in some form that had its origin as satellite data. In America, cable television carries at least one channel in every major city devoted exclusively to weather information. In addition, more than 300 National Weather Service radio stations broadcast weather data across the country 24 hours a day. This lesson about weather, satellites, and hurricanes brings together a number of important concepts in technology and the natural sciences. It is important to take your time to consider each separate concept in detail and then tie them together at the end of the lesson.TIROS-1, which stands for Television Infrared Observation Satellite, was the first weather satellite and one of America's first working satellites. It was launched April 1, 1960. One of the first (and very primitive) images from that satellite is shown below. Since 1960, satellite weather tracking for the United States has evolved into a "data-on-demand" system of such importance that it is considered a matter of national security. Weather satellite data, for example, was a major tool of the Coalition during the Persian Gulf War in 1991. Weather has been a strategic element in military campaigns for as long as wars have been fought. In the civilian community, we usually plan our daily lives according to the weather forecast.
 TIROS-1 Courtesy of NASA.
 Image of Hurricane Diana '84 Courtesy of NOAA/NESDIS.
This lesson is not intended as an in-depth review of weather; that would require an entire book itself. There are references, however, to a number of important weather systems and most of these books are illustrated. For the most part, though, this lesson has to do with hurricanes, especially those that develop in the Atlantic Ocean north of the equator. British Hurricane '87 is an anomaly— event that seldom happens and so the phenomenon is unusual enough to study on its own merit. If you are interested in reading more about hurricanes and weather in general, consult Appendix B. The following publications are also important for learning some of the basic mechanics and technologies of weather watching. If you have a serious interest in weather and hurricanes, these fine books should become part of your permanent library.
An excellent general introduction to weather phenomena is contained in each of the following two books:
It is difficult to speak of hurricanes in generalities because each hurricane has its own individual characteristics. You need only review the hurricanes over the past several decades to see how one differs from another in size, direction, forward velocity, internal velocity, life span, landfall strike, etc. You should keep this "personality aspect" in mind as you study this lesson. Hurricane PatternsDay turns into night and back into day again as the earth rolls over every 24 hours in its endless cycle. As the earth tilts upward and the seasons change, the sun's rays strike the earth at a more perpendicular angle and heat our planet to a higher temperature. Summer comes to the northern hemisphere, and the lands and oceans grow hotter as the sun beats down directly on the equator.Along America's Atlantic and Gulf coasts, the annual hurricane season runs from June through the end of November. Early in this season in June, most potential hurricanes are whipped up in the western Caribbean and the Gulf of Mexico (Hurricane Allison in the Gulf in the first week of June 1995 is a good example). In July and August, however, the storm center undergoes a decided eastward shift, and by early August a few storms are being born as far east as the Cape Verde Islands off Africa's west coast. They travel across the Atlantic, gathering force and size with each passing mile. After mid-September, the Atlantic storm center shifts back westward and most storms once more originate in the western Caribbean and the Gulf of Mexico. It is very important to understand this predictable shift of the cradles of hurricane activity. The shift enables you to know where to expect developing hurricanes and where to begin monitoring specific areas of the Atlantic-Caribbean complex.
