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Tropical Cyclone

Tropical cyclone

:Hurricane and Typhoon redirect here. For other uses, see Hurricane (disambiguation) and Typhoon (disambiguation). Typhoon (disambiguation) on March 26, 2004.]] In meteorology, a tropical cyclone (or tropical depression, tropical storm, typhoon, or hurricane, depending on strength and geographical context) is a type of low pressure system which generally forms in the tropics. While they can be highly destructive, tropical cyclones are an important part of the atmospheric circulation system, which moves heat from the equatorial region toward the higher latitudes.

Terms for tropical cyclones

equatorial region]] Depending on the region, different terms are used to describe tropical cyclones with maximum sustained winds exceeding 33 meters per second (63 knots, 73 mph, or 117 km/h):
- hurricane in the North Atlantic Ocean, North Pacific Ocean east of the dateline
- typhoon in the Northwest Pacific Ocean west of the dateline
- severe tropical cyclone in the Southwest Pacific Ocean west of 160°E or Southeast Indian Ocean east of 90°E
- severe cyclonic storm in the North Indian Ocean
- tropical cyclone in the Southwest Indian Ocean and South Pacific Ocean east of 160°E.
- cyclone unofficially in the South Atlantic Ocean In other areas, hurricanes have been called Baguio in the Philippines and Taino in Haiti.

Etymology

The word typhoon has two possible origins:
- From the Chinese 大風 (daaih fūng (Cantonese); dà fēng (Mandarin)) which means "great wind". (The Chinese term as 颱風 táifēng, and 台風 taifu in Japanese, has an independent origin traceable variously to 風颱, 風篩 or 風癡 hongthai, going back to Song 宋 (960-1278) and Yuan 元(1260-1341) dynasties. The first record of the character 颱 appeared in 1685's edition of Summary of Taiwan 臺灣記略).
- From Urdu, Persian or Arabic ţūfān (طوفان) < Greek tuphōn (Τυφών). Portuguese tufão is also related to typhoon. See tuphōn for more information. The word hurricane is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracán. The word cyclone came from the Greek word "κύκλος", meaning "circle".

Mechanics of a tropical cyclone

Spanish. The air heats up, rising further, which leads to more condensation. The air flowing out of the top of this “chimney” drops towards the ground, forming powerful winds.]] Structurally, a tropical cyclone is a large, rotating system of clouds, wind and thunderstorm activity. Its primary energy source is the release of the heat of condensation from water vapor condensing at high altitudes, the heat ultimately derived from the sun. Therefore, a tropical cyclone can be thought of as a giant vertical heat engine supported by mechanics driven by physical forces such as the orbital revolution and gravity of the Earth. Continued condensation leads to higher winds, continued evaporation, and continued condensation, feeding back into itself. This gives rise to factors that give the system enough energy to be self-sufficient and cause a positive feedback loop where it can draw more energy as long as the source of heat, warm water, remains. Factors such as a continued lack of equilibrium in air mass distribution would also give supporting energy to the cyclone. The orbital revolution of the Earth causes the system to spin, giving it a cyclone characteristic and affecting the trajectory of the storm. The factors to form a tropical cyclone include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft. If the right conditions persist and allow it to create a feedback loop by maximizing the energy intake possible, for example, such as high winds to increase the rate of evaporation, they can combine to produce the violent winds, incredible waves, torrential rains, and floods associated with this phenomenon. Condensation as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena, and because this is strongest in a tropical climate, this defines the initial domain of the tropical cyclone. By contrast, mid-latitude cyclones, for example, draw their energy mostly from pre-existing horizontal temperature gradients in the atmosphere. In order to continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the atmospheric moisture needed. The condensation of this moisture is driven by the high winds and reduced atmospheric pressure in the storm, resulting in a sustaining cycle. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly. Scientists at the National Center for Atmospheric Research estimate that a hurricane releases heat energy at the rate of 50 to 200 trillion watts -- about the amount of energy released by exploding a 10-megaton nuclear bomb every 20 minutes [http://www.ucar.edu/news/features/hurricanes/index.shtml].

