Notable Tropical Cyclones and Unusual Areas of Tropical Cyclone Formation

A flood is an overflow of an expanse of water that submerges land.[1] The EU Floods directive defines a flood as a temporary covering by water of land not normally covered by water.[2] In the sense of "flowing water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows or breaks levees, with the result that some of the water escapes its usual boundaries.[3]

While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless such escapes of water endanger land areas used by man like a village, city or other inhabited area.

Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bends or meanders. Floods often cause damage to homes and businesses if they are placed in natural flood plains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other bodies of water, since time out of mind, people have lived and worked by the water to seek sustenance and capitalize on the gains of cheap and easy travel and commerce by being near water. That humans continue to inhabit areas threatened by flood damage is evidence that the perceived value of living near the water exceeds the cost of repeated periodic flooding.

The word "flood" comes from the Old English flod, a word common to Germanic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float; also compare with Latin fluctus, flumen). Deluge myths are mythical stories of a great flood sent by a deity or deities to destroy civilization as an act of divine retribution, and are featured in the mythology of many cultures.

[edit]Principal types and causes

[edit]Riverine

. Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river's channel. Causes include heavy rains frommonsoons, hurricanes and tropical depressions, foreign winds and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice, or debris can cause slow flooding upstream of the obstruction. . Fast kinds: include flash floods resulting from convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, orglacier.

[edit]Estuarine

. Commonly caused by a combination of sea tidal surges caused by storm-force winds. A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

[edit]Coastal

. Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

[edit]Catastrophic . Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard (e.g. earthquake or volcanic eruption).

[edit]Muddy

. A muddy flood is generated by run off on crop land.

A muddy flood is produced by an accumulation of runoff generated on cropland. Sediments are then detached by runoff and carried as suspended matter or bed load. Muddy runoff is more likely detected when it reaches inhabited areas.

Muddy floods are therefore a hill slope process, and confusion with mudflows produced by mass movements should be avoided. [edit]Other

. Floods can occur if water accumulates across an impermeable surface (e.g. from rainfall) and cannot rapidly dissipate (i.e. gentle orientation or low evaporation). . A series of storms moving over the same area. . Dam -building beavers can flood low-lying urban and rural areas, often causing significant damage.

[edit]Effects

[edit]Primary effects

. Physical damage - Can damage any type of structure, including bridges, cars, buildings, sewerage systems, roadways, and canals. . Casualties - People and livestock die due to drowning. It can also lead to epidemics and waterborne diseases.

[edit]Secondary effects

. Water supplies - Contamination of water. Clean drinking water becomes scarce. . Diseases - Unhygienic conditions. Spread of water-borne diseases. . Crops and food supplies - Shortage of food crops can be caused due to loss of entire harvest.[4] However, lowlands near rivers depend upon river silt deposited by floods in order to add nutrients to the local soil. . Trees - Non-tolerant species can die from suffocation.[5] [edit]Tertiary/long-term effects

Economic - Economic hardship, due to: temporary decline in tourism, rebuilding costs, food shortage leading to price increase ,etc.

[edit]Control

[1] A tsunami (Japanese: 津波 [tsɯnami], lit. 'harbor wave'; English pronunciation: /suːˈnɑːmi/ s oo - [2] NAH -mee ) or tidal wave is a series of water waves (called a tsunami wave train ) caused by the displacement of a large volume of a body of water, usually an ocean, but can occur in large lakes. Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded.[3] Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions.

Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices),landslides and other mass movements, meteorite ocean impacts or similar impact events, and other disturbances above or below water all have the potential to generate a tsunami.

The Greek historian Thucydides was the first to relate tsunami to submarine earthquakes,[4][5] but understanding of tsunami's nature remained slim until the 20th century and is the subject of ongoing research. Many early geological, geographical, andoceanographic texts refer to tsunamis as "seismic sea waves."

Some meteorological conditions, such as deep depressions that cause tropical cyclones, can generate a storm surge, called ameteotsunami, which can raise tides several metres above normal levels. The displacement comes from low atmospheric pressure within the centre of the depression. As these storm surges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such a storm surge inundated Burma in May 2008. Etymology

The term tsunami comes from the Japanese, meaning "harbor" (tsu, 津) and "wave" (nami, 波). (For the plural, one can either follow ordinary English practice and add an s, or use an invariable plural as in the Japanese.[6])

Tsunami are sometimes referred to as tidal waves. In recent years, this term has fallen out of favor, especially in the scientific community, because tsunami actually have nothing to do with tides. The once-popular term derives from their most common appearance, which is that of an extraordinarily high tidal bore. Tsunami and tides both produce waves of water that move inland, but in the case of tsunami the inland movement of water is much greater and lasts for a longer period, giving the impression of an incredibly high tide. Although the meanings of "tidal" include "resembling"[7] or "having the form or character of"[8] the tides, and the term tsunami is no more accurate because tsunami are not limited to harbours, use of the termtidal wave is discouraged by geologists and oceanographers.

There are only a few other languages that have a native word for this disastrous wave. In the Tamil language, the word is aazhi peralai. In the Acehnese language, it is ië beuna or alôn buluëk [9] (Depending on the dialect. Note that in the fellow Austronesian language of Tagalog, a major language in the Philippines, alon means "wave".) On Simeulue island, off the western coast of Sumatra in Indonesia, in the Defayan language the word is smong, while in the Sigulai language it is emong.[10] Generation mechanisms

The principal generation mechanism (or cause) of a tsunami is the displacement of a substantial volume of water or perturbation of the sea.[11] This displacement of water is usually attributed to either earthquakes, landslides, volcanic eruptions, or more rarely by meteorites and nuclear tests.[12][13] The waves formed in this way are then sustained by gravity. It is important to note that tides do not play any part in the generation of tsunamis, hence referring to tsunamis as 'tidal waves' is inaccurate. Seismicity generated tsunamis

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position.[14] More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, due to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami.

Drawing of tectonic plate Overriding plate bulges boundary beforeearthquake. Plate slips, The energy released under strain, causingsubsidence and produces tsunami waves. causingtectonic uplift. releasing energy into water. Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds of kilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usually about 300 millimetres (12 in) above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.

On April 1, 1946, a magnitude-7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska. It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 metres (46 ft) high surge. The area where the earthquake occurred is where the Pacific Ocean floor is subducting (or being pushed downwards) under Alaska.

Examples of tsunami at locations away from convergent boundaries include Storegga about 8,000 years ago, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). The Grand Banks and Papua New Guinea tsunamis came from earthquakes which destabilized sediments, causing them to flow into the ocean and generate a tsunami. They dissipated before traveling transoceanic distances.

The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of the sediments, an earthquake or a release of gas hydrates (methane etc.)

The 1960 Valdivia earthquake (Mw 9.5) (19:11 hrs UTC), 1964 Alaska earthquake (Mw 9.2), and 2004 Indian Ocean earthquake (Mw 9.2) (00:58:53 UTC) are recent examples of powerfulmegathrust earthquakes that generated tsunamis (known as teletsunamis) that can cross entire oceans. Smaller (Mw 4.2) earthquakes in Japan can trigger tsunamis (called local andregional tsunamis) that can only devastate nearby coasts, but can do so in only a few minutes.

In the 1950s, it was discovered that larger tsunamis than had previously been believed possible could be caused by giant landslides. These phenomena rapidly displace large water volumes, as energy from falling debris or expansion transfers to the water at a rate faster than the water can absorb. Their existence was confirmed in 1958, when a giant landslide inLituya Bay, Alaska, caused the highest wave ever recorded, which had a height of 524 metres (over 1700 feet). The wave didn't travel far, as it struck land almost immediately. Two people fishing in the bay were killed, but another boat amazingly managed to ride the wave. Scientists named these waves megatsunami.

Scientists discovered that extremely large landslides from volcanic island collapses can generate megatsunami, that can travel trans-oceanic distances. Characteristics

When the wave enters shallow water, it slows down and its amplitude (height) increases.

The wave further slows and amplifies as it hits land. Only the largest waves crest.

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and a height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres (120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but due to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 metre (3.3 ft).[15] This makes tsunamis difficult to detect over deep water. Ships rarely notice their passage.

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses the wave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than 20 kilometres (12 mi) and its amplitude grows enormously, producing a distinctly visible wave. Since the wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for the very largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like a fast moving tidal bore.[16] Open bays and coastlines adjacent to very deep water may shape the tsunami further into a step-like wave with a steep-breaking front.

When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed 'run up'. Run up is measured in metres above a reference sea level.[16] A large tsunami may feature multiple waves arriving over a period of hours, with significant time between the wave crests. The first wave to reach the shore may not have the highest run up.[17]

About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies of water, including lakes. They are caused by earthquakes, landslides, volcanic explosions, and bolides. Drawback

If the first part of a tsunami to reach land is a trough—called a drawback—rather than a wave crest, the water along the shoreline recedes dramatically, exposing normally submerged areas.