During a typical year, more than 100 disturbances of hurricane potential are seen in the Atlantic, Gulf, and Caribbean, but usually only about ten reach the tropical storm stage and only five or six grow into full hurricanes. Of these, probably only two hurricanes will strike the United States, killing from 50 to 100 people anywhere from Texas to Maine and causing hundreds of millions of dollars worth of property damage. In an unusually active year, the same storms can cause several hundred deaths and property damage mounting into the billions of dollars. Storms born in the North Atlantic usually stay in the Atlantic. On a rare occasion, an Atlantic storm will retain enough forward momentum to rush over southern Mexico and emerge in the Pacific Ocean. These unusual storms are encouraged by the Mexican deserts' warm air and the Intertropical Convergence Zone— place along the equator where tropical winds north and south of the equator meet. On such an occasion, a storm may reintensify itself or it may fall apart, depending on variable conditions. Meteorologists continually pore over massive amounts of data that might show early indications of a developing storm somewhere out over the warm Atlantic sea. Meteorological data from many diverse surface stations, balloon probes of the atmosphere, and information from hurricane-stalking aircraft are all tools of the hurricane watcher. As a hurricane develops and matures, meteorologists depend heavily on each measuring tool to help assess various characteristics of a hurricane's behavior. The data is put into computers and the hurricane tracking process begins as computer models are created. Life Cycle of a HurricaneWarm water feeds the hurricane. A developing severe storm usually begins as a strong whirlwind just above the ocean's surface as rays from the sun heat the ocean waves. Low-pressure atmospheric waves known as tropical waves emanate from the Caribbean and from Africa's west coast, continue to develop into tropical depressions (winds 0 to 38 miles per hour), and are carried across the ocean to the Caribbean and the United States. This process is depicted in Figures 3.1 and 3.2. Early in its life, a future hurricane may be nothing more than a slight drop in barometric pressure (when it is labeled a tropical depression) as the warm ocean water begins to swirl, twist, and spin.
 Computer graphics by T.W. Becker
 Computer graphics by T.W. Becker If the depression's internal mechanism is strong enough, and other variable conditions are present, the simple tropical depression develops into a tropical storm (39 to 73 miles per hour) as winds begin to pick up and grow, covering immense areas of often hundreds of thousands of square miles. In contrast, the winds and size of a mature hurricane are more compact and, though more deadly, are not as large in area but nevertheless may also cover thousands of square miles. In the photo below, Tropical Storm Barry '95 appears as an unorganized mass with all the early signs of a potential hurricane. Note that an eye has not yet formed.
 Tropical Storm Barry '95 Courtesy of NOAA/NESDIS/NCDC and Meteorologist Doug Ross.
Inside the tropical storm, rain bands of dense clouds form with driving torrential rainfall. These spiral rain bands ascend in layers of cumulus and cumulonimbus clouds to the high, upper atmosphere where condensing water vapor is swept away by high-altitude winds. These winds, often called shears (because they tend to shear off parts of the hurricane complex), appear in the form of ice crystals veiled in thin cirrus clouds. Lightning flares up in the rain bands as the cloudy landscape is lashed by ferocious turbulence. In the lower several thousand feet, air flows are pulled inward toward the center of the hurricane and are thrown upward through towering columns of air near the center. Above 40,000 feet, the pattern changes to a reverse action circulation, which becomes the hurricane's exhaust system, tossing out heat and excess moisture. At lower levels in the intense hurricane, winds out in the storm's perimeter take on a broad pattern like the slower currents at the rim of a whirlpool. These winds, like those currents, move faster as they approach the central vortex. This inner band is the eyewall where the storm's toughest winds are located and where moist air entering down at the ocean's surface moves upward, releasing heat to continue driving the storm. In most hurricanes, these winds exceed 90 knots (about twice that speed in extreme cases). The winds of a hurricane are caused by differences in atmospheric pressure. Atmospheric pressure is usually expressed as the height of a column of mercury that can be supported by the weight of the overlying air at a given time. In North America, barometric measurements at sea level seldom fall below 29 inches (982 millibars) of mercury, and in the tropics it is generally close to 30 inches (1016 millibars) under normal conditions. Hurricanes send these readings sharply downward far below normal categories. The change is swift, and pressure can drop an inch (33.8 millibars) with each mile of storm height. A mature hurricane operates across more than a million cubic miles of atmosphere. Over the deep ocean, waves generated by hurricane winds can reach heights of 50 feet or more. Beneath the storm's center, the ocean's surface is pulled upward like water through a straw, forming a mound one to three feet or so higher than the surrounding ocean surface. This mound can create coastal surges of 20 feet or more. Besides this surge, massive swells pulsate in and out of the upper levels of the sea. Ship captains at sea call these pulsations "ground swells." A hurricane can disrupt the sea down to a depth of several hundred feet. While a hurricane lives, the buildup of energy within its circulation is immense. The condensation and heat energy released by a hurricane in one day can be the equivalent of energy released by the explosion of four hundred 20-megaton hydrogen bombs. One day's released energy, converted to electricity, could supply the entire United States' electrical needs for about six months.
