How Hurricanes & Tornados Work

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

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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



• 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.



• 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.



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



(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.



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:



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.


How Hurricanes & Tornados Work

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