A drawback occurs because the water propagates outwards with the trough of the wave at its front. Drawback begins before the wave arrives at an interval equal to half of the wave's period. Drawback can exceed hundreds of metres, and people unaware of the danger sometimes remain near the shore to satisfy their curiosity or to collect fish from the exposed seabed. During the Indian Ocean tsunami, the sea withdrew and many people went onto the exposed sea bed to investigate.[citation needed] Photos show people walking on the normally submerged areas with the advancing wave in the background.[citation needed] Few survived.[citation needed] Scales of intensity and magnitude

As with earthquakes, several attempts have been made to set up scales of tsunami intensity or magnitude to allow comparison between different events.[18] Intensity scales

The first scales used routinely to measure the intensity of tsunami were the Sieberg-Ambraseys scale, used in the Mediterranean Sea and the Imamura-Iida intensity scale, used in thePacific Ocean. The latter scale was modified by Soloviev, who calculated the Tsunami intensity I according to the formula

where Hav is the average wave height along the nearest coast. This scale, known as the Soloviev-Imamura tsunami intensity scale, is used in the global tsunami catalogues compiled by the NGDC/NOAA and the Novosibirsk Tsunami Laboratory as the main parameter for the size of the tsunami. Magnitude scales

The first scale that genuinely calculated a magnitude for a tsunami, rather than an intensity at a particular location was the ML scale proposed by Murty & Loomis based on the potential energy.[18] Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely used. Abe introduced the tsunami magnitude scale Mt, calculated from,

where h is the maximum tsunami-wave amplitude (in m) measured by a tide gauge at

a distance R from the epicenter, a, b & D are constants used to make the Mt scale match as closely as possible with the moment magnitude scale.[19] Warnings and predictions

See also: Tropical cyclone forecasting

The National Hurricane Center in the US, forecasts storm surge using the SLOSH model, which stands for Sea, Lake and Overland Surges from Hurricanes. The model is accurate to within 20 percent.[18] SLOSH inputs include the central pressure of a tropical cyclone, storm size, the cyclone's forward motion, its track, and maximum sustained winds. Local topography, bay and river orientation, depth of the sea bottom, astronomical tides, as well as other physical features are taken into account, in a predefined grid referred to as a SLOSH basin. Overlapping SLOSH basins are defined for the southern and eastern coastline of the continental U.S.[19] Some storm simulations use more than one SLOSH basin; for instance, Katrina SLOSH model runs used both the Lake Ponchartrain / New Orleans basin, and the Mississippi Sound basin, for the northern Gulf of Mexico landfall. The final output from the model run will display the maximum envelope of water, or MEOW, that occurred at each location. To allow for track or forecast uncertainties, usually several model runs with varying input parameters are generated to create a map of MOMs, or Maximum of Maximums.[20] And for hurricane evacuation studies, a family of storms with representative tracks for the region, and varying intensity, eye diameter, and speed, are modeled to produce worst-case water heights for any tropical cyclone occurrence. The results of these studies are typically generated from several thousand SLOSH runs. These studies have been completed by USACE, under contract to the Federal Emergency Management Agency, for several states and are available on their Hurricane Evacuation Studies (HES) website.[21] They include coastal county maps, shaded to identify the minimum SSHS category of hurricane that will result in flooding, in each area of the county.[22]

[edit]Mitigation

Although meteorological surveys alert about hurricanes or severe storms, in the areas where the risk of coastal flooding is particularly high, there are specific storm surge warnings. These have been implemented, for instance, in Holland,[23] Spain,[24][25] the United States,[26] [27] and Great Britain.[28]

A prophylactic method introduced after the North Sea Flood of 1953 is the construction of dams and floodgates (storm surge barriers). They are open and allow free passage but close when the land is under threat of a storm surge. Major storm surge barriers are the Oosterscheldekering and Maeslantkering in the Netherlands which are part of the Delta Works project, and the Thames Barrier protecting London.

Another modern development (in use in the Netherlands) is the creation of housing communities at the edges of wetlands with floating structures, restrained in position by vertical pylons. [29] Such wetlands can then be used to accommodate runoff and surges without causing damage to the structures while also protecting conventional structures at somewhat higher low-lying elevations, provided that dikes prevent major surge intrusion.

For mainland areas, storm surge is more of a threat when the storm strikes land from seaward, rather than approaching from landwards.[30] [edit]Notes

Limnic eruption From Wikipedia, the free encyclopedia

Lake Nyos, silty after a limnic eruption

A limnic eruption, also referred to as a lake overturn, is a rare type of natural disaster in which carbon dioxide (CO2) suddenly erupts from deep lake water, suffocating wildlife, livestock and humans. Such an eruption may also cause tsunamis in the lake as the rising CO2 displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption. Lakes in which such activity occurs may be known as limnically active lakes or exploding lakes. Some features of limnically active lakes include:

. CO2-saturated incoming water . A cool lake bottom indicating an absence of direct volcanic interaction with lake waters

. An upper and lower thermal layer with differing CO2 saturations . Proximity to areas with volcanic activity

Scientists have recently determined, from investigations into the mass casualties in the 1980s at Lake Monoun and Lake Nyos, that limnic eruptions and volcanic eruptions, although indirectly related, are actually separate types of disaster events.[1]

[edit]Historical occurrences Cow killed by the limnic eruption at Lake Nyos

To date, this phenomenon has been observed only twice. The first was in Cameroon at Lake Monoun in 1984, causing the asphyxiation and death of 37 people living nearby[2]. A second, deadlier eruption happened at neighbouring Lake Nyos in 1986, this time releasing over 80 [3] millioncubic meters of CO2 and killing between 1,700 and 1,800 people, again by asphyxiation .

Due to the nature of the event, it is hard to determine if limnic eruptions have happened elsewhere. However, a third lake — Lake Kivu — containing massive amounts of dissolved

CO2 exists on the border between the Democratic Republic of the Congo and Rwanda. Sample sediments from the lake were taken by Professor Robert Hecky from the University of Michigan which showed that an event caused living creatures in the lake to go extinct approximately every thousand years, and caused nearby vegetation to be swept back into the lake.

The Messel pit fossil deposits of Messel, Germany, show evidence of a limnic eruption there in the early Eocene. Among the victims are perfectly preserved insects, frogs, turtles, crocodiles, birds, anteaters, insectivores, early primates and paleotheres.

[edit]Causes For a limnic eruption to occur, the lake must be nearly saturated with gas. In the two known cases, the major component was CO2, however, in Lake Kivu, scientists are concerned about the concentrations of methane gas as well. This CO2 may come from volcanic gas emitted from under the lake or from decomposition of organic material. Before a lake is saturated, it behaves like an unopened carbonated beverage (soft drink): the CO2 is dissolved in the water. In both the lake and the soft drink, CO2 dissolves much more readily at higher pressure. This is why bubbles in a can of soda only form after the drink is open; the pressure is released and the

CO2 comes out of solution. In the case of lakes, the bottom is at a much higher pressure; the deeper it is, the higher the pressure at the bottom. This means that huge amounts of CO2 can be dissolved in large, deep lakes. Also, CO2 dissolves more readily in cooler water, such as that at the bottom of a lake. A small rise in watertemperature can lead to the release of a large amount of CO2.

Once the lake is saturated with CO2, it is very unstable. A trigger is all that is needed to set off an eruption. In the case of the 1986 eruption at Lake Nyos, landslides were the suspected triggers, but an actual volcanic eruption, an earthquake, or even wind and rain storms are other possible triggers. In any case, the trigger pushes some of the saturated water higher in the lake, where the pressure is insufficient to keep the CO2 in solution. Bubbles start forming and the water is lifted even higher in the lake (buoyancy), where even more of the CO2 comes out of solution. This process forms a column of gas. At this point the water at the bottom of this column is pulled up by suction, and it too loses its CO2 in a runaway process. This eruption pours

CO2 into the air and can also displace water to form a tsunami.

There are several reasons this type of eruption is very rare. First, there must be a source of the

CO2, so only regions with volcanic activity are at risk. Second, temperate lakes, such as North America's Great Lakes, turn over each spring and fall as a result of seasonal air temperature changes, mixing water from the bottom and top of the lake, so CO2 that builds up at the bottom of the lake is brought to the top where the pressure is too low for it to stay in solution and it escapes into the atmosphere. Finally, a lake must be quite deep to have enough pressure to dissolve large volumes of CO2. So only in deep, stable, tropical, volcanic lakes such as Lake Nyos are limnic eruptions possible.

[edit]Consequences

Once an eruption occurs, a large CO2 cloud forms above the lake and expands to the neighbouring region. Because CO2 is denser than air, it has a tendency to sink to the ground while pushing breathable air up. As a result, life forms that need to breathe oxygen suffocate once the CO2 cloud reaches them, as there is very little oxygen in the cloud. The CO2 can make human bodily fluids very acidic, potentially causing CO2 poisoning. As victims gasp for air they actually hurt themselves more by sucking in the CO2 gas.

At Lake Nyos, the gas cloud descended from the lake into a nearby village where it settled, killing nearly everyone. In this eruption, some people as far as 25 km (15.5 miles) from the lake died. A change in skin color on some bodies led scientists to think that the gas cloud may have contained a dissolved acid such as hydrogen chloride as well, but that hypothesis is disputed. [4] Many victims were found with blisters on their skin. This is believed to have been caused by pressure ulcers, which likely formed from the low levels of oxygen present in the blood of those asphyxiated by the carbon dioxide.[5] Thousands of cattle and wild animals were also asphyxiated, but no official counts were made. On the other hand, vegetation nearby was mostly unaffected except for that which grew immediately adjacent to the lake. There the vegetation was damaged or destroyed by a 5-meter (16.4 ft.) tsunami from the violent eruption.

[edit]A possible solution: Degassing lakes

A siphon used by French scientists in an attempt to degas Lake Nyos.

Efforts have been under way for several years to develop a solution to remove the gas from these lakes and prevent a build-up that could lead to another catastrophe. A team of French scientists began experimenting at Lake Monoun and Lake Nyos in 1990 using siphons to degas the waters of these lakes in a controlled manner. A pipe is positioned vertically in the lake with its upper end above the water's surface. Water saturated with CO2 enters the bottom of the pipe and rises to the top. The lower pressure at the surface allows the gas to come out of solution. Interestingly, only a small amount of water has to initially be mechanically pumped through the pipe to start the flow. As the saturated water rises, the CO2 comes out of solution and forms bubbles. The natural buoyancy of the bubbles draws the water up the pipe at high velocity causing a large fountain at the surface. The degassifying water acts as a pump, drawing more water into the bottom of the pipe, and creating a self-sustaining flow. This is the same process that leads to a natural eruption, but in this case it is controlled by the size of the pipe. Each pipe has a limited pumping capacity and several would be required for both Lake Monoun and Lake Nyos to degas a significant fraction of the deep lake water and render the lakes safe.

The deep lake waters are slightly acidic due to the dissolved CO2 which causes corrosion to the pipes and electronics, necessitating ongoing maintenance. There are also fears that the

CO2 from the pipes could settle on the surface of the lake forming a thin layer of unbreathable air and thus causing problems for wildlife.

In January 2001, a single pipe was installed on Lake Nyos. A second pipe was installed at Lake Monoun in late 2002. These two pipes are thought to be sufficient to prevent an increase in

CO2 levels, removing approximately the same amount of gas as that naturally entering at the lake bed. In January 2003, an 18-month project had been given approval to fully degas Lake Monoun.[6] The project appears to have been subsequently cancelled.[citation needed]

[edit]Lake Kivu's potential danger

Satellite image of Lake Kivu.