A hurricane has no guidance system of its own. The paths and directions it takes depend on four main steering mechanisms.
Making LandfallThe hurricane can survive as long as it is over warm water. But its movement is controlled by upper atmosphere forces that drive the storm ashore or push it over colder water north of the tropics where it will collapse and die. In this movement away from the tropics in the Atlantic, upper atmospherics and wind behaviors shove Atlantic hurricanes northward and frequently force them against the eastern U.S. coastline, where they often strike coastal cities and barrier islands.An embryonic storm has forward motion, or velocity, driven by the easterly flow in which it is embedded. As long as this drift is slow, the young hurricane may intensify. Rapid forward motion generally prevents intensification in the storm's early stages. Entering the temperate latitudes, some storms have thundered along at better than 40 miles per hour, but such fast moving storms weaken quickly. At middle latitudes, the hurricane's death is swift. Colder air penetrates the storm's vortex; the warm core cools down and acts as a thermal brake. Water temperature below 80º does not contribute much energy to a hurricane. Even though some large hurricanes can travel for days over cold North Atlantic water, all storms are destined to die once they leave the warm tropical waters that keep them alive. The farther they move into higher latitudes, the less fuel they receive from the sea and the faster they will die. Hurricanes fall apart quickly over land. Cut off from their source of energy in the sea, and with the added effect of frictional drag, circulation rapidly slows down and becomes more disorganized. Torrential rains, however, can continue even after the winds subside. In the southeastern United States, about one-fourth of the annual rainfall comes from dissipating hurricanes.
Hurricanes often renew themselves as extratropical (outside the tropics) cyclones at higher latitudes, or they combine with existing temperate-zone disturbances. Many storms moving up America's Atlantic coast are in the throes of this transformation when they hit New England, and large continental lows are often energized by the remains of tropical storms. Hurricanes disturb the entire weather pattern across large regions of the United States. A hurricane on the East Coast at the Carolinas, for example, often disrupts existing weather conditions as far inland as Missouri, Kansas, and Illinois. Some hurricanes that erupt in the Gulf of Mexico speed onto landfall on the coast with continuing severe storms that flow up through Alabama or Mississippi, past Kentucky, Arkansas, and Missouri and swerve eastward to blast their way through Ohio, Pennsylvania, and other northeast states. Storms in the Gulf cause quite widespread disruptions throughout the entire southern and midwestern parts of the United States. Storm SurgeEarlier in this lesson, it was said that a hurricane will raise the ocean surface several feet as it travels across the sea. In fact, an intense hurricane will gather surface water and begin to push it in front of itself, building a wall of water that slightly precedes the hurricane, which is known as a storm surge. The stronger the hurricane, the higher this wall will be; it can reach up to as much as 20 feet or even more. If the tide is coming into shore, the surge is added to the strength of the tide and the sum of the forces can be catastrophic. It is this storm surge that causes a large amount of damage to structures on the shore, as illustrated in Figure 3.3. Storm surge has been known to completely redesign the ecological system and the shoreline during a powerful hurricane, striking the shoreline at 50 to 60 miles per hour.
 Computer graphics by T.W. Becker.
