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How Hurricanes & Tornados Work


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Semi-technical reading for German university students with key words translated into German.

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How Hurricanes & Tornados Work

  1. 1. Technical English for Native German Speakers How Hurricanes (Wirbelstürme) Work by Marshall Brain and Craig C. Freudenrich, Ph.D., as modified by Harvey Utech (German translation of word stems added by Utech) Every year between June 1 and November 30 (commonly called “hurricane season”), hurricanes threaten (bedrohen) the Eastern and Gulf coasts of the United States, Mexico, Central America and the Caribbean. In other parts of the world, the same types of storms are called typhoons or cyclones. Hurricanes wreak havoc (verheerenden Schaden anrichten) when they make landfall (Land erreichen), and they can kill thousands of people and cause billions of dollars of property damage when they hit heavily populated (dicht besiedelt) areas. In this article, we'll discuss how hurricanes form and move, and look at the destruction and damage (Schaden) they can cause. We'll also examine how meteorologists track hurricanes. You'll be amazed at the power and impact of these storms! Defining a Hurricane According to the National Hurricane Center, "hurricane" is a name for a tropical cyclone that occurs in the Atlantic Ocean. "Tropical cyclone" is the generic (generisch) term used for low-pressure systems that develop in the tropics. Tropical cyclones with maximum sustained (ununterbrochen) surface winds of less than 17 meters per second (39 mph / 62.7 kph / 34 knots) are called tropical depressions (Tiefdruckgebiet). Once the tropical cyclone reaches winds of at least 17 meters per second (m/s), it is typically called a tropical storm and assigned a name. If winds reach 33 m/s (74 mph / 119 kph), then it is called a "hurricane." Hurricanes are defined by the following characteristics: • They are tropical, meaning that they are generated (entwickeln) in tropical areas of the ocean near the Equator. • They are cyclonic (zyklonal), meaning that their winds swirl (wirbeln) around a central eye. Wind direction is counterclockwise (west to east) in the Northern Hemisphere and clockwise (east to west) in the Southern Hemisphere (more about this later). • They are low-pressure systems (Tiefdrucksystem). The eye of a hurricane is always a low-pressure area. The lowest barometric pressures ever recorded have occurred inside hurricanes. • The winds swirling around the center of the storm have a sustained speed (Dauergeschwindigkeit) of at least 74 mph (119 kph / 64 knots). How a Hurricane Forms Hurricanes form in tropical regions where there is warm water (at least 80 degrees Fahrenheit / 27 degrees Celsius), moist air and converging (zusammenlaufen) equatorial (äquatorial) winds. Most Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms (Gewitter) that licensed under * Page 1 of 8
  2. 2. Technical English for Native German Speakers move out over the warm, tropical ocean waters. A thunderstorm reaches hurricane status in three stages: • Tropical depression - swirling clouds and rain with wind speeds of less than 38 mph (61.15 kph) • Tropical storm - wind speeds of 39 to 73 mph (54.7 to 117.5 kph) • Hurricane - wind speeds greater than 74 mph (119 kph) It can take anywhere from hours to several days for a thunderstorm to develop into a hurricane. Although the whole process of hurricane formation is not entirely understood, three events must happen for hurricanes to form: • A continuing evaporation (Verdunstung)-condensation (Kondensation) cycle of warm, humid (feucht) ocean air • Patterns of wind characterized by converging winds at the surface and strong, uniform-speed winds at higher altitudes (Höhe) • A difference in air pressure (pressure gradient) between the surface and high altitude Warm, moist air from the ocean surface begins to rise rapidly. As this warm air rises, its water vapor (Dunst) condenses to form storm clouds and droplets of rain. The condensation releases heat called latent heat (latente Wärme) of condensation. This latent heat warms the cool air aloft (hoch droben), thereby causing it to rise. This rising air is replaced by more warm, humid air from the ocean below. This cycle continues, drawing more warm, moist air into the developing storm and continuously moving heat from the surface to the atmosphere. This exchange of heat from the surface creates a pattern of wind that circulates around a center. This circulation is similar to that of water