Formation

nuclear bomb The formation of tropical cyclones is the topic of extensive ongoing research, and is still not fully understood. Five factors are necessary to make tropical cyclone formation possible: # Sea surface temperatures above 26.5 degrees Celsius (79.7 degrees Fahrenheit) to at least a depth of 50 meters (164 feet). The moisture in the air above the warm water is the energy source for tropical cyclones. # Upper-atmosphere conditions conducive to thunderstorm formation. Temperature in the atmosphere must decrease quickly with height, and the mid-troposphere must be relatively moist. # A pre-existing weather disturbance. This is most frequently provided by tropical waves—non-rotating areas of thunderstorms that move through tropical oceans. # A distance of approximately 10 degrees or more from the equator, so that the Coriolis effect is strong enough to initiate the cyclone's rotation. (2004's Hurricane Ivan was the strongest storm to form closer than 10 degrees from the equator; it started forming at 9.7 degrees north.) # Low vertical wind shear (change in wind speed or direction over height). High wind shear can break apart the vertical structure of a tropical cyclone. Tropical cyclones occasionally form despite not meeting these conditions. Only specific weather disturbances can result in tropical cyclones. These include: # Tropical waves, or easterly waves, which, as mentioned above, are westward moving areas of convergent winds. This often assists in the development of thunderstorms, which can develop into tropical cyclones. Most tropical cyclones form from these. A similar phenomenon to tropical waves are West African disturbance lines, which are squally lines of convection that form over Africa and move into the Atlantic. # Tropical upper tropospheric troughs, which are cold-core upper level lows. A warm-core tropical cyclone may result when one of these (on occasion) works down to the lower levels and produces deep convection. # Decaying frontal boundaries may occasionally stall over warm waters and produce lines of active convection. If a low level circulation forms under this convection, it may develop into a tropical cyclone.

When do tropical cyclones form?

Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest. However, each particular basin has its own seasonal patterns. In the North Atlantic, a distinct hurricane season occurs from June 1 to November 30, sharply peaking from late August through September. The statistical peak of the North Atlantic hurricane season is September 10. The Northeast Pacific has a broader period of activity, but in a similar timeframe to the Atlantic. The Northwest Pacific sees tropical cyclones year-round, with a minimum in February and a peak in early September. In the North Indian basin, storms are most common from April to December, with peaks in May and November. In the Southern Hemisphere, tropical cyclone activity begins in late October and ends in May. Southern Hemisphere activity peaks in mid-February to early March. Worldwide, an average of 80 tropical cyclones form each year.

Where do tropical cyclones form?

Most tropical cyclones form in a worldwide band of thunderstorm activity called the Intertropical convergence zone (ITCZ). Nearly all of them form between 10 and 30 degrees of the equator and 87% form within 20 degrees of it. Because the Coriolis effect initiates and maintains tropical cyclone rotation, such cyclones almost never form or move within about 10 degrees of the equator [http://www.bom.gov.au/bmrc/pubs/tcguide/ch1/figures_ch1/figure1.9.htm], where the Coriolis effect is weakest. However, it is possible for tropical cyclones to form within this boundary if there is another source of initial rotation. These conditions are extremely rare, and such storms are believed to form at most once per century. Hurricane Ivan of 2004 developed within 10 degrees of the equator. A combination of a pre-existing disturbance, upper level divergence and a monsoon-related cold spell led to Typhoon Vamei at only 1.5 degrees north of the equator in 2001. It is estimated that such conditions occur only once every 400 years.