Lake Kivu is not only 2,000 times larger than Lake Nyos — it is also located in a far more densely populated area, with over two million people living along its shores. Fortunately, it has not reached a high level of CO2 saturation yet. If the water were to become heavily saturated, it could become an even greater risk to human and animal life, as it is located very close to a potential trigger, Mount Nyiragongo, an active volcano that erupted in January 2002. It is also located in an active earthquake zone and close to other active volcanoes.

While the lake could be degassed in a manner similar to Lake Monoun and Lake Nyos, due to the size of the lake and the volume of gas involved such an operation would be expensive, running into millions of dollars. A scheme initiated in 2010 to utilise methane trapped in the lake as a fuel source to generate electricity in Rwanda has led to a degree of CO2 degassing. [7] During the procedure for extracting the flammable gas used to fire power stations on the shore, some CO2 is removed in a process known as scrubbing. It is as yet unclear whether enough of the gas will be removed in this way to ensure the danger of a limnic eruption posed by Lake Kivu will be completely eliminated

Tropical Storm Allison From Wikipedia, the free encyclopedia

This article is about the Atlantic tropical storm of 2001. For other storms of the same name, see Tropical Storm Allison (disambiguation).

Tropical Storm Allison

tropical storm (SSHS)

Tropical Storm Allison on June 5, 2001

Formed June 4, 2001

Dissipat June 18, 2001 ed Highest winds

Lowest 1000 mbar (hPa; 29.53 inHg) pressure

Fatalitie 41 direct, 14 indirect s

Damage $5.5 billion (2001 USD) $6.82 billion (2010 USD)

Areas Texas (particularly affected aroundHouston), Louisiana, most of the Eastern United States

Part of the 2001 Atlantic hurricane season

Tropical Storm Allison was a tropical storm that devastated southeast Texas in June of the 2001 Atlantic hurricane season. The first storm of the season, Allison lasted unusually long for a June storm, remaining tropical or subtropical for 15 days. The storm developed from atropical wave in the northern Gulf of Mexico on June 4, 2001, and struck the upper Texas coast shortly thereafter. It drifted northward through the state, turned back to the south, and re- entered the Gulf of Mexico. The storm continued to the east-northeast, made landfall on Louisiana, then moved across the southeast United States and Mid-Atlantic. Allison was the first storm since Tropical Storm Frances in 1998 to strike the northern Texas coastline.[1]

The storm dropped heavy rainfall along its path, peaking at over 40 inches (1,000 mm) in Texas. The worst flooding occurred in Houston, where most of Allison's damage occurred: 30,000 became homeless after the storm flooded over 70,000 houses and destroyed 2,744 homes. Downtown Houston was inundated with flooding, causing severe damage to hospitals and businesses. Twenty-three people died in Texas. Throughout its entire path, Allison caused $5.5 billion ($6.7 billion 2008 USD) in damage and 41 deaths. Aside from Texas, the places worst hit were Louisiana and southeastern Pennsylvania.

Following the storm, President George W. Bush designated 75 counties along Allison's path as disaster areas (the first time he had to do so), which enabled the citizens affected to apply for aid. Allison is the only tropical storm to have its name retired without ever having reached hurricane strength.

[edit]Meteorological history Storm path

A tropical wave moved off the coast of Africa on May 21, 2001. It moved westward across theAtlantic Ocean, retaining little convection on its way. After moving across South America and the southwestern Caribbean Sea, the wave entered the eastern North Pacific Ocean on June 1. A low-level circulation developed on June 2, while it was about 230 miles (370 km) south- southeast of Salina Cruz, Mexico. Southerly flow forced the system northward, and the wave moved inland on June 3. The low-level circulation dissipated, though the mid-level circulation persisted. It emerged into the Gulf of Mexico on June 4, and developed deep convection on its eastern side.[2] Early on June 5, satellite imagery suggested that a tropical depression was forming in the northwest Gulf of Mexico, which was furthered by reports of wind gusts as high as 60 mph (95 km/h) just a few hundred feet above the surface, towards the east side of the system.[3]

Tropical Storm Allison on June 5 at peak strength

At 1200 UTC on June 5, the disturbance developed a broad, low-level circulation, and was classified as Tropical Storm Allison, the first storm of the 2001 Atlantic hurricane season. Some intensification was projected, though it was expected to be hindered by cool offshore sea surface temperatures.[4] Due to the cold-core nature of the center, Allison initially contained subtropical characteristics. Despite this, the storm quickly strengthened to attain peak sustained winds of 60 mph (95 km/h), with tropical storm-force winds extending up to 230 miles (370 km) east of the center, and a minimum central pressure of 1000 mbar.[2] The storm initially moved very little, and the presence of several small vortices from within the deep convection caused difficulty in determining the exact center location.[5]

Later in the day, several different track forecasts arose. One scenario had the cyclone tracking westward in to Mexico. Another projected the storm moving east towards southern Louisiana. At the time, it was noted that little rain or wind persisted near the center, but rather to the north and east.[6] Under the steering currents of a subtropical ridge that extended in an east–west orientation across the southeast United States,[5]Allison weakened while nearing the Texas coastline, and struck near Freeport, Texas with 50 mph (80 km/h) winds.[2] Inland, the storm rapidly weakened, and the National Hurricane Center discontinued advisories early on June 6. [7] Shortly after being downgraded to a tropical depression, surface observations showed an elongated circulation with a poorly defined center, which had reformed closer to the deep convection.[8]

Tropical Storm Allison with an eye-like feature over Mississippi

The depression drifted northward until reaching Lufkin, Texas, where it stalled due to a high pressure system to its north.[2] While stalling over Texas, the storm dropped excessive rainfall, peaking at just over 40 inches (1,033 mm) in northwestern Jefferson County.[9] On June 7, the subtropical ridge off Florida weakened, while the ridge west of Texas intensified. This steered Tropical Depression Allison to make a clockwise loop, and the storm began drifting to the southwest. As the center reached Huntsville, Texas, a heavy rain band began to back build from Louisiana westward into Liberty County, Texas, which had caused additional flooding.[10] At the time, the system had a minimum central pressure of about 1004 mb and maximum sustained winds of about 10 mph (16 km/h).[11]

Late on June 9 and early on June 10, Allison's remnants once again reached the Gulf of Mexico and emerged over open waters.[12] The low once again became nearly stationary about 60 mi (100 km) south of Galveston, Texas, and despite more favorable upper-level winds, it showed no signs of redevelopment.[13] Due to dry air and moderate westerly wind shear, the storm transformed into a subtropical cyclone. While the subtropical depression moved eastward, a new low level circulation redeveloped to the east, and Allison quickly made landfall on Morgan City, Louisiana on June 11.[2] At around the same time, the surface center reformed to the east- northeast of its previous location, aligning with the mid-level circulation. [14] Strong thunderstorms redeveloped over the circulation, and Allison strengthened into a subtropical storm over southeastern Louisiana.[2] The storm intensified further to attain sustained winds of 45 mph (70 km/h) and a minimum barometric pressure of about 1000 mb near Mclain, Mississippi, accompanied by a well-defined eye-like feature.[15]

Tropical Storm Allison over North Carolina

The storm was officially downgraded to a subtropical depression at 0000 UTC on June 12. Somewhat accelerating, the depression tracked to the east-northeast through Mississippi, Alabama, Georgia, and South Carolina before becoming nearly stationary near Wilmington, North Carolina.[2] The depression drifted through North Carolina and sped to the northeast for a time in response to an approaching cold front.[16]Though satellite and radar imagery show the system was well-organized, the system slowed and moved erratically for a period of time,[17]executing what appeared to be a small counterclockwise loop.[18]

The storm began tracking in a generally northeasterly direction, and crossed into the southern Delmarva Peninsula on June 16.[19] The subtropical remnants reached the Atlantic on June 17, and while located east of Atlantic City, New Jersey, winds began to restrengthen, and heavy rains formed to the north of the circulation. The low was interacting with a frontal boundary, and started merging with it, as it accelerated to the northeast at 13 mph (21 km/h). [20] The remnants of Allison briefly reintensified to a subtropical storm through baroclinic processes, though it became extratropical while south of Long Island. [2] By later on June 17, the low was situated off the coast of Rhode Island, spreading a swath of precipitation over New England.[21] The remnants of the tropical storm were then absorbed by the frontal boundary by June 18, and eventually passed south of Cape Race, Newfoundland on June 20, where the extratropical cyclone dissipated.[2]

[edit]Preparations

Main article: Effects of Tropical Storm Allison in Texas

Shortly after the storm formed, officials in Galveston County, Texas issued a voluntary evacuation for the western end of Galveston Island, as the area was not protected by the Galveston Seawall.[2] The ferry from the island to the Bolivar Peninsula was closed, while voluntary evacuations were issued in Surfside in Brazoria County.[22] When the National Hurricane Centerissued the first advisory on Allison, officials issued Tropical Storm Warnings from Sargent, Texas to Morgan City, Louisiana.[23] After the storm made landfall, flash flood watches and warnings were issued for numerous areas in eastern Texas.[24] During the flood event, the National Weather Service in Houston issued 99 flash flood warnings with an average lead time of 40 minutes. With an average lead time of 24 minutes, the National Weather Service in Lake Charles, Louisiana issued 47 flash flood warnings. With an average lead time of 39 minutes, the National Weather Service in New Orleans/Baton Rouge issued 87 flash flood warnings, of which 30 were not followed by a flash flood.[25]

In Tallahassee, Florida, a shelter opened the day prior to Allison's movement northward through the area, seven staff members housing 12 people. Two other shelters were on standby. Teams informed citizens in the Florida Panhandle of flood dangers.[26]

[edit]Impact

Tropical Storm Allison was a major flood disaster throughout its path from Texas to the Mid- Atlantic. The worst of the flooding occurred in Houston, Texas, where over 35 inches (890 mm) of rain fell. Allison killed 41 people, of which 27 drowned. The storm also caused over $5 billion in damage (2001 USD, $6.4 billion 2007 USD), making Allison the deadliest and costliest tropical storm on record in the United States.[2] [edit]Texas Main article: Effects of Tropical Storm Allison in Texas

