Tropical Storm /
Cyclone
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What is a Tropical Storm or Tropical Cyclone?
In
meteorology, a tropical cyclone (or
tropical storm, typhoon
or hurricane, depending on
strength and location) is a type of
low-pressure system which generally forms in the tropics. While some,
particularly those that make landfall in populated areas, are regarded
as highly destructive, tropical cyclones are an important part of the
atmospheric circulation system, which moves heat from the equatorial
region toward the higher latitudes.
A heat engine
Structurally,
a tropical cyclone is a large,
rotating area of clouds,
wind, and thunderstorm activity. The primary energy source of a
tropical cyclone is the release of heat of condensation from water
vapor condensing at high altitudes. Because of this, a tropical cyclone
can be thought of as a giant vertical heat engine.
The ingredients
for a tropical cyclone include a pre-existing weather
disturbance, warm tropical oceans, moisture, and relatively light winds
aloft. If the right conditions persist long enough, they can combine to
produce the violent winds, incredible waves, torrential rains, and
floods associated with this phenomenon.
This use of
condensation as a driving force is the primary difference
setting tropical cyclones apart from other meteorological phenomena,
such as mid-latitude cyclones, which draw energy mostly from
pre-existing temperature gradients in the atmosphere. To drive its heat
engine, a tropical cyclone must stay over warm water, which provides
the atmospheric moisture needed. The evaporation of this moisture is
driven by the high winds and reduced atmospheric pressure present in
the storm, resulting in a sustaining cycle.
Classification and
terminology
Tropical
cyclones are classified into three
main groups: tropical
depressions, tropical storms, and a third group whose name depends on
the region.
A tropical
depression is an organized system of clouds and
thunderstorms with a defined surface circulation and maximum sustained
winds of less than 17 metres per second (33 knots, 38 mph, or 62 km/h).
It has no eye, and does not typically have the spiral shape of more
powerful storms.
A tropical storm
is an organized system of strong thunderstorms with a
defined surface circulation and maximum sustained winds between 17 and
33 meters per second (34 to 63 knots, 39 to 73 mph, or 62 to 117 km/h).
At this point, the distinctive cyclonic shape starts to develop, though
an eye is usually not present.
The term used to
describe tropical cyclones with maximum sustained
winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h)
varies depending on region of origin, as follows:
- hurricane in
the North Atlantic Ocean, North Pacific Ocean east of the dateline, and
the South Pacific Ocean east of 160°E
typhoon in the Northwest Pacific Ocean west of the dateline
- severe tropical cyclone in the Southwest
Pacific Ocean west of 160°E or Southeast Indian Ocean east of
90°E
- severe cyclonic storm in the North Indian Ocean
- tropical cyclone in the Southwest Indian Ocean
This is the
intensity at which tropical cyclones tend to develop an
eye, which is an area of relative calm surrounded by the strongest
winds of the storm, in the eyewall. The strongest of these storms have
had maximum sustained windspeeds recorded at 85 meters per second (165
knot, 190 mph, 305 km/h).
In other places
in the world, hurricanes
have been called Bagyo in the Philippines, Chubasco in Mexico, and
Taino in Haiti.
Hurricanes
are categorized on a 1-to-5 scale according to the strength of their
winds, using the Saffir-Simpson Hurricane Scale. A Category 1 storm has
the lowest wind speeds, while a Category 5 hurricane has the strongest.
These are relative terms, because lower category storms can sometimes
inflict greater damage than higher category storms, depending on where
they strike and the particular hazards they bring. In fact, tropical
storms can also produce significant damage and loss of life, mainly due
to flooding.
The U.S. National
Hurricane
Center classifies hurricanes of Category 3 or above as Major
Hurricanes. The Joint Typhoon Warning Center classifies typhoons with
wind speeds of at least 150 mi/h (67 m/s or 241 km/h; a strong Category
4 storm) as Super Typhoons.
The definition of
sustained winds recommended by the World
Meteorological Organization (WMO) is that of a ten-minute average, and
that definition is adopted by most countries. However, a few countries
use different definitions: the United States, for example, defines
sustained winds based on a 1-minute average wind measured at about 10
meters (33 ft) above the surface.
An extratropical
cyclone is a storm that was once tropical in nature.
However, once it passed over land or cool waters, its energy source
changed from released heat from condensing water to the difference in
temperature between air masses. From space, these storms resemble a
comma. Extratropical cyclones still can be dangerous because their
continuing low pressure causes powerful winds.
In the United
Kingdom and Europe, some severe northeast Atlantic
cyclonic depressions are referred to as "hurricanes," even though they
rarely originate in the tropics. These European windstorms can generate
hurricane-force windspeeds but are not given individual names. In
British shipping forecasts, winds of force 12 on the Beaufort scale are
described as "hurricane force".
There is also a
polar counterpart to the tropical cyclone, called an
arctic cyclone.
Location
Hurricane Anita
approaching landfall on the coast of Mexico in September 1977.