Major basins

There are seven main basins of tropical cyclone formation:
- North Atlantic Basin: The most-studied of all tropical basins, it includes the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Tropical cyclone formation here varies widely from year to year, ranging from over twenty to one per year. The average is about ten. The United States Atlantic coast, Mexico, Central America, the Caribbean Islands and Bermuda are frequently affected by storms in this basin. Venezuela, the south-east of Canada and Atlantic "Macaronesian" islands are also occasionally affected. The U.S. National Hurricane Center (NHC) based in Miami, Florida, issues forecasts for storms for all nations in the region; the Canadian Hurricane Centre, based in Halifax, Nova Scotia, also issues forecasts and warnings for storms expected to affect Canadian territory and waters. Hurricanes that strike Mexico, Central America, and Caribbean island nations, often do intense damage, as hurricanes are deadlier over warmer water. Additionally, they can hit the coast of the U.S., especially Florida, North Carolina, the U.S. Gulf Coast and occasionally New Jersey, New York and New England (usually hurricanes weaken to tropical storms before they reach these northern regions). The coast of Atlantic Canada receives hurricane landfalls on rare occasion, such as Hurricane Juan in 2003. Many of the more intense Atlantic storms are Cape Verde-type hurricanes, which form off the west coast of Africa near the Cape Verde islands.
- Western North Pacific Ocean: Tropical storm activity in this region frequently affects China, Japan, the Philippines, and Taiwan, but also many other countries in South-East Asia, such as Vietnam, South Korea and Indonesia, plus numerous Oceanian islands. This is by far the most active basin, accounting for one-third of all tropical cyclone activity in the world. The eastern coasts of Taiwan and Philippines also have the highest tropical cyclone landfall frequency in the world. National meteorology organizations and the Joint Typhoon Warning Center (JTWC) are responsible for issuing forecasts and warnings in this basin.
- Eastern North Pacific Ocean: This is the second most active basin in the world, and the most dense (a large number of storms for a small area of ocean). Storms that form here can affect western Mexico, Hawaii, northern Central America, and on extremely rare occasions, California. In the U.S., the Central Pacific Hurricane Center is responsible for forecasting the western part of this area while the National Hurricane Center is responsible for the eastern part.
- South Western Pacific Ocean: Tropical activity in this region largely affects Australia and Oceania, and is forecast by Australia and Papua New Guinea.
- Northern Indian Ocean: This basin is divided into two areas, the Bay of Bengal and the Arabian Sea, with the Bay of Bengal dominating (5 to 6 times more activity). This basin's season has an interesting double peak; one in April and May before the onset of the monsoon, and another in October and November just after. Hurricanes which form in this basin have historically cost the most lives — most notably, the 1970 Bhola cyclone killed 200,000. Nations affected by this basin include India, Bangladesh, Sri Lanka, Thailand, Myanmar, and Pakistan, and all of these countries issue regional forecasts and warnings. Rarely, a tropical cyclone formed in this basin will affect the Arabian Peninsula.
- Southeastern Indian Ocean: Tropical activity in this region affects Australia and Indonesia, and is forecast by those nations.
- Southwestern Indian Ocean: This basin is the least understood, due to a lack of historical data. Cyclones forming here impact Madagascar, Mozambique, Mauritius, and Kenya, and these nations issue forecasts and warnings for the basin.