Combined with waves on top, areas of Galveston Island experienced a wall of water 8 feet (2.5 m) in height, creating overwash along the coastline. The storm caused winds of up to 43 mph (69 km/h) at the Galveston Pier. While Allison was stalling over Texas, it dropped very heavy rainfall across the state.[2] Minimal beach erosion was reported.[27] Flash flooding continued for days,[10] with rainfall amounts across the state peaking at just over 40 inches (1,033 mm) in northwestern Jefferson County. In the Port of Houston, a total of 36.99 inches (940 mm) was reported.[9] Houston experienced torrential rainfall in a short amount of time. The six-day rainfall in Houston amounted to 38.6 inches (980 mm).[28] The deluge of rainfall flooded 95,000 automobiles and 73,000 houses throughout Harris County.[1] Tropical Storm Allison destroyed 2,744 homes, leaving 30,000 homeless with residential damages totaling to $1.76 billion (2001 USD, $2.05 billion 2007 USD).[28]

The Southwest Freeway, near Downtown Houston, lies under water due to flooding from Tropical Storm Allison

Several hospitals in the Texas Medical Center, the largest medical complex in the world, experienced severe damage from the storm, which hit quickly and with unexpected fury on a Friday evening. The Baylor College of Medicine experienced major damage, totaling $495 million (2001 USD, $577 million 2007 USD). The medical school lost 90,000 research animals, 60,000 tumor samples, and 25 years of research data.The University of Texas Health Science Center at Houston, across the street, lost thousands of laboratory animals. Throughout the Medical Center, damage totaled to over $2 billion (2001 USD, $2.3 billion 2007 USD).[28] Buffalo Bayou and White Oak Bayou at Main Street after Tropical Storm Allison hitHouston

The underground tunnel system, which connects most large office buildings in downtown Houston, was submerged, as were many streets and parking garages adjacent to Buffalo Bayou. At the Theatre District, also in downtown, the Houston Symphony, Houston Grand Opera, and Alley Theater lost millions of dollars of costumes, musical instruments, sheet music, archives and other artifacts. By midnight on June 9 nearly every freeway and major road in the city was under several feet of water, forcing hundreds of motorists to abandon their vehicles for higher ground.[28]

Despite massive flooding damage to entire neighborhoods there were no drowning deaths in flooded homes. In the area, there were 12 deaths from driving, 6 from walking, 3 from electrocution, and 1 in an elevator.[1] Elsewhere in Texas, a man drowned when swimming in a ditch in Mauriceville.[25] Damage totaled to $5.2 billion (2001 USD, $6 billion 2007 USD) throughout Texas.[29]

[edit]Louisiana

Flooding in Chackbay, Louisiana While making its first landfall, Allison's large circulation dropped severe rains on southwest Louisiana.[30] Days later, Allison hit the state as a subtropical storm, dropping more heavy rains to the area. Rainfall totals peaked at 29.86 inches (758 mm) in Thibodaux, the highest rainfall total in Louisiana from a tropical cyclone since another Tropical Storm Allison in 1989.[31] Most of the southeastern portion of the state experienced over 10 inches of rain (255 mm).[9] Winds were generally light, peaking at 38 mph (61 km/h) sustained in Lakefront with gusts to 53 mph (85 km/h) in Bay Gardene. The storm produced a storm surge of 2.5 feet (0.75 m) in Cameron as it was making landfall in Texas.[2] While moving northward through Texas, the outer bands of the storm produced anF1 tornado near Zachary, damaging several trees and a power line. A man was killed when a damaged power line hit his truck.[32]

When Allison first made landfall, heavy rainfall flooded numerous houses and businesses. Minor wind gusts caused minor roof damage to 10 houses in Cameron Parish, while its storm surge flooded portions of Louisiana Highway 82.[33] When the system returned, more rainfall occurred, flooding over 1,000 houses in St. Tammany Parish,[34] 80 houses in Saint Bernard Parish,[35] and hundreds of houses elsewhere in the state. The flooding also forced 1,800 residents from their homes in East Baton Rouge Parish.[36] The deluge left numerous roads impassable, while runoff resulted in severe river flooding. The Bogue Falaya River in Covington crested past its peak twice to near record levels.[34] The Amite and Comite Rivers reached their highest levels since 1983. In addition, the levee along the Bayou Manchac broke, flooding roadways and more houses.[36] Damage in Louisiana totaled to $65 million (2001 USD, $75 million 2007 USD).[28]

[edit]Southeast United States

Rainfall totals from Allison In Mississippi, Allison produced heavy rainfall of over 10 inches (255 mm) in one night,[37] while some areas in the southwestern portion of the state received over 15 inches (380 mm).[9] The flooding damaged numerous houses and flooded many roadways.[37] Thunderstorms from the storm produced four tornadoes,[2] including one in Gulfport, Mississippi that damaged 10 houses.[38] Severe thunderstorms in George Countydamaged 15 houses, destroyed 10, and injured five people.[39] Damage in Mississippi totaled to over $1 million (2001 USD, $1.2 million 2007 USD).[37][38][39] Rainfall in Alabama was moderate, with areas near Mobile experiencing more than 10 inches (255 mm).[9] Heavy rainfall closed several roads in Crenshaw County. [40] The storm, combined with a high pressure, produced coastal flooding in southern Alabama. [41]Allison produced an F0 tornado in southwest Mobile County that caused minor roof damage[42] and another F0 tornado in Covington Countythat caused minor damage to six homes and a church.[42]

The storm, combined with a high pressure system, produced a strong pressure gradient, resulting in strong rip currents off the coast of Florida. The currents prompted sirens, which are normally used for storm warnings, to be activated in Pensacola Beach.[43] The rip currents killed 5 off the coast of Florida.[44] Outer rain bands from the storm dropped heavy rainfall across the Florida Panhandle of over 11 inches (280 mm) in one day. The Tallahassee Regional Airport recorded 10.13 inches (257 mm) in 24 hours, breaking the old 24 hour record set in 1969.[45] Throughout the state, Allison destroyed 10 homes and damaged 599, 196 severely, primarily in Leon County.[46] Including the deaths from rip currents, Allison killed eight people in Florida[2] and caused $20 million (2001 USD, $23 million 2007 USD) in damage.[45]

Over Georgia, the storm dropped heavy rainfall of 10 inches (255 mm) in 24 hours in various locations. The deluge caused rivers to crest past their banks, including the Oconee River at Milledgeville which peaked at 33.7 feet (10.3 m). The rainfall, which was heaviest across the southwestern portion of the state, washed out several bridges and roads, and flooded many other roads. Georgia governor Roy Barnes declared a state of emergency for seven counties in the state.[47] The storm also spawned two tornadoes.[2] In South Carolina, Allison's outer bands produced 10 tornadoes [2] and several funnel clouds, though most only caused minor damage limited to a damaged courthouse, snapped trees[48] and downed power lines.[49] Allison produced from 12 to 16 inches (305 to 406 mm) of rainfall in North Carolina, closing nearly all roads in Martin County and damaging 25 homes.[50] The severe flooding washed out a bridge in eastern Halifax County [51] and flooded numerous cars.[31] Wet roads caused nine traffic accidents throughout the state.[2] [edit]Mid-Atlantic and Northeast United States

Damage from flooding in Pennsylvania

In Virginia, Allison produced light rainfall, with the southeastern and south-central portions of the state experiencing over 3 inches (76 mm).[9] A tree in a saturated ground fell over and killed one person.[52] Allison also produced one tornado in the state.[2] Washington, D.C. experienced moderate rainfall from the storm, totaling to 2.59 inches (66 mm) in Georgetown.[53] In Maryland, rainfall from Tropical Depression Allison totaled to 7.5 inches (190 mm) in Denton, closing eleven roads and causing washouts on 41 others. The Maryland Eastern Shore experienced only minor rainfall from one to two inches (25 to 50 mm). Damage was light, and no deaths were reported.[54] In Delaware, the storm produced moderate rainfall, peaking at 4.2 inches (106 mm) in Greenwood. No damage was reported.[55]

Allison, in combination with an approaching frontal boundary, dropped heavy rainfall across southeastern Pennsylvania, peaking at 10.17 inches (258 mm) in Chalfont in Bucks County and over 3 inches (76 mm) in portions of Philadelphia. The rainfall caused rivers to rise, with the Neshaminy Creek in Langhorne peaking at 16.87 feet (5.1 m). Several other rivers and creeks in southeastern Pennsylvania crested at over 10 feet (3 m). The rainfall downed numerous weak trees and power lines, leaving 70,000 without power during the storm. The flooding washed out several roads and bridges, including a few SEPTA rail lines. In addition, the rainfall destroyed 241 homes and damaged 1,386 others. Flooding at a Dodge dealership totaled 150 vehicles. Hundreds of people were forced to be rescued from damaged buildings from flood waters. The flooding dislodged a clothes dryer in the basement of the "A" building of the Village Green Apartment Complex in Upper Moreland Township, breaking a natural gas line. The gas leak resulted in an explosion and an ensuing fire that killed six people. Firefighters were unable to render assistance as the building was completely surrounded by floodwaters. Additionally, one man drowned in his vehicle in a river.[56] Damage in Pennsylvania totaled to $215 million (2001 USD, $250 million 2007 USD).[28] In New Jersey, the storm produced heavy rainfall, peaking at 8.1 inches (205 mm) in Tuckerton. The rains also caused river flooding, including the north branch of the Metedeconk River inLakewood which crested at 8 feet (2.5 m). The flooding, severe at places, closed several roads, including numerous state highways.[57] Gusty winds of up to 44 mph (71 km/h) in Atlantic City downed weak trees and power lines, leaving over 13,000 without power. Several people had to be rescued from high waters, though no fatalities occurred in the state. Overall damage was minimal.[58]

