Every year between June 1 and November 30 (commonly called
hurricane season), hurricanes threaten 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. These huge storms wreak havoc when they make
landfall. They can kill thousands of people and cause billions
of dollars of property damage when they hit heavily populated
areas.
Photo courtesy Weather.com,
photographer Stuart Livingston Destructive waves from Hurricane Opal (1995)
at the State Pier in Gulf Shores,
AL
In this edition of HowStuffWorks,
we'll define a hurricane and its parts. We'll discuss how
hurricanes differ from other tropical storms, how they move,
the destruction and damage they cause, and how hurricanes are
tracked. 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 term used for low-pressure systems
that develop in the tropics.
"Tropical cyclones with maximum sustained
surface winds of less than 17 meters per second (39 mph /
62.7 kph / 34 knots) are called tropical depressions.
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 / 64
kt), then it is called a "hurricane."
"Hurricane"
According to the National Hurricane Center, the
word "hurricane" comes from the name "Hurican," the
Caribbean god of
evil.
Hurricanes are defined by the following characteristics:
They are tropical, meaning that they are
generated in tropical areas of the ocean near the Equator.
They are cyclonic, meaning that their winds swirl
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. 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 of at least 74 mph (119 kph / 64 kt).
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 equatorial winds. Most Atlantic hurricanes begin
off the west coast of Africa, starting as thunderstorms that
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 / 33 kt)
Tropical storm - wind speeds of 39 to 73 mph
(54.7 to 117.5 kph / 34 to 63 kt)
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-condensation cycle of warm,
humid ocean air
Patterns of wind characterized by converging
winds at the surface and strong, uniform-speed winds at
higher altitudes
A difference in air pressure (pressure gradient)
between the surface and high altitude
Warm, Humid Ocean
Air Warm, moist air from the ocean surface begins to
rise rapidly. As this warm air rises, its water vapor
condenses to form storm clouds and droplets of rain. The
condensation releases heat called latent heat of
condensation. This latent heat warms the cool air aloft,
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.
Photo courtesy NASA This photo is a composite of three days'
views (August 23, 24 and 25, 1992) of Hurricane Andrew
as it slowly moved across south Florida from east to
west.
Patterns of
Wind "Converging winds" are winds moving in
different directions that run into each other. Converging
winds at the surface collide and push warm, moist air upward.
This rising air reinforces 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 are
present -- the storm loses organization and weakens.
Pressure
Gradient 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 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 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
Source: NASA
Observatorium
For an interesting look at the anatomy and birth of a
hurricane, see Hurricane
Creation. Also, you can Create
a Hurricane and experiment with the various factors that
affect hurricane formation.
Size and Location Hurricanes vary widely in
physical size. Some storms are very compact and have only a
few trailing bands of wind and rain behind them. Other storms
are looser, so the bands 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.
Photo courtesy NASA/GSFC Hurricane Floyd was a Category 3 storm that
brought intense rains and record flooding to the eastern
United States and Canada. Nearly 90 percent of the
fatalities associated with this storm were drownings due
to inland flooding.
Photo courtesy National Oceanic &
Atmospheric Administration (NOAA) Hurricane Bertha (July 1996) was also a
Category 3 storm, but Bertha's power and impact were
contained in a much smaller area than
Floyd's.
Weather Alerts According
to The
Weather Channel Online, there are a four weather alerts
for tropical storms and hurricanes. Depending on where you are
located in proximity to the storm, you may find yourself under
one of these alerts:
Tropical-storm watch
Tropical-storm warning
Hurricane watch
Hurricane warning
A tropical-storm watch is issued when sustained
winds from 39 to 73 mph (54.7 to 117.48 kph / 34 to 63 kt) are
possible in your area within 36 hours. A tropical-storm
warning indicates that these conditions are likely in your
area within 24 hours.
A hurricane watch is issued when hurricane
conditions (sustained winds greater than 74 mph / 119 kph / 64
kt) are possible in your area within 36 hours. A hurricane
warning is issued when these conditions are likely in your
area within 24 hours.
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 dozens of inches of rain in just a
day of two, much of it inland. That amount of rain can
create inland flooding that can totally devastate a large
area around the hurricane's center.
Photo courtesy U.S. Army Corps of
Engineers Many communities
and small towns in eastern North Carolina found
themselves afloat following Hurricane Floyd's massive
rainfall and the ensuing floods. Streams and waterways
were already swollen from Hurricane Dennis, which struck
eastern North Carolina (twice) just weeks before
Hurricane
Floyd.
High sustained winds cause structural damage.
These winds can also roll cars, blow over trees and erode
beaches (both by blowing sand and by blowing
the waves into the beach).
Photo courtesy U.S. Federal Emergency Management
Agency (FEMA) Hurricanes
often cause severe structural damage. This building, in
the U.S. Virgin Islands, was
leveled.
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, it causes beach erosion and significant inland
flooding.
Photo courtesy FEMA Ocean-front property is particularly
susceptible to damage from hurricane storm surges, when
ocean waves are very high and strong because of the
strength of the hurricane's
winds.
Hurricane winds often spawn tornadoes,
which are smaller, more intense cyclonic storms that cause
additional damage.
Whether the storm comes ashore head-on or just grazes
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
there. On the left side, the hurricane's speed of motion
subtracts 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 of dollars.
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
1
74 to 95
mph (119 to 153 kph)
Storm surge 4 to 5 ft (1.2 to 1.5 m) above normal
Some flooding
Little or no structural
damage
2
96 to 110
mph (155 to 177 kph)
Storm surge 6 to 8 ft (1.8 to 2.4 m) above normal
Trees down
Roof damage (shingles ripped
off)
3
111 to 130
mph (178.6 to 209 kph)
Storm surge 9 to 12 ft (2.7 to 3.7 m) above normal
Structural damage in houses
Mobile homes destroyed
Severe flooding
4
131 to 154
mph (210 to 247.8 kph)
Storm surge 13 to 18 ft (4 to 5.5 m) above normal
Severe flooding inland
Some roofs ripped off
Major structural damage
5
>155
mph (> 249.4 kph)
Storm surge at least 18 ft (5.5 m) above normal
Severe flooding further 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.
Tracking a Hurricane
Photo courtesy U.S. Air Force / Hurricane
Hunters The eye of a
hurricane, as seen from the window of a
hurricane-surveillance
plane.
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 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.
Photo courtesy U.S. Air Force / Hurricane
Hunters This WC-130H is one
of the planes used by the Hurricane Hunters to fly into
storms and hurricanes. The plane is equipped with
special weather equipment to gather data inside the
storm.
Weather satellites
use different sensors to gather different types of information
about hurricanes:
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. Our
modern system (tracking, early detection, warnings) has
greatly reduced the loss of life during a hurricane. For an
account of hurricanes before our modern system, read "Isaac's
Storm: a Man, a Time, and the Deadliest Hurricane in
History," by Erik Larson.
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 saints
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 2001 hurricane season was a Pacific Ocean
storm near Acapulco, Mexico, named Adolf. The first Atlantic
storm of the 2001 season will be named Allison. A list of
names through 2006 is available from the National
Hurricane Center.
According to the NOAA:
"Whenever a hurricane has had a major impact,
any country affected by the storm can request that the name
of the hurricane be retired by agreement of the World
Meteorological Organization (WMO). Retiring a name actually
means that it cannot be reused for at least 10 years, to
facilitate historic references, legal actions, insurance
claim activities, etc. and avoid public confusion with
another storm of the same name."
For a list of hurricane names that have been retired, check
out this
site.