The storm surge dome can be from 50 to 100 miles wide depending on other conditions, and it can spread itself across as many miles of coastal beach. Hurricane Camille in 1969 carried a 25-foot surge that completely inundated the town of Pass Christian in southern Mississippi. Nine out of ten fatalities during a hurricane are the result of storm surge because of human miscalculation of its severity. There is a great difference between undulating ocean swells that sweep up onto the beach and a true storm surge. Large swells are usually rather low and predictably repetitive, but a surge can be almost as high as a three-storey building and arrive with such force that it smashes anything in its path. A very deep continental shelf can allow the sea to absorb some of the surge's energy, but a high shelf (making the water more shallow) will simply throw the surge upward, encouraging more and more height. The gathering of forces (energy) in a hurricane is the result of the arithmetic of the storm. Storm surge plus tidal force plus spin force plus shallowness of the continental shelf can add up to an enormous storm impact. For some reason not completely understood, this impact is carried in the hurricane's right front quadrant illustrated in Figure 3.4. Most likely, this right quadrant phenomenon occurs because of the counterclockwise rotation of the hurricane.
 Computer graphics by T.W. Becker.
Measurements of the sea surface, especially in the Atlantic Ocean, have been conducted by European Space Agency scientists using the ERS-1 and ERS-2 synthetic aperture radar satellites. In general, their findings indicate that the surface of the ocean is not flat but rather consists of high and low places corresponding to the ridges and mountains of the ocean floor. The findings are illustrated in photo below.
 Computer Model of Relationship of Ocean Floor Topography to Ocean Surface Courtesy of European Space Agency, Paris.
Worse still are flash floods, strong walls of water that race downstream or quickly cover over low-level land areas. Flash floods can be surprisingly destructive because they move swiftly, taking us by surprise before we can set up barriers. In some cases, there is no opportunity to erect barriers during flash floods. Hurricanes approaching a coastline send out early surges, or swells, that strike the beach and bounce back; this is like the waves of water from a pebble tossed into a clear pool that sends out rings of little swells. The waves that bounce back, called rip tides, can be strong and deadly to swimmers and surfers because they also exist unseen underwater, often pulling the swimmers under and drowning them. Rip tides are deceptive and often not noticeable to people on the shore.
Causes of Hurricane DamageThere are three major causes of destruction during a hurricane:
The Saffir-Simpson scale (see Figure 3.5) describes a hurricane's potential for destruction. Herbert S. Saffir, a Miami engineer, designed the scale in 1971 and donated it to the National Weather Service. Robert Simpson, a former director of the National Hurricane Center, added storm surge and barometric pressure.
The Eye of the StormThe eye of the hurricane is a tunnel with thick walls of stored, potentially violent energy and water. It is through this tunnel that the hurricane breathes, sending warm air upwards to mingle in the atmosphere that then plummets downward again as cooled air. Barometric pressure drops significantly. At ground level, the diameter of the eye is usually calm and people can walk around in it unmolested. Air at the top of the eye is thrown outward in an anticyclonic motion to form clouds and feeder bands. But some of the air moves inward, sinking back into the eye to be warmed all over again and increasing its ability to hold water. As the reheated air dries out, the eye/tunnel becomes cloud-free and clear.The diameter of the eye is a measure of the hurricane's intensity and its ability to do damage; the smaller the eye's diameter, the more compact and deadly the storm is. The principle here has to do with the conservation of angular momentum— the principle that makes an ice skater spin faster with his/her arms folded than with arms outstretched. When the eye begins to shrink, meteorologists know that the hurricane is building its energy force and spinning faster around the central eye. A diameter of 10 to 12 miles (normal) indicates a highly compact storm; diameters of 25 to 35 miles (normal) indicate a far weaker hurricane system. In order to assess the hurricane's strength, it is necessary to take measurements not just in the storm's perimeter, but at the eyewall as well. This difficult and highly dangerous task, which is a usual research procedure, makes up the work of "hurricane hunters," meteorologists and other professional technicians who fly through the hurricane in an instrument-laden airplane. Pressure, temperature, cyclonic activity, and distances inside the storm are all documented to provide a working profile of the hurricane. This profile leads to greater understanding of the mechanisms that drive the hurricane. The airplane actually penetrates the eyewall and, if there is sufficient space, can fly around inside the storm's eye accumulating research data. The view of Hurricane Emily '93 in the photograph below shows a medium to large eye, probably a sufficient size in which to fly around in a research aircraft. Hurricane Luis '95 at Force 4 had a 35-mile diameter eye, and research aircraft flew missions inside the eye.