Tropical Storm Allison caused flash flooding in New York, dropping up to 3 inches (75 mm) of rain in one hour in several locations and peaking at 5.73 inches (146 mm) in Granite Springs. The rains also caused river flooding, including the Mahwah River which crested at 3.79 feet (1.2 m). Allison's rainfall damaged 24 houses and several stores, while the flooding closed several major highways in the New York City area. Overall damage was light, and no fatalities occurred in New York due to Allison.[59] Similarly, rainfall in Connecticut peaked at 7.2 inches (183 mm) in Pomfret,[60] closing several roads and causing minor damage to numerous houses. The Yantic River at Yantic crested at 11.1 feet (3.4 m),[60] while a state road was closed when a private dam in Hampton failed from the rainfall.[60] In Rhode Island, Allison produced up to 7.1 inches (180 mm) of rainfall in North Smithfield, washing out several roads and houses, and destroying a log house in Foster.[61]

An isolated severe thunderstorm in the outer bands of Allison produced an F1 tornado in Worcester and Middlesex Counties in Massachusetts, impacting over 100 trees and damaging one house and one small camper. A microburst in Leominster and another in Shirley damaged several trees. Lightning from the storm hit two houses, causing significant damage there but little elsewhere. Allison also produced moderate rainfall in the state, mainly ranging from 3 to 5 inches (75 to 125 mm). The rainfall caused drainage and traffic problems. Damage in Massachusetts totaled to $400,000 (2001 USD, $466,000 2007 USD).[62]

[edit]Aftermath

Main article: Effects of Tropical Storm Allison in Texas

Within weeks of the disaster, President George W. Bush declared 75 counties in Texas, [63] southern Louisiana,[64] southern Mississippi,[65] northwestern Florida,[66] and southeastern Pennsylvania as disaster areas.[67] The declarations allowed affected citizens to receive aid for temporary housing, emergency home repairs, and other serious disaster-related expenses. The Federal Emergency Management Agency (FEMA) also provided 75% for the cost of debris removal, emergency services related to the disaster, and repairing or replacing damaged public facilities, such as roads, bridges and utilities.[63]

Aid from the American Red Cross

A few weeks after Allison, FEMA opened six disaster recovery centers across southeast Texas, which provided recovery information to those who applied for disaster assistance. [68] The American Red Cross and the Salvation Army opened 48 shelters at the peak of need for people driven from their homes, which served nearly 300,000 meals. The National Disaster Medical System deployed a temporary hospital to Houston with 88 professionals, aiding nearly 500 people.[69] Thirty-five volunteer services provided aid for the flood victims in Texas, including food, clothing, and volunteers to help repair the houses.[70] After nearly 50,000 cars were flooded and ruined, many people attempted to sell the cars across the country without telling of the car's history.[71] Following the extreme flooding, a mosquito outbreak occurred, though FEMA provided aid to control the problem.[72] By six months after the storm, around 120,000 Texas citizens applied for federal disaster aid, totaling to $1.05 billion (2001 USD, $1.22 billion 2007 USD).[73]

Like in Texas, a mosquito outbreak occurred in Louisiana. Only pesticides acceptable to the US Environmental Protection Agency and the US Fish and Wildlife Service were allowed to be used.[74] FEMA officials warned homeowners of the dangers of floodwaters, including mold, mildew, and bacteria.[75] By three months after the storm, just under 100,000 Louisiana citizens applied for federal aid, totaling to over $110 million (2001 USD, $128 million 2007 USD). $25 million (2001 USD, $29 million 2007 USD) of the total was for business loans, while an additional $8 million was for public assistance for communities and state agencies.[76] More than 750 flood victims in Florida applied for governmental aid, totaling to $1.29 million (2001 USD, $1.5 million 2007 USD).[77] In Pennsylvania, 1,670 flood victims applied for federal aid, totaling to $11.5 million (2001 USD, $13.4 million 2007 USD). $3.4 million (2001 USD, $4 million 2007 USD) of the total was to replace a SEPTA rail bridge over the Sandy Run in Fort Washington.[78]

Tropical cyclone From Wikipedia, the free encyclopedia

"Hurricane" redirects here. For other uses, see Hurricane (disambiguation).

Hurricane Isabel (2003) as seen from orbit duringExpedition 7 of the International Space Station. The eye, eyewall and surrounding rainbands that are characteristics of tropical cyclones are clearly visible in this view from space.

A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated airrises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor'easters, European windstorms, and polar lows. The characteristic that separates tropical cyclones from other cyclonic systems is that any height in the atmosphere, the center of a tropical cyclone will be warmer than its surrounds; a phenomenon called "warm core" storm systems.

The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in maritime tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, withcounterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere. The opposite direction of spin is a result of the Coriolis force. Depending on its location and strength, a tropical cyclone is referred to by names such ashurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone.

While tropical cyclones can produce extremely powerful winds and torrential rain, they are also able to produce high waves and damaging storm surge as well as spawning tornadoes. They develop over large bodies of warm water, and lose their strength if they move over land due to increased surface friction and loss of the warm ocean as an energy source. This is why coastal regions can receive significant damage from a tropical cyclone, while inland regions are relatively safe from receiving strong winds. Heavy rains, however, can produce significant flooding inland, and storm surges can produce extensive coastal flooding up to 40 kilometres (25 mi) from the coastline. Although their effects on human populations can be devastating, tropical cyclones can also relieve drought conditions. They also carry heat and energy away from the tropics and transport it toward temperate latitudes, which makes them an important part of the global atmospheric circulation mechanism. As a result, tropical cyclones help to maintain equilibrium in the Earth's troposphere, and to maintain a relatively stable and warm temperature worldwide.

Many tropical cyclones develop when the atmospheric conditions around a weak disturbance in the atmosphere are favorable. The background environment is modulated by climatological cycles and patterns such as the Madden-Julian oscillation, El Niño-Southern Oscillation, and the Atlantic multidecadal oscillation. Others form when other types of cyclones acquire tropical characteristics. Tropical systems are then moved by steering winds in the troposphere; if the conditions remain favorable, the tropical disturbance intensifies, and can even develop an eye. On the other end of the spectrum, if the conditions around the system deteriorate or the tropical cyclone makes landfall, the system weakens and eventually dissipates. It is not possible to artificially induce the dissipation of these systems with current technology. Physical structure

See also: Eye (cyclone) Structure of a tropical cyclone

All tropical cyclones are areas of low atmospheric pressure in the Earth's atmosphere. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.[1] Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation, which occurs when moist air is carried upwards and its water vapor condenses. This heat is distributed vertically around the center of the storm. Thus, at any given altitude (except close to the surface, where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.[2] Eye and center

A strong tropical cyclone will harbor an area of sinking air at the center of circulation. If this area is strong enough, it can develop into a large "eye". Weather in the eye is normally calm and free of clouds, although the sea may be extremely violent.[3] The eye is normally circular in shape, and may range in size from 3 kilometres (1.9 mi) to 370 kilometres (230 mi) in diameter.[4] [5] Intense, mature tropical cyclones can sometimes exhibit an outward curving of the eyewall's top, making it resemble a football stadium; this phenomenon is thus sometimes referred to as the stadium effect.[6]

There are other features that either surround the eye, or cover it. The central dense overcast is the concentrated area of strong thunderstorm activity near the center of a tropical cyclone;[7] in weaker tropical cyclones, the CDO may cover the center completely.[8] The eyewall is a circle of strong thunderstorms that surrounds the eye; here is where the greatest wind speeds are found, where clouds reach the highest, and precipitation is the heaviest. The heaviest wind damage occurs where a tropical cyclone's eyewall passes over land. [3] Eyewall replacement cycles occur naturally in intense tropical cyclones. When cyclones reach peak intensity they usually have an eyewall and radius of maximum winds that contract to a very small size, around 10 kilometres (6.2 mi) to 25 kilometres (16 mi). Outer rainbands can organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum. When the inner eyewall weakens, the tropical cyclone weakens (in other words, the maximum sustained winds weaken and the central pressure rises.) The outer eyewall replaces the inner one completely at the end of the cycle. The storm can be of the same intensity as it was previously or even stronger after the eyewall replacement cycle finishes. The storm may strengthen again as it builds a new outer ring for the next eyewall replacement.[9]

Size descriptions of tropical cyclones ROCI Type

Less than 2 degrees Very latitude small/midget

2 to 3 degrees of Small latitude

3 to 6 degrees of Medium/Avera latitude ge

6 to 8 degrees of Large anti- latitude dwarf

Over 8 degrees of Very large[10] latitude

Size

One measure of the size of a tropical cyclone is determined by measuring the distance from its center of circulation to its outermost closed isobar, also known as its ROCI. If the radius is less than two degrees of latitude or 222 kilometres (138 mi), then the cyclone is "very small" or a "midget". A radius between 3 and 6 latitude degrees or 333 kilometres (207 mi) to 670 kilometres (420 mi) are considered "average-sized". "Very large" tropical cyclones have a radius of greater than 8 degrees or 888 kilometres (552 mi).[10] Use of this measure has objectively determined that tropical cyclones in the northwest Pacific Ocean are the largest on earth on average, withAtlantic tropical cyclones roughly half their size.[11] Other methods of determining a tropical cyclone's size include measuring the radius of gale force winds and measuring the radius at which its relative vorticity field decreases to 1×10−5 s−1 from its center.[12] [13] Mechanics Tropical cyclones form when the energy released by the condensation of moisture in rising air causes a positive feedback loop over warm ocean waters.[14]

A tropical cyclone's primary energy source is the release of the heat of condensation from water vapor condensing, with solar heatingbeing the initial source for evaporation. Therefore, a tropical cyclone can be visualized as a giant vertical heat engine supported by mechanics driven by physical forces such as the rotation and gravity of the Earth.[15] In another way, tropical cyclones could be viewed as a special type of mesoscale convective complex, which continues to develop over a vast source of relative warmth and moisture. While an initial warm core system, such as an organized thunderstorm complex, is necessary for the formation of a tropical cyclone, a large flux of energy is needed to lower atmospheric pressure more than a few millibars (0.10 inch of mercury). The inflow of warmth and moisture from the underlying ocean surface is critical for tropical cyclone strengthening.[16] A significant amount of the inflow in the cyclone is in the lowest 1 kilometre (3,300 ft) of the atmosphere.[17]

Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy;[18] the faster winds and lower pressure associated with them in turn cause increased surface evaporation and thus even more condensation. Much of the released energy drives updrafts that increase the height of the storm clouds, speeding up condensation.[19] This positive feedback loop, called the Wind-induced surface heat exchange, continues for as long as conditions are favorable for tropical cyclone development. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The rotation of the Earth causes the system to spin, an effect known as the Coriolis effect, giving it a cyclonic characteristic and affecting the trajectory of the storm.[20][21][22]

