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Get answers to general questions about tropical cyclones.


*DEFINITION ...*

Cyclone is the generic term used for all weather systems generating converging spiraling winds around a low atmospheric pressure area such as:
      polar low, mid-latitude low, extra-tropical cyclone, sub-tropical cyclone, torndado and, water spout, tropical cyclone.

Tropical cyclones are classed as cyclones forming only over warm tropical waters and having their highest winds concentrated right around the center, producing flooding rains and srtong winds.
They are classed into different levels of intensity:
                                                          -Tropical depression (35 m.p.h winds)
                                                          -Tropical storm (40 to 73 m.p.h winds)
                                                          -Hurricane (winds above 73 m.p.h)

The names of tropical cyclones with sustained winds of 74 m.p.h and higher can differe from one oceanic bassin to another:
                                -Atlantic: HURRICANES
                                -Eastern Pacific: HURRICANES
                                -Western Pacific: TYPHONS
                                -Australia: TROPICAL CYCLONES

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*CLASSIFICATION ...*

Tropical cyclones undergo different stages of intensification:
      - Tropical waves
      - Tropical depression
      - Tropical storm
      - and finaly, Hurricanes.

The National Hurricane Center at Miami uses the SAFFIR-SIMPSON scale to classify the intensities and related potential damage of hurricanes, both in the Atlantic and Eastern Pacific.


Saffir-Simpson category	Max. sust. winds (m.p.h)	Min. surface pressure (mb)	Storm surge (feet)	Level	Potential damage	Example
I	74-95	980 and higher	3-5	minimal	1	Marco (1996)
II	96-110	979-965	6-8	moderate	10	Jose (1999)
III	111-130	964-945	9-12	extensive	50	Bonnie (1998)
IV	131-155	944-920	13-18	extreme	100	Lenny (1999)
V	156 and higher	lower than 920	greater than 18	catastrophic	250	Ivan (2004)

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*PHASES OF DEVELOPMENT ...*


Tropical cyclones can develop from different weather disturbances: - a *cold front stalled over the warm waters of the Gulf-Stream - occluded front* - *upper tropospheric systems* that can slowy develop surface features - mid latitude *Mesoscale Convective Complexe* (MCC), - and *tropical waves. TROPICAL WAVE* Note that 60% of tropical storms and 85% of intense hurricanes come from tropical waves. Tropical waves is cause by in instability (called *Barotropic-baroclinic instability) of the eastern African Jet Stream, due to the difference in temperature between the Sahara (very hot and dry) and the Gulf of Guinea (cooler but humid). These thunderstorm complexes will originate in the heart land of central equatorial Africa then travel westwards towards the coast. As the MCC* moves over the waters, the tropical wave will likely loose some of its convection but is identifiable by a curvature of the isobars, so as the winds and an area where humidity is relatively higher than the surrounding air. Well developed T. waves can have a low pressure area embeded in the thunderstorm zone. If the pressure is low enough (lower than 1010 mb) the wave takes the name of a *Tropical Low. Winds begin to spiral towards the low pressure area, a cyclonic spin* is then observe in the wind field. *TROPICAL DEPRESSION* If there is a persistancy in convection and several *spiral bands* organizing around the center of low pressure, and cyclonic spin (rotation) becomes evident on satellite imagery, then the Tropical Low will be upgrade to a Tropical depression. Maximum sustained winds near the center of circulation will be 30 or 35 m.p.h. Higher gusts can be felt in *squalls, rough seas (up to 14 foot sweels) and mainly heavy rain (up to 20 inches of rain can be produce by a slowly moving well-developed tropical depression) TROPICAL STORM* Convection (thunderstorms) gets organize around the center of low pressure, forming a cluster of dense and high cumulonimbus clouds (*Central Dense Overcast : CDO). The inflow* (converging air at sea level) and *outflow aloft* (diverging air in the upper tropospher) become visible in all quadrent of the cyclone, the clouds get taller (this is indicated by cooler cloud tops read from infra-red satellite imagery) and estimated pressure drops (generaly below 1004 mb) then this cyclone will be classified as a tropical storm. Maximum sustained winds range from 39 to 73 m.p.h. Strong wind gusts are possible. Storms can produce torrential rain (up to 15 inches of rain has already been observed during the passage of a tropical storm). Due to lower atmospheric pressure within the center and strong swirling winds, the sea level rises 1 foot or two above normal. Added to that *storm surge, tropical storms can generate hazadous sea conditions with waves reaching up to 20 feet. HURRICANE* The cyclone continues to move into a favorable environment and continues to intensify. The number of spiral bands increases and the CDO get a more *circular shape* (this indicates that the system is getting better *organize). Cloud tops temperature continue to drop. Infra-red imagery begins to show a ring of dense and very high cumulonimbus* clouds developing around the center of circulation. The NHC at Miami classifies the cyclone as a hurricane. Maximum sustained winds reach a minimal value of 74 m.p.h, and pressure lower than 990 mb. (click here to read about a Cap Verde cyclone: Alberto) (click here to read about a Barbadian cyclone: Debby)