going down a drain (Abflussrohr). "Converging winds" are winds moving in different directions that run into each other. Converging winds at the surface collide (zusammenstoßen) and push warm, moist air upward. This rising air reinforces (verstärken) the air that is already rising from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds blowing at uniform speeds at higher altitudes (up to 30,000 ft / 9,000 m) help to remove the rising hot air from the storm's center, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds do not blow at the same speed at all levels -- if wind shears (Scherung) are present -- the storm loses organization and weakens. High-pressure air in the upper atmosphere (above 30,000 ft / 9,000 m) over the storm's center also removes heat from the rising air, further driving the air cycle and the hurricane's growth. As high-pressure air is sucked (saugen) into the low-pressure center of the storm, wind speeds increase. Parts of a Hurricane Once a hurricane forms, it has three main parts: • Eye - the low-pressure, calm (ruhig) center of circulation • Eye wall - area around the eye with the fastest, most violent winds licensed under * Page 2 of 8
  3. 3. Technical English for Native German Speakers • Rain bands - bands of thunderstorms circulating outward from the eye that are part of the evaporation/condensation cycle that feeds the storm Hurricane Size Hurricanes vary (sich ändern) widely in physical size. Some storms are very compact and have only a few trailing (hinter sich herziehen) bands of wind and rain behind them. Other storms are looser (lockerer), so the bands (Streifen) of wind and rain spread out over hundreds or thousands of miles. Hurricane Floyd, which hit the eastern United States in September 1999, was felt from the Caribbean islands to New England. Hurricane Categories Once a hurricane forms, it is rated on the Saffir-Simpson Hurricane Scale. There are five categories in this rating system. Saffir-Simpson Hurricane Scale Category Wind Speed Effects • Storm surge (Sturmflut) 4 to 5 ft (1.2 to 74 to 95 mph 1.5 m) above normal 1 (119 to 153 kph) • Some flooding (Überschwemmung) • Little or no structural damage • Storm surge 6 to 8 ft (1.8 to 2.4 m) above normal 96 to 110 mph 2 • Trees down (155 to 177 kph) • Roof damage (shingles (Schindel) ripped off) • Storm surge 9 to 12 ft (2.7 to 3.7 m) above 111 to 130 mph normal 3 (178.6 to 209 • Structural damage in houses kph) • Mobile homes (Wohnwagen) destroyed • Severe flooding • Storm surge 13 to 18 ft (4 to 5.5 m) above 131 to 154 mph normal 4 (210 to 247.8 • Severe flooding inland kph) • Some roofs ripped off (wegreißen) • Major structural damage • Storm surge at least 18 ft (5.5 m) above >155 mph normal 5 (> 249.4 kph) • Severe flooding further (ferner) inland • Serious damage to most wooden structures Hurricanes in categories 3, 4 and 5 can cause widespread damage, from severe inland flooding to the loss of life, property, agriculture and livestock (Vieh). In the next section, we'll look at how this damage occurs. Hurricane Damages The damage caused by a hurricane results from a number of aspects of the storm. licensed under * Page 3 of 8
  4. 4. Technical English for Native German Speakers • Hurricanes bring with them huge amounts of rain. A big hurricane can dump (verschleudern) dozens (Dutzend) of inches of rain in just a day or two, much of it inland. That amount of rain can create inland flooding that can totally devastate (verwüsten) a large area around the hurricane's center. • High sustained winds cause structural damage. These winds can also roll cars, blow over trees and erode (auswaschen) beaches (both by blowing sand and by blowing the waves into the beach). • The prevailing winds of a hurricane push a wall of water, called a storm surge, in front of it. If the storm surge happens to synchronize with a high tide (Hochwasser), it causes beach erosion (Auswaschung) and significant inland flooding. • Hurricane winds often spawn (hervorbringen) tornadoes, which are smaller, more intense cyclonic storms that cause additional damage. The extent of damage depends on a few things: • The category of the hurricane (see below) • Whether the storm comes ashore head-on (frontal) or just grazes (streifen) the coastline • Whether the right or left side of the hurricane strikes a given area The right side of a hurricane packs more punch because the wind speed and the hurricane speed-of-motion are complimentary (einander ergänzend) there. On the left side, the hurricane's speed of motion subtracts (abziehen) from the wind speed. This combination of winds, rain and flooding can level