What primarily distinguishes tropical cyclones from other meteorological phenomena is deep convection as a driving force.[23] Because convection is strongest in a tropical climate, it defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere.[23] To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the needed atmospheric moisture to keep the positive feedback loop running. When a tropical cyclone passes over land, it is cut off from its heat source and its strength diminishes rapidly.[24]

Chart displaying the drop in surface temperature in the Gulf of Mexico as HurricanesKatrina and Rita passed over

The passage of a tropical cyclone over the ocean causes the upper layers of the ocean to cool substantially, which can influence subsequent cyclone development. This cooling is primarily caused by wind-driven mixing of cold water from deeper in the ocean and the warm surface waters. This effect results in a negative feedback process which can inhibit further development or lead to weakening. Additional cooling may come in the form of cold water from falling raindrops (this is because the atmosphere is cooler at higher altitudes). Cloud cover may also play a role in cooling the ocean, by shielding the ocean surface from direct sunlight before and slightly after the storm passage. All these effects can combine to produce a dramatic drop in sea surface temperature over a large area in just a few days.[25]

Scientists estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 exajoules (1018 J) per day,[19] equivalent to about 1 PW (1015 watt). This rate of energy release is equivalent to 70 times the world energy consumption of humans and 200 times the worldwide electrical generating capacity, or to exploding a 10-megaton nuclear bomb every 20 minutes.[19][26]

In the lower troposphere, the most obvious motion of clouds is toward the center. However tropical cyclones also develop an upper-level (high-altitude) outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through the "chimney" of the storm engine.[15] This outflow produces high, cirrus clouds that spiral away from the center. The clouds thin as they move outwards from the center of the system and are evaporated. They may be thin enough for the sun to be visible through them. These high cirrus clouds may be the first signs of an approaching tropical cyclone.[27] As air parcels are lifted within the eye of the storm the vorticity is reduced, causing the outflow from a tropical cyclone to have anti-cyclonic motion. Major basins and related warning centers

Main articles: Tropical cyclone basins and Regional Specialized Meteorological Center

Basins and WMO Monitoring Institutions[28]

Basin Responsible RSMCs and TCWCs

North Atlantic National Hurricane Center (United States)

North-East Pacific National Hurricane Center (United States)

Central Pacific Hurricane Center (United North-Central Pacific States)

North-West Pacific Japan Meteorological Agency

North Indian Ocean India Meteorological Department

South-West Indian Météo-France Ocean

Bureau of Meteorology† (Australia) Meteorological and Geophysical Australian region Agency† (Indonesia) Papua New Guinea National Weather Service†

Fiji Meteorological Service Southern Pacific Meteorological Service of New Zealand†

†: Indicates a Tropical Cyclone Warning Center There are six Regional Specialized Meteorological Centers (RSMCs) worldwide. These organizations are designated by the World Meteorological Organization and are responsible for tracking and issuing bulletins, warnings, and advisories about tropical cyclones in their designated areas of responsibility. Additionally, there are six Tropical Cyclone Warning Centers (TCWCs) that provide information to smaller regions.[29]The RSMCs and TCWCs are not the only organizations that provide information about tropical cyclones to the public. The Joint Typhoon Warning Center (JTWC) issues advisories in all basins except the Northern Atlantic for the purposes of the United States Government.[30] The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues advisories and names for tropical cyclones that approach the Philippines in the Northwestern Pacific to protect the life and property of its citizens.[31]The Canadian Hurricane Center (CHC) issues advisories on hurricanes and their remnants for Canadian citizens when they affect Canada.[32]

On 26 March 2004, Cyclone Catarina became the first recorded South Atlantic cyclone and subsequently struck southern Brazil with winds equivalent to Category 2 on the Saffir-Simpson Hurricane Scale. As the cyclone formed outside the authority of another warning center, Brazilian meteorologists initially treated the system as an extratropical cyclone, although subsequently classified it as tropical.[33] Formation

Main article: Tropical cyclogenesis

Map of the cumulative tracks of all tropical cyclones during the 1985–2005 time period. ThePacific Ocean west of the International Date Linesees more tropical cyclones than any other basin, while there is almost no activity in the Atlantic Ocean south of the Equator. Map of all tropical cyclone tracks from 1945 to 2006. Equal-area projection.

Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month, while September is the most active whilst November is the only month with all the tropical cyclone basins active.[34] Times

In the Northern Atlantic Ocean, a distinct cyclone season occurs from June 1 to November 30, sharply peaking from late August through September.[34] The statistical peak of the Atlantic hurricane season is 10 September. The Northeast Pacific Ocean has a broader period of activity, but in a similar time frame to the Atlantic.[35] The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and March 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.[34] In the Southern Hemisphere, the tropical cyclone year begins on July 1 and runs all year round and encompasses the tropical cyclone seasons which run from November 1 until the end of April with peaks in mid-February to early March.[34][36]

Season lengths and seasonal averages[34][37]

Tropical Tropical Category 3+ Season Season Basin Storms Cyclones TCs start end (>34 knots) (>63 knots) (>95 knots)

Northwest Pacific April January 26.7 16.9 8.5 South Indian November April 20.6 10.3 4.3

Novembe Northeast Pacific May 16.3 9.0 4.1 r

Novembe North Atlantic June 10.6 5.9 2.0 r

Australia Southwest November April 9 4.8 1.9 Pacific

Decembe North Indian April 5.4 2.2 0.4 r

Factors

Waves in the trade winds in the Atlantic Ocean—areas of converging winds that move along the same track as the prevailing wind—create instabilities in the atmosphere that may lead to the formation of hurricanes.

The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood.[38] While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the following conditions. In most situations,water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft); [39] waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.[40] Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.[39] High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.[39] Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.[39] Tropical cyclones generally need to form more than 555 km (345 mi) or 5 degrees of latitude away from the equator, allowing the Coriolis effect to deflect winds blowing towards the low pressure center and creating a circulation.[39] Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place. [39] Low-latitude and low-level westerly wind bursts associated with the Madden-Julian oscillation can create favorable conditions for tropical cyclogenesis by initiating tropical disturbances.[41] Locations

Most tropical cyclones form in a worldwide band of thunderstorm activity called by several names: the Intertropical Front (ITF), theIntertropical Convergence Zone (ITCZ), or the monsoon trough.[42][43][44] Another important source of atmospheric instability is found intropical waves, which cause about 85% of intense tropical cyclones in the Atlantic ocean, and become most of the tropical cyclones in the Eastern Pacific basin.[45][46][47]

Tropical cyclones move westward when equatorward of the subtropical ridge, intensifying as they move. Most of these systems form between 10 and 30 degrees away of the equator, and 87% form no farther away than 20 degrees of latitude, north or south.[48][49] Because the Coriolis effect initiates and maintains tropical cyclone rotation, tropical cyclones rarely form or move within about 5 degrees of the equator, where the Coriolis effect is weakest.[48] However, it is possible for tropical cyclones to form within this boundary as Tropical Storm Vamei did in 2001 and Cyclone Agni in 2004.[50][51] Movement and track Steering winds See also: Prevailing winds

Although tropical cyclones are large systems generating enormous energy, their movements over the Earth's surface are controlled by large-scale winds—the streams in the Earth's atmosphere. The path of motion is referred to as a tropical cyclone's track and has been compared by Dr. Neil Frank, former director of the National Hurricane Center, to "leaves carried along by a stream".[52]

Tropical systems, while generally located equatorward of the 20th parallel, are steered primarily westward by the east-to-west winds on the equatorward side of the subtropical ridge—a persistent high pressure area over the world's oceans.[52] In the tropical North Atlantic and Northeast Pacific oceans, trade winds—another name for the westward-moving wind currents— steer tropical waves westward from the African coast and towards the Caribbean Sea, North America, and ultimately into the central Pacific ocean before the waves dampen out.[46]These waves are the precursors to many tropical cyclones within this region.[45] In the Indian Ocean and Western Pacific (both north and south of the equator), tropical cyclogenesis is strongly influenced by the seasonal movement of the Intertropical Convergence Zone and the monsoon trough, rather than by easterly waves.[53] Tropical cyclones can also be steered by other systems, such as other low pressure systems, high pressure systems, warm fronts, and cold fronts. Coriolis effect

Infrared image of a powerful southern hemisphere cyclone, Monica, near peak intensity, showing clockwise rotation due to the Coriolis effect

The Earth's rotation imparts an acceleration known as the Coriolis effect, Coriolis acceleration, or colloquially, Coriolis force. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents.[54] The poleward portion of a tropical cyclone contains easterly winds, and the Coriolis effect pulls them slightly more poleward. The westerly winds on the equatorward portion of the cyclone pull slightly towards the equator, but, because the Coriolis effect weakens toward the equator, the net drag on the cyclone is poleward. Thus, tropical cyclones in the Northern Hemisphere usually turn north (before being blown east), and tropical cyclones in the Southern Hemisphere usually turn south (before being blown east) when no other effects counteract the Coriolis effect.[21]

The Coriolis effect also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds – that force is theheat of condensation.[19] Interaction with the mid-latitude westerlies See also: Westerlies

Storm track of Typhoon Ioke, showing recurvature off the Japanese coast in 2006

When a tropical cyclone crosses the subtropical ridge axis, its general track around the high- pressure area is deflected significantly by winds moving towards the general low-pressure area to its north. When the cyclone track becomes strongly poleward with an easterly component, the cyclone has begun recurvature.[55] A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve offshore of Japan to the north, and then to the northeast, if the typhoon encounters southwesterly winds (blowing northeastward) around a low-pressure system passing over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by extratropical cyclones in this manner, which move from west to east to the north of the subtropical ridge. An example of a tropical cyclone in recurvature was Typhoon Ioke in 2006, which took a similar trajectory.[56] Landfall See also: List of notable tropical cyclones and Unusual areas of tropical cyclone formation