 GOES Image of Hurricane Emily '93 Courtesy of NOAA/NESDIS/NCDC.
If weather monitoring is to be useful, we must become much more accurate in prediction. Enormous strides have been taken in the later half of the twentieth century because of faster and more sophisticated instruments; yet, at best our predictions are accurate over only about three to five days. Perhaps we do not yet really understand the earth's very complicated weather machine sufficiently despite the modern explosion of knowledge, methods, and technology. Someday, we will be able to change weather patterns at will: stop hurricanes before they really get started, tailor the weather to our own daily needs, etc. While such abilities are many decades off in the future, today's continuing research assures us these things eventually are possible. Weather SatellitesThe National Oceanic and Atmospheric Administration (NOAA), which was founded in 1970, is a branch of the U.S. Department of Commerce, a Cabinet post headed by the secretary of commerce. NOAA is responsible for monitoring the atmosphere, weather, and weather events in and around the United States; it is also responsible for the gathering, analyzing, reporting, archiving, and disseminating information about all this data. That's a very large responsibility and one that the employees of NOAA take very seriously. One of the NOAA sub-branches is the National Environmental Satellite, Data, and Information Services division (NESDIS), which operates three major locations (see Figure 3.6).
The most important of the NESDIS operations in regard to hurricane data is the National Climatic Data Center (NCDC) in Asheville, North Carolina. This is NOAA's and the nation's major archive for weather information. NCDC supplies pictures, charts, diagrams, storm analysis information, and reports to a wide group of users throughout the global community. The photo below shows a meteorologist at the National Climatic Data Center at Asheville tracking a storm on the brand new WSR-88 Doppler Radar equipment, called NEXRAD (next generation radar).The image on the screen shows a major storm off the coast of Florida.
 Meteorologist Tracking a Storm Courtesy of T. W. Becker.
The eastern GOES can image all the way across the Atlantic Ocean to the western coast of Africa, and the western GOES can image almost all the way to Australia including New Zealand, Aleutian Islands of Alaska, and Hawaii. In addition, the satellite receives data from remotely located collection platforms such as buoys, vessels at sea, and land-monitoring stations.
 Computer graphics by T.W. Becker.
The second type of satellite is known as TIROS (Television Infrared Operational Satellite), a polar-orbiting vehicle in low Earth orbit that orbits from pole to pole covering about 80 percent of the earth's surface four times a day. Global data are tape recorded onboard the polar satellites and later played back on command as the satellite passes over a ground station. The TIROS satellites measure temperature and humidity in the earth's atmosphere, Earth surface temperature, cloud cover, water-ice boundaries, and proton and electron flux near the earth. TIROS satellites also carry a search-and-rescue transponder (receiver), which receives signals from portable transmitters on an emergency radio frequency carried by lost explorers, victims of disasters at sea, and mountain climbers, for example. America's weather satellite operation is one of the most advanced in the world in terms of how data is treated and handled. The U.S. weather center is located outside Washington, D.C., at the World Weather Building. Data is received and retransmitted to the NOAA network at various NOAA/NESDIS centers around the country. At each center, the data is analyzed and entered into numerous ready-files for use by users. Other nations also operate weather satellites. The Japanese Geostationary Operational Meteorological Satellite, or GOMS, was patterned after America's GOES and does about the same kind of imaging job. INSAT, a meteorological satellite lofted by India's space program, images the Indian sub-continent, part of Europe, and part of Asia. For some time, the former Soviet Union maintained a number of Meteor weather satellites on a regular basis. Soviet optics and imaging and receiving equipment are often not as advanced as that of many other nations, and in recent years (since the breakup of the Soviet Union) much of its space operations programs have fallen into serious disrepair. It remains to be seen if the Soviet national space program can keep abreast of other nations in the future. By far, the most valuable foreign weather satellite, however, is Meteosat (see Figure 3.8), which is operated by the European Space Agency. This is valuable because of the series of operating systems set up by the Europeans to monitor weather on a constant basis. Ground receiving stations, equipment, methods of manipulating and disseminating data, and the training of operations personnel are all highly advanced and are the most reliable in the world. The European main receiving center for satellite weather data is located at Darmstadt, Germany, where ESA maintains a world-class operations center.