a coastal town and cause significant damage to cities far from the coast. In 1996, Hurricane Fran swept 150 miles (241 km) inland to hit Raleigh, N.C. Tens of thousands of homes were damaged or destroyed, millions of trees fell, power was out for weeks in some areas and the total damage was measured in the billions (Milliarden) of dollars. Tracking a Hurricane Hurricanes in the Northern Hemisphere rotate counterclockwise (west to east) and move through the ocean clockwise (east to west). In the Southern Hemisphere, hurricanes rotate clockwise (east to west) and move counterclockwise (west to east). These motions, known as the Coriolis effect, are caused by the Earth's rotation. To monitor and track (verfolgen) the development and movement of a hurricane, we rely on remote sensing by satellites, as well as data gathered by the Hurricane Hunters. The Hurricane Hunters are members of the 53rd Weather Reconnaissance Squadron/403rd Wing, based at Keesler Air Force Base in Biloxi, Mississippi. Since 1944, the U.S. Department of Defense (which oversees the U.S. military) has been the only organization to fly into tropical storms and hurricanes. Since 1965, the Hurricane Hunters team has used the C-130 Hercules, a very sturdy turboprop plane. The only difference between this plane and the cargo version is the specialized, highly sensitive weather equipment installed on the WC-130. The team can cover up to five storm missions per day, anywhere from the mid- Atlantic to Hawaii. licensed under * Page 4 of 8
  5. 5. Technical English for Native German Speakers Weather satellites use different sensors to gather different types of information about hurricanes: • Visible - clouds, circulation patterns • Radar / Doppler radar - rain, wind speeds, precipitation amounts • Infrared - temperature differences, cloud heights The Hurricane Hunters gather information about wind speeds, rainfall and barometric pressures within the storm. The information is relayed back to the National Hurricane Center in Miami, FL, where it is interpreted and distributed to national and local news media. The National Hurricane Center predicts the hurricane's movement and intensity using various weather models and issues hurricane watches and warnings to areas in the storm's path. The modern system (tracking, early detection, warnings) has greatly reduced the loss of life during a hurricane. Hurricane Names To better track hurricanes, weather officials decided to name them. The names are chosen by the World Meteorological Organization. According to the National Oceanic & Atmospheric Administration (NOAA): "For several hundred years, hurricanes in the West Indies were often named after the particular saint’s day on which the hurricane occurred. For example, 'Hurricane San Felipe' struck Puerto Rico on September 13, 1876. Another storm struck Puerto Rico on the same day in 1928, and this storm was named 'Hurricane San Felipe the Second.'" Until World War II, hurricanes were given only masculine names. In the early 1950s, weather services began naming storms alphabetically and with only feminine names. By the late 1970s, this practice was replaced with alternating masculine and feminine names. The first hurricane of the season is given a name starting with the letter A, the second with the letter B and so on. According to NOAA, "the name lists... have an international flavor because hurricanes affect other nations and are tracked by the public and weather services of many countries." Hurricanes in the Pacific Ocean are assigned a different set of names than Atlantic storms. For example, the first hurricane of the 2007 hurricane season was a Pacific Ocean storm near Acapulco, Mexico, named Alvin. The first Atlantic storm of the 2001 season was named Andrea. A list of names through 2014 is available from the National Hurricane Center. How Tornadoes Work by Marshall Brain, as modified by Havey Utech (German translations of word stems added by Utech) A tornado is one of those amazing, awesome (eindrucksvoll) acts of nature that simply leave you dumbfounded (sprachlos) -- a huge, swirling, 200-mph beast licensed under * Page 5 of 8