Officially, landfall is when a storm's center (the center of its circulation, not its edge) crosses the coastline.[57] Storm conditions may be experienced on the coast and inland hours before landfall; in fact, a tropical cyclone can launch its strongest winds over land, yet not make landfall; if this occurs, then it is said that the storm made a direct hit on the coast.[57] As a result of the narrowness of this definition, the landfall area experiences half of a land-bound storm by the time the actual landfall occurs. For emergency preparedness, actions should be timed from when a certain wind speed or intensity of rainfall will reach land, not from when landfall will occur.[57] Multiple storm interaction Main article: Fujiwhara effect

When two cyclones approach one another, their centers will begin orbiting cyclonically about a point between the two systems. The two vortices will be attracted to each other, and eventually spiral into the center point and merge. When the two vortices are of unequal size, the larger vortex will tend to dominate the interaction, and the smaller vortex will orbit around it. This phenomenon is called the Fujiwhara effect, after Sakuhei Fujiwhara.[58] Dissipation Factors

Tropical Storm Franklin, an example of a strongly sheared tropical cyclone in the Atlantic Basin during 2005

A tropical cyclone can cease to have tropical characteristics in several different ways. One such way is if it moves over land, thus depriving it of the warm water it needs to power itself, quickly losing strength.[59] Most strong storms lose their strength very rapidly after landfall and become disorganized areas of low pressure within a day or two, or evolve into extratropical cyclones. There is a chance a tropical cyclone could regenerate if it managed to get back over open warm water, such as with Hurricane Ivan. If it remains over mountains for even a short time, weakening will accelerate.[60] Many storm fatalities occur in mountainous terrain, as the dying storm unleashes torrential rainfall,[61] leading to deadly floods and mudslides, similar to those that happened with Hurricane Mitch in 1998.[62]Additionally, dissipation can occur if a storm remains in the same area of ocean for too long, mixing the upper 60 metres (200 ft) of water, dropping sea surface temperatures more than 5 °C (9 °F).[63] Without warm surface water, the storm cannot survive.[64]

A tropical cyclone can dissipate when it moves over waters significantly below 26.5 °C (79.7 °F). This will cause the storm to lose its tropical characteristics (i.e. thunderstorms near the center and warm core) and become a remnant low pressure area, which can persist for several days. This is the main dissipation mechanism in the Northeast Pacific ocean.[65] Weakening or dissipation can occur if it experiences vertical wind shear, causing the convection and heat engine to move away from the center; this normally ceases development of a tropical cyclone. [66] Additionally, its interaction with the main belt of the Westerlies, by means of merging with a nearby frontal zone, can cause tropical cyclones to evolve into extratropical cyclones. This transition can take 1–3 days.[67] Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane/typhoon force) winds and drop several inches of rainfall. In the Pacific ocean and Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane or typhoon-force wind speeds when they reach the west coast of North America. These phenomena can also affect Europe, where they are known as European windstorms; Hurricane Iris's extratropical remnants are an example of such a windstorm from 1995.[68] Additionally, a cyclone can merge with another area of low pressure, becoming a larger area of low pressure. This can strengthen the resultant system, although it may no longer be a tropical cyclone. [66] Studies in the 2000s have given rise to the hypothesis that large amounts of dust reduce the strength of tropical cyclones.[69] Artificial dissipation

In the 1960s and 1970s, the United States government attempted to weaken hurricanes through Project Stormfury by seeding selected storms with silver iodide. It was thought that the seeding would cause supercooled water in the outer rainbands to freeze, causing the inner eyewall to collapse and thus reducing the winds.[70] The winds of Hurricane Debbie—a hurricane seeded in Project Stormfury—dropped as much as 31%, but Debbie regained its strength after each of two seeding forays.[71] In an earlier episode in 1947, disaster struck when a hurricane east of Jacksonville, Florida promptly changed its course after being seeded, and smashed into Savannah, Georgia.[72] Because there was so much uncertainty about the behavior of these storms, the federal government would not approve seeding operations unless the hurricane had a less than 10% chance of making landfall within 48 hours, greatly reducing the number of possible test storms. The project was dropped after it was discovered that eyewall replacement cycles occur naturally in strong hurricanes, casting doubt on the result of the earlier attempts. Today, it is known that silver iodide seeding is not likely to have an effect because the amount of supercooled water in the rainbands of a tropical cyclone is too low.[73]

Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans.[74] Other ideas range from covering the ocean in a substance that inhibits evaporation,[75] dropping large quantities of ice into the eye at very early stages of development (so that the latent heat is absorbed by the ice, instead of being converted to kinetic energy that would feed the positive feedback loop),[74] or blasting the cyclone apart with nuclear weapons.[18] Project Cirrus even involved throwing dry ice on a cyclone.[76] These approaches all suffer from one flaw above many others: tropical cyclones are simply too large and short-lived for any of the weakening techniques to be practical.[77] Effects

The aftermath of Hurricane Katrina in Gulfport, Mississippi.

Main article: Effects of tropical cyclones

Tropical cyclones out at sea cause large waves, heavy rain, and high winds, disrupting international shipping and, at times, causing shipwrecks.[78] Tropical cyclones stir up water, leaving a cool wake behind them, which causes the region to be less favorable for subsequent tropical cyclones.[25] On land, strong winds can damage or destroy vehicles, buildings, bridges, and other outside objects, turning loose debris into deadly flying projectiles. The storm surge, or the increase in sea level due to the cyclone, is typically the worst effect from landfalling tropical cyclones, historically resulting in 90% of tropical cyclone deaths.[79] The broad rotation of a landfalling tropical cyclone, and vertical wind shear at its periphery, spawns tornadoes. Tornadoes can also be spawned as a result of eyewall mesovortices, which persist until landfall. [80] Over the past two centuries, tropical cyclones have been responsible for the deaths of about 1.9 million people worldwide. Large areas of standing water caused by flooding lead to infection, as well as contributing to mosquito-borne illnesses. Crowded evacuees in sheltersincrease the risk of disease propagation.[81] Tropical cyclones significantly interrupt infrastructure, leading to power outages, bridge destruction, and the hampering of reconstruction efforts.[81][82]

Although cyclones take an enormous toll in lives and personal property, they may be important factors in the precipitation regimes of places they impact, as they may bring much-needed precipitation to otherwise dry regions.[83] Tropical cyclones also help maintain the global heat balance by moving warm, moist tropical air to the middle latitudes and polar regions.[84] The storm surge and winds of hurricanes may be destructive to human-made structures, but they also stir up the waters of coastal estuaries, which are typically important fish breeding locales. Tropical cyclone destruction spurs redevelopment, greatly increasing local property values.[85] Observation and forecasting Observation Main article: Tropical cyclone observation

Sunset view of Hurricane Isidore's rainbands photographed at 7,000 feet (2,100 m)

Intense tropical cyclones pose a particular observation challenge, as they are a dangerous oceanic phenomenon, and weather stations, being relatively sparse, are rarely available on the site of the storm itself. Surface observations are generally available only if the storm is passing over an island or a coastal area, or if there is a nearby ship. Usually, real-time measurements are taken in the periphery of the cyclone, where conditions are less catastrophic and its true strength cannot be evaluated. For this reason, there are teams of meteorologists that move into the path of tropical cyclones to help evaluate their strength at the point of landfall.[86] Tropical cyclones far from land are tracked by weather satellites capturing visible and infrared images from space, usually at half-hour to quarter-hour intervals. As a storm approaches land, it can be observed by land-based Doppler radar. Radar plays a crucial role around landfall by showing a storm's location and intensity every several minutes.[87]

In-situ measurements, in real-time, can be taken by sending specially equipped reconnaissance flights into the cyclone. In the Atlantic basin, these flights are regularly flown by United States government hurricane hunters.[88] The aircraft used are WC-130 Hercules andWP-3D Orions, both four-engine turboprop cargo aircraft. These aircraft fly directly into the cyclone and take direct and remote-sensing measurements. The aircraft also launch GPS dropsondes inside the cyclone. These sondes measure temperature, humidity, pressure, and especially winds between flight level and the ocean's surface. A new era in hurricane observation began when a remotely piloted Aerosonde, a small drone aircraft, was flown through Tropical Storm Ophelia as it passed Virginia's Eastern Shore during the 2005 hurricane season. A similar mission was also completed successfully in the western Pacific ocean. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare.[89]

A general decrease in error trends in tropical cyclone path prediction is evident since the 1970s Forecasting See also: Tropical cyclone track forecasting, Tropical cyclone prediction model, and Tropical cyclone rainfall forecasting

Because of the forces that affect tropical cyclone tracks, accurate track predictions depend on determining the position and strength of high- and low-pressure areas, and predicting how those areas will change during the life of a tropical system. The deep layer mean flow, or average wind through the depth of the troposphere, is considered the best tool in determining track direction and speed. If storms are significantly sheared, use of wind speed measurements at a lower altitude, such as at the 700 hPa pressure surface (3,000 metres / 9,800 feet above sea level) will produce better predictions. Tropical forecasters also consider smoothing out short- term wobbles of the storm as it allows them to determine a more accurate long-term trajectory. [90] High-speed computers and sophisticated simulation software allow forecasters to produce computer models that predict tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. Combining forecast models with increased understanding of the forces that act on tropical cyclones, as well as with a wealth of data from Earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades.[91] However, scientists are not as skillful at predicting the intensity of tropical cyclones.[92] The lack of improvement in intensity forecasting is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development. Classifications, terminology, and naming Intensity classifications Main article: Tropical cyclone scales Three tropical cyclones at different stages of development. The weakest (left) demonstrates only the most basic circular shape. A stronger storm (top right) demonstrates spiral banding and increased centralization, while the strongest (lower right) has developed an eye.

Tropical cyclones are classified into three main groups, based on intensity: tropical depressions, tropical storms, and a third group of more intense storms, whose name depends on the region. For example, if a tropical storm in the Northwestern Pacific reaches hurricane-strength winds on the Beaufort scale, it is referred to as a typhoon; if a tropical storm passes the same benchmark in the Northeast Pacific Basin, or in the Atlantic, it is called a hurricane.[57] Neither "hurricane" nor "typhoon" is used in either the Southern Hemisphere or the Indian Ocean. In these basins, storms of tropical nature are referred to simply as "cyclones".