Nearly all
tropical cyclones form within 30 degrees of the equator and
87% form within 20 degrees of it. Since the Coriolis effect initiates
and maintains tropical cyclone rotation, such cyclones almost never
form or move within about 10 degrees of the equator [1] (where the
Coriolis effect is weakest). However, it is possible for tropical
cyclones to form within this boundary if another source of initial
rotation is provided. These conditions are extremely rare and such
storms are believed to form at a rate of less than one a century.
Most tropical
cyclones form in a worldwide band of thunderstorm
activity known as the Intertropical convergence zone (ITCZ).
Worldwide, an
average of 80 tropical cyclones form each year.
Major basins
There are
seven main basins of tropical
cyclone formation:
- Western North Pacific Ocean: Tropical storm
activity in this region
frequently affects China, Japan, the Philippines, and Taiwan. This is
by far the most active basin, accounting for one third of all tropical
cyclone activity in the world. National meteorology organizations, as
well as the Joint Typhoon Warning Center (JTWC) are responsible for
issuing forecasts and warnings in this basin.
- Eastern North Pacific Ocean: This is the second
most active basin in
the world, and is also the most dense (a large number of storms for a
small area of ocean). Storms which form in this basin can affect
western Mexico, Hawaii and on extremely rare occasions, California. The
Central Pacific Hurricane Center is responsible for forecasting the
western part of this area, and the National Hurricane Center for the
eastern part.
- South Western Pacific Ocean: Tropical activity
in this region largely
affects Australia and Oceania, and is forecast by Australia and New
Guinea.
- Northern Indian Ocean: This basin is actually
divided into two areas,
the Bay of Bengal and the Arabian Sea, with the Bay of Bengal
dominating (5 to 6 times more activity). Hurricanes which form in this
basin have historically cost the most lives — most notably, the Bhola
Cyclone of 1970 killed 200,000. Nations affected by this basin include
India, Bangladesh, Sri Lanka, Thailand, Burma, and Pakistan, and all of
these countries issue region forecasts and warnings. Rarely, a tropical
cyclone formed in this basin will affect the Arabian Peninsula.
- Southeastern Indian Ocean: Tropical activity in
this region affects Australia and Indonesia, and is forecast by those
nations.
- Southwestern Indian Ocean: This basin is the
least understood, due to a
lack of historical data. Cyclones forming here impact Madagascar,
Mozambique, Mauritius, and Kenya, and these nations issue forecasts and
warnings for the basin.
- North Atlantic Basin: The most well studied of
all tropical basins, the
North Atlantic includes the Atlantic Ocean, the Caribbean Sea, and the
Gulf of Mexico. Tropical cyclone formation here varies widely year to
year, ranging from over twenty to just one. The average is ten. The
United States, Mexico, Central America, the Caribbean Islands and
Canada are affected by storms in this basin. Forecasts for all storms
are issued by the National Hurricane Center based in Miami, Florida;
the Canadian Hurricane Centre, based in Halifax, Nova Scotia, also
issues forecasts and warnings for storms expected to affect Canadian
territory and waters. Hurricanes that strike Mexico, Central America,
and Caribbean island nations, often do intense damage: they are
deadlier when over warmer water, and the United States is better able
to evacuate people from threatened areas than many other nations. Many
of the more intense Atlantic storms are Cape Verde-type hurricanes,
forming just west of Africa near the Cape Verde islands.
Unusual formation
areas
The
following areas spawn tropical cyclones
only very rarely.
- Southern Atlantic Ocean: A combination of
cooler waters, the lack of an
Inter-tropical Convergence Zone, and wind shear makes it very difficult
for the Southern Atlantic to support tropical activity. However, three
tropical cyclones have been observed here — a weak tropical storm in
1991 off the coast of Africa, Cyclone Catarina (sometimes also referred
to as Aldonça), which made landfall in Brazil in 2004, and a
smaller
storm in January of 2004, east of Salvador, Brazil. The January storm
is thought to have reached tropical storm intensity based on
scatterometre winds.
- Central North Pacific: Shear in this area of
the Pacific Ocean severely
limits tropical development. However, this region is commonly
frequented by tropical cyclones that form in the much more favorable
Eastern North Pacific Basin.
- Mediterranean Sea: Storms which appear similar
to tropical cyclones in
structure sometimes occur in the Mediterranean basin. Such cyclones
formed in September 1947, September 1969, January 1982, September 1983,
and January 1995. There is debate on whether these storms were tropical
in nature.
Timing
Worldwide,
tropical cyclone activity peaks in
late summer when water
temperatures are warmest. However, each particular basin has its own
seasonal patterns.
In the north
Atlantic, a distinct hurricane season occurs from June 1
to November 30, sharply peaking in early September. The northeast
Pacific has a broader period of activity, but in a similar timeframe to
the Atlantic. The northwest Pacific sees tropical cyclones year-round,
with a minimum in February and a peak in early September. In the north
Indian basin, storms are most common from April to December, with peaks
in May and November.
In the southern
hemisphere, tropical cyclone activity begins in late
October, and ends in May. Southern hemisphere activity peaks in
mid-February to early March.
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