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*STRUCTURE ...*

A hurricane is a huge thermodynamic machine composed of spiral bands of thick clouds which get more dense as you reach closer to the center of the storm.
Here is the structure of a fully developed (category 4 to 5) hurricane:


1: peripheral cumulus and cumulonimbus clouds.
These are isolated clouds that are located on the edge or periphery of the cyclone. Some cumulonimbus can produce a squall or two ahead of the system. Generally this bad weather is announced by a "vail" of fine elongated clouds high in the sky forming the cirrus layer (seen here by number 7).
2: body of the cyclone.

Spiral bands of cumulonimbus get closer to each other. Because of the dynamics and air instability these clouds protuberate above the cirrus

layer, forming powerful thundersom cells producing tropical storm like condtions (intermitent showers with winds gusting up to 60 mph).


3: core of the cyclone.
Cyclonic type cumulonimbus can reach as high as 18 000 meters high (59 000 feet). The associated pressure gradient (or difference between the high pressure zone outside and beyond the cyclone and the low pressure area within the storm) produce stiff winds, sustained up to 70 mph and gusting to 90 mph. Battering heavy rain can reduce visibility down to 50%.
4: inner core of the cyclone.
This is usually 2 or 3 tightly bonded rings of intense and mature cyclonic type cumulonimbus clouds producing up to 70% of the total rainfall of the cyclone. Atmospheric pressure can drop as low as 930 hPa (930 mb) within this section of the system and winds can hurl up to 140 mph with gusts to 160.
5: eye wall of the cyclone
This is the most inner ring of cyclonic cumulonimbus of the cyclone. Within this inner ring winds rotate at warping speeds before being projected upward. This creates a tremendous suction force or low pressure near the sea surface. Pressure has know to have droped to as low as 888 hPa (compare to the usual 1014 hPa air pressure of everyday life). Reading wind speeds with anemometers become difficult to near impossible when the air charged with rain moves at speeds in excess of 185 mph. Some "super cyclones" are known to have sustained winds of 190 mph and dreadful blasts of up to 220 mph within their eye wall !
6: eye of the cyclone
Inside this zone there is little to no clouds. Weather is calm and the air actually sinks. The eye of powerful hurricanes is sahpe like a "V" (wide at the top and narrow at the surface). The smaller the eye is in strong cyclones the more powerful it will be.
7: exaust of the cyclone.
The large quantity of rising air mass within the eye wall is spread out horizontally in a clockwise motion on top of the cyclone.

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*CONDITIONS ...*

A hurricane does not form spontaniously, it goes through different developing stages. But some minimal conditions are required:


The tropical wave or tropical low must have a low pressure area and a well defined cyclonic spin at sea level. To preserve the low pressure center and to induce barometric fall a cerain concentration of thunderstorms (convection) has to develop around and over the center. This can only be done if there is 0 vertical shear, a diverging (or weak ridge) must be in place over the tropical system.

For convection to be "explosive" (dynamic) warm sea surface temperature (at least 26.5°C, 28°C is ideal), very moist (humid) air in the lower and mid troposphere (above 60% relative humidity) and cool air aloft, are required

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