  6. 6. Technical English for Native German Speakers (Bestie) of a storm that appears to have a mind of its own. You have to actually see one with your own eyes to believe it. In certain places, tornadoes appear with amazing regularity. That's why we see them in the news all the time. In this article, we will take a look at tornadoes to learn what they are, how they form and just how powerful they can be. Tornadoes and Your Bathtub If you have ever seen a whirlpool form in your bathtub, sink (Spülbecken) or toilet when the water is draining, you have seen the fundamentals of a tornado at work. A drain's whirlpool, also known as a vortex (Wirbel), forms because of the downdraft ((Luft)zug von oben) that the drain creates in the body of water. The downward flow of the water into the drain begins to rotate, and as the rotation speeds up, the vortex forms. Because you see vortexes all the time in tubs and sinks, it is obviously a fairly common phenomenon. In a tornado, the same sort of thing happens, except with air instead of water. Tornadoes and Thunderstorms With a tornado there is no drain. Instead, there is a thunderstorm cloud. A typical thunderstorm cloud can accumulate a huge amount of energy. If the conditions are right, this energy creates a huge updraft ((Luft)zug von unten) into the cloud. But where does the energy come from? Clouds are formed when water vapor condenses in the air. This change in physical state releases heat, and heat is a form of energy. A good deal of a thunderstorm's energy is a result of the condensation that forms the cloud. According to Encyclopedia Britannica: For every gram of water condensed, about 600 calories of heat are made available. When the water freezes in the upper parts of the cloud, another 80 calories of heat per gram of water are released. This energy goes to increase the temperature of the updraft and, in part, is converted to kinetic energy of upward and downward air movement. If the quantity of water that is condensed in and subsequently precipitated (ausfällen) from a cloud is known, then the total energy of a thunderstorm can be calculated. In an average thunderstorm, the energy released amounts to about 10,000,000 kilowatt-hours, which is equivalent to a 20-kiloton nuclear warhead. A large, severe thunderstorm might be 10 to 100 times more energetic. In supercell thunderstorms, the updrafts are particularly strong (see the links at the end of this article for information on supercells). If they are strong enough, a vortex of air can form just like a vortex of water forms in a sink. An air vortex under a thunderstorm cloud is a tornado. The tornado reaches down out of a thundercloud as a huge, swirling rope of air. Wind speeds in the range of 200 to 300 mph are not uncommon. If the vortex touches ground, the speed of the whirling wind (as well as the updraft and the pressure differences) can cause tremendous damage. licensed under * Page 6 of 8
  7. 7. Technical English for Native German Speakers The tornado follows a path that is controlled by the path of its parent thundercloud (Gewitterwolke), and it will often appear to hop (hüpfen). The hops occur when the vortex is disturbed. You have probably seen that it is easy to disturb a vortex in the tub, but then it will reform. The same thing can happen to a tornado's vortex, causing it to form and collapse (zusammenbrechen) along its path. Tornado Ratings Tornadoes are rated on what is called the Fujita Scale, named for the inventor T. Theodore Fujita. There are six levels in this scale: licensed under * Page 7 of 8
  8. 8. Technical English for Native German Speakers Level Wind Speed Possible Damage Light damage: Tears (reißen) branches from trees; rips shallow-rooted trees from the F0 40 - 72 mph ground; can damage signposts, traffic signals and chimneys (Schornstein) Moderate damage: Roofing (bedachend) materials and vinyl siding can be displaced (vertreiben); mobile homes are highly vulnerable (gefährdet) and can easily be F1 73 - 112 mph knocked from the foundation (Fundament) or toppled (kippen); motorists can be sent careening (schlittern) off road and possibly flipped over (umdrehen) Considerable damage: Well established (gegründet) trees are easily uprooted 113 - 157 (entwurzelt); mobile homes are torn apart; F2 mph entire roofs can be ripped off houses; train cars and trucks are knocked over; small objects become dangerous missiles Severe damage: Forests are destroyed as a majority of trees are ripped from the ground; 158 - 206 F3 entire trains are derailed (entgleisen) and mph knocked over; walls and roofs are torn from houses Devastating (verwüstend) damage: Houses and 207 - 260 other small structures can be razed F4 mph (niederreißen) entirely; automobiles are propelled (antreiben) through the air. Incredible damage: Cars become projectiles as they are hurled (schleudern) through the air; entire houses are completely destroyed after 261 - 318 F5 being ripped from the foundation and sent mph tumbling (purzeln) into the distance; steel- reinforced concrete structures (Stahlbetonbau) can be seriously damaged. licensed under * Page 8 of 8