 Computer graphics by T. W. Becker.
The 1995 North Atlantic Hurricane SeasonBy any standard, the 1995 hurricane season in the North Atlantic, Caribbean, and Gulf of Mexico regions was one of the most active and unusual on record. By the end of October, 19 named storms had developed, including 11 hurricanes and 8 tropical storms. The names of these storms are in Figure 3.9:
The wake-up call for the season was Hurricane Allison on June 3, which is unusually early in the season for a full-blown hurricane to develop. As the weeks and months progressed, it quickly became evident that hurricane activity was building in frequency. During the week of August 22-29, a highly irregular "procession" of waves, depressions, storms, and hurricanes off the African coast all occurred at one time. On August 24, the procession was lined up like cars on a highway; by August 28, the storms and waves had scattered and been joined by a new storm, Luis. See Figures 3.10-3.13 for a graphic explanation of events. Why and how did this dispersion of storms and waves happen? Even more to the point, why did such an active 1995 hurricane season occur at all? Generally, there are four reasons:
Figure 3.10 shows how the 1995 hurricane season was an especially active one in the tropics. Depressions, storms, and hurricanes continuously emerged from the Caribbean and from the west coast of Africa. Look at the intense "march of disturbances" all the way from the Cape Verde Islands off the African coast into the Caribbean. There are also disturbances near Florida and off the west coast of Mexico in the Pacific, while the remnant of Hurricane Felix still lingers on its way across the Atlantic toward Europe. This march of disturbances continued throughout July, August, and September, producing tropical depressions and tropical storms (some of which became full-fledged hurricanes). The tropical wave #1 dissipated and fell apart; Hurricane Humberto (#2) kept organizing itself and turned north to the sea while Hurricane Iris (#3) eventually disorganized itself and turned southward. The tropical wave #4 never became a full hurricane, but it nevertheless caused a great amount of trouble by dumping huge amounts of rainfall onto Georgia, the Carolinas, and the Florida peninsula— one time, as much as 16 inches of rain in a single day, causing widespread flooding. It then flowed into the plains region. Tropical Storm Gil (#6) wavered for a time and then fell apart without ever organizing itself into a hurricane.
 Data courtesy of the National Hurricane Center and The Weather Channel. Computer graphics by T. W. Becker.
 Satellite data courtesy of Doug Ross, NCDC. Satellite image tracing by T. W. Becker.
 Data courtesy of the National Hurricane Center. Computer graphics by T. W. Becker.
September 1, 1995, 1700 EDT  Data courtesy of the National Hurricane Center. Computer graphics by T. W. Becker.
The FutureThe current forecast is for continued hurricane activity over the next decade. The past 12 years have been relatively quiet because of the African drought. The drought that ended in 1994 gave rise to the record-setting hurricane activity in 1995 and portends similar highly active seasons in the immediate future.Figure 3.14 is a computer sketch of new NOAA weather satellites—GOES 8 and 9— now stand guard over America's coasts and adjacent oceans. Launched in 1994 and 1995 respectively, these two sentinels now provide timely imagery using improved electronics and optics. Referred to as "the next generation of weather satellites," the satellites' lifetimes are estimated to be at least eight years; they will probably last longer. America is now ready to conduct much more sophisticated study and research of weather phenomena and to take our country confidently into the next millennium.
 Computer graphics by T. W. Becker, from NOAA sketches. Note: You should complete Activity 3 before you take the Lesson 03 Progress Evaluation
Copyright (c) 1997
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