Additionally, as indicated in the table below, each basin uses a separate system of terminology, making comparisons between different basins difficult. In the Pacific Ocean, hurricanes from the Central North Pacific sometimes cross the International Date Line into the Northwest Pacific, becoming typhoons (such as Hurricane/Typhoon Ioke in 2006); on rare occasions, the reverse will occur.[93] It should also be noted that typhoons with sustained winds greater than 67 metres per second (130 kn) or 150 miles per hour (240 km/h) are calledSuper Typhoons by the Joint Typhoon Warning Center.[94] Tropical depression

A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 17 metres per second (33 kn) or 38 miles per hour (61 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression".[15]The practice of the Philippines is to name tropical depressions from their own naming convention when the depressions are within the Philippines' area of responsibility.[95] Tropical storm

A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 metres per second (33 kn) (39 miles per hour (63 km/h)) and 32 metres per second (62 kn) (73 miles per hour (117 km/h)). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Government weather services, other than the Philippines, first assign names to systems that reach this intensity (thus the term named storm).[15] Hurricane or typhoon A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 33 metres per second (64 kn) or 74 miles per hour (119 km/h).[15] A cyclone of this intensity tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of circulation. The eye is often visible in satellite images as a small, circular, cloud-free spot. Surrounding the eye is the eyewall, an area about 16 kilometres (9.9 mi) to 80 kilometres (50 mi) wide in which the strongest thunderstorms and winds circulate around the storm's center. Maximum sustained winds in the strongest tropical cyclones have been estimated at about 85 metres per second (165 kn) or 195 miles per hour (314 km/h).[96]

[hide]Tropical Cyclone Classifications (all winds are 10-minute averages)[97][98]

10- SW NE Pacific & Beauf minute N Indian Austral SW NW NW Indian N Atlantic ort sustaine Ocean ia Pacific Pacific Pacific Ocean NHC, CHC &C scale d winds IMD BOM FMS JMA JTWC MF PHC (knots)

<28 knot Trop. s (32 Depressio 0–6 Disturban mph; 52 n ce km/h) Tropical Tropical Depressi Depression 28–29 on knots Tropical Tropical (32–33 Tropical Depressio Depressi mph; 52– Low n on 54 km/h) Deep Depressio 7 Depressio n n 30–33 Tropical Tropical Storm knots Storm (35–38 mph; 56– 61 km/h)

8–9 34–47 Cyclonic Moderate Tropical Tropical Tropical knots Storm Tropical Cyclone Cyclone Storm (39–54 Storm (1) (1) mph; 63– 87 km/h)

48–55 knots (55–63 10 mph; 89– 102 km/h) Severe Severe Tropical Tropical Severe Cyclonic Tropical Cyclone Cyclone Tropical Storm Storm (2) (2) Storm 56–63 Typhoon knots (64–72 11 mph; 104–117 km/h)

Hurricane (1)

12 64–72 Very Typhoon knots Severe (74–83 Cyclonic mph; Storm 119–133 km/h) Severe Severe Tropical Tropical Cyclone Cyclone 73–85 (3) (3) knots (84–98 Tropical Hurricane (2) mph; Cyclone 135–157 km/h)

86–89 Severe Severe Major knots Tropica Tropical Hurricane (3) (99–102 l Cyclone mph; Cyclon (4) 159–165 e (4) km/h)

90–99 Intense knots Tropical (100–114 mph; 170–183 km/h)

100–106 knots (120–122 mph; 190–196 Cyclone km/h)

107–114 knots (123–131 Major mph; Hurricane (4) 198–211 km/h)

115–119 Severe Severe knots Tropica Tropical (132–137 l Cyclone mph; Cyclon (5) 213–220 e (5) Very km/h) Intense Super Tropical Typhoon Cyclone >120 kn ots (140 Super Major mph; Cyclonic Hurricane (5) 220 Storm km/h) Origin of storm terms

Taipei 101 endures a typhoon in 2005

The word typhoon, which is used today in the Northwest Pacific, may be derived which in turn originates ,(طوفان) from Urdu, Persian and Arabic ţūfān from Greek Typhon (Τυφών), a monster from Greek mythology associated with storms.[99] The related Portuguese word tufão, used in Portuguese for typhoons, is also derived from Typhon. [100] The word is also similar to Chinese "taifeng" ("toifung" in Cantonese) (颱風 – literally great winds), and also to the Japanese "taifu" (台風), which may explain why "typhoon" came to be used for East Asian cyclones.[citation needed]

The word hurricane, used in the North Atlantic and Northeast Pacific, is probably derived from the name of a Mayan storm god, Huracan, via the Spanish, huracán.[101] Huracan is also the source of the word Orcan, another word for a European windstorm. Another possible source is Hyrrokkin, a Jotun or giantess in Norse mythology, called upon by the Aesir to launch the ship bearing the body of the godBalder, which was too heavy for even the gods to move.[102] Naming Main articles: Tropical cyclone naming and Lists of tropical cyclone names

Storms reaching tropical storm strength were initially given names to eliminate confusion when there are multiple systems in any individual basin at the same time, which assists in warning people of the coming storm.[103] In most cases, a tropical cyclone retains its name throughout its life; however, under special circumstances, tropical cyclones may be renamed while active. These names are taken from lists that vary from region to region and are usually drafted a few years ahead of time. The lists are decided on, depending on the regions, either by committees of the World Meteorological Organization (called primarily to discuss many other issues), or by national weather offices involved in the forecasting of the storms. Each year, the names of particularly destructive storms (if there are any) are "retired" and new names are chosen to take their place. Different countries have different local conventions; for example, in Japan, storms are referred to by number (each year), such as 台風第 9 号 (Typhoon #9). Notable tropical cyclones

Main articles: List of notable tropical cyclones, List of Atlantic hurricanes, and List of Pacific hurricanes

Tropical cyclones that cause extreme destruction are rare, although when they occur, they can cause great amounts of damage or thousands of fatalities. The 1970 Bhola cyclone is the deadliest tropical cyclone on record, killing more than 300,000 people[104] and potentially as many as 1 million[105] after striking the densely populated Ganges Delta region ofBangladesh on 13 November 1970. Its powerful storm surge was responsible for the high death toll. [104] The North Indian cyclone basin has historically been the deadliest basin.[81] [106]Elsewhere, Typhoon Nina killed nearly 100,000 in China in 1975 due to a 100-year flood that caused 62 dams including the Banqiao Dam to fail.[107] The Great Hurricane of 1780 is the deadliest Atlantic hurricane on record, killing about 22,000 people in the Lesser Antilles.[108] A tropical cyclone does need not be particularly strong to cause memorable damage, primarily if the deaths are from rainfall or mudslides. Tropical Storm Thelma in November 1991 killed thousands in the Philippines,[109] while in 1982, the unnamed tropical depression that eventually became Hurricane Paul killed around 1,000 people in Central America.[110]

Hurricane Katrina is estimated as the costliest tropical cyclone worldwide,[111] causing $81.2 billion in property damage (2008 USD)[112] with overall damage estimates exceeding $100 billion (2005 USD).[111] Katrina killed at least 1,836 people after striking Louisiana and Mississippi as a major hurricane in August 2005.[112] Hurricane Andrew is the second most destructive tropical cyclone in U.S history, with damages totaling $40.7 billion (2008 USD), and with damage costs at $31.5 billion (2008 USD), Hurricane Ike is the third most destructive tropical cyclone in U.S history. The Galveston Hurricane of 1900 is the deadliest natural disaster in the United States, killing an estimated 6,000 to 12,000 people in Galveston, Texas.[113] Hurricane Mitch caused more than 10, 000 fatalities in Latin America. Hurricane Iniki in 1992 was the most powerful storm to strike Hawaii in recorded history, hitting Kauai as a Category 4 hurricane, killing six people, and causing U.S. $3 billion in damage.[114] Other destructive Eastern Pacific hurricanes include Pauline and Kenna, both causing severe damage after striking Mexico as major hurricanes.[115][116] In March 2004, Cyclone Gafilo struck northeastern Madagascar as a powerful cyclone, killing 74, affecting more than 200,000, and becoming the worst cyclone to affect the nation for more than 20 years.[117]

The relative sizes of Typhoon Tip, Cyclone Tracy, and theContiguous United States

The most intense storm on record was Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of 870 mbar (25.69 inHg) and maximum sustained wind speeds of 165 knots (85 m/s) or 190 miles per hour (310 km/h).[118] Tip, however, does not solely hold the record for fastest sustained winds in a cyclone. Typhoon Keith in the Pacific and HurricanesCamille and Allen in the North Atlantic currently share this record with Tip.[119] Camille was the only storm to actually strike land while at that intensity, making it, with 165 knots (85 m/s) or 190 miles per hour (310 km/h) sustained winds and 183 knots (94 m/s) or 210 miles per hour (340 km/h) gusts, the strongest tropical cyclone on record at landfall.[120] Typhoon Nancy in 1961 had recorded wind speeds of 185 knots (95 m/s) or 215 miles per hour (346 km/h), but recent research indicates that wind speeds from the 1940s to the 1960s were gauged too high, and this is no longer considered the storm with the highest wind speeds on record.[96] Similarly, a surface-level gust caused by Typhoon Paka on Guam was recorded at 205 knots (105 m/s) or 235 miles per hour (378 km/h). Had it been confirmed, it would be the strongest non- tornadic wind ever recorded on the Earth's surface, but the reading had to be discarded since the anemometer was damaged by the storm.[121]

In addition to being the most intense tropical cyclone on record, Tip was the largest cyclone on record, with tropical storm-force winds 2,170 kilometres (1,350 mi) in diameter. The smallest storm on record, Tropical Storm Marco, formed during October 2008, and made landfall in Veracruz. Marco generated tropical storm-force winds only 37 kilometres (23 mi) in diameter. [122]

Hurricane John is the longest-lasting tropical cyclone on record, lasting 31 days in 1994. Before the advent of satellite imagery in 1961, however, many tropical cyclones were underestimated in their durations.[123] John is also the longest-tracked tropical cyclone in the Northern Hemisphere on record, which had a path of 7,165 miles (13,280 km). Reliable data for Southern Hemisphere cyclones is unavailable.[124] Changes due to El Niño-Southern Oscillation