Unusual formation areas

Kenya at 2300 UTC near the Madeira Islands.]] The following areas spawn tropical cyclones only very rarely.
- Southern Atlantic Ocean: A combination of cooler waters, the lack of an ITCZ, and wind shear makes it very difficult for the Southern Atlantic to support tropical activity. However, three tropical cyclones have been observed here — a weak tropical storm in 1991 off the coast of Africa, Hurricane Catarina (sometimes also referred to as Aldonça), which made landfall in Brazil in 2004 as a Category 1 hurricane, and a smaller storm in January 2004, east of Salvador, Brazil. The January storm is thought to have reached tropical storm intensity based on scatterometre winds.
- Central North Pacific: Shear in this area of the Pacific Ocean severely limits tropical development. However, this region is commonly frequented by tropical cyclones that form in the much more favorable Eastern North Pacific Basin.
- Eastern South Pacific: Tropical cyclones are rare in this region; activity is frequently linked to El Niño episodes. When they do form, they can affect the islands of Polynesia.
- Mediterranean Sea: Storms which appear similar to tropical cyclones in structure sometimes occur in the Mediterranean basin. Such cyclones formed in September 1947, September 1969, January 1982, September 1983, and January 1995. However, there is debate on whether these storms were tropical in nature.
- Northeastern Atlantic Ocean: In October 2005, Hurricane Vince formed near Madeira, then moved northeastward, passing south of the Portuguese south coast, and made landfall in southwestern Spain as a tropical storm. Vince's origin was the northernmost in the eastern Atlantic ever recorded, and Vince was the first storm in recorded history to reach the Iberian Peninsula as a tropical cyclone, i.e. before being transformed into an extratropical low or absorbed into other systems of low pressure.
- Australia: SW Pacific Basin includes the eastern part of Australia and the Fiji area.
- Australia: SE Indian Basin includes the eastern part of the Indian ocean and the northern and western part of the Australian basin.
- Southern South China Sea Tropical cyclones normally do not develop in the Southern South China Sea due to its close proximity to the equator. Areas within ten degrees laditude of the equator do not experience a significant coriolis force, a vital ingredient in tropical cyclone formation. However, in December 2001, Typhoon Vamei formed in the Southern South China Sea and made landfall in Malaysia. It caused flooding in southern Malaysia and some damage in Singapore. It formed from a thunderstorm formation in Borneo that moved into the South China Sea.

Average Season

Structure and classification

Borneo A strong tropical cyclone consists of the following components.
- Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that occur on Earth's surface at sea level.
- Warm core: Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as 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.
- Central Dense Overcast (CDO): The Central Dense Overcast is a dense shield of very intense thunderstorm activity that make up the inner portion of the hurricane. This contains the eye wall, and the eye itself. The classic hurricane contains a symmetrical CDO, which means that it is perfectly circular and round on all sides.
- Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). Eyes are home to the coldest temperatures of the storm at the surface, and the warmest temperatures at the upper levels. The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
- Eyewall: It is the area directly around the eye of the cyclone where the winds are the highest, the clouds reach furthest into the atmosphere and the precipitation is the heaviest. The heaviest damage caused by tropical cyclones occurs where the eyewall crosses over land.
- Outflow: The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.

Types of tropical cyclones

Tropical cyclones are classified into three main groups: tropical depressions, tropical storms, and a third group whose name depends on the region. A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 metres per second (33 knots, 38 mph, or 62 km/h). It has no eye, and does not typically have the spiral shape of more powerful storms. It is already becoming a low-pressure system, however, hence the name "depression". A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 and 33 meters per second (34–63 knots, 39–73 mph, or 62–117 km/h). At this point, the distinctive cyclonic shape starts to develop, though an eye is usually not present. Government weather services assign first names to systems that reach this intensity (thus the term named storm). At hurricane intensity, a tropical cyclone tends to develop an eye, an area of relative calm (and lowest atmospheric pressure) at the center of the 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 10 to 50 miles (16 to 80 kilometers) wide in which the strongest thunderstorms and winds circulate around the storm's center. The circulation of clouds around a cyclone's center imparts a distinct spiral shape to the system. Bands or arms may extend over great distances as clouds are drawn toward the cyclone. The direction of the cyclonic circulation depends on the hemisphere; it is counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 85 m/s (165 knots, 190 mph, 305 km/h). Intense, mature hurricanes can sometimes exhibit an inward curving of the eyewall top that resembles a football stadium: this phenomenon is thus sometimes referred to as stadium effect. Eyewall replacement cycles naturally occur in intense tropical cyclones. When cyclones reach peak intensity they usually - but not always - have an eyewall and radius of maximum winds that contract to a very small size, around 5 to 15 miles. At this point, some of the outer rainbands may organize into an outer ring of thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and momentum. During this phase, the tropical cyclone is weakening (i.e. the maximum winds die off a bit and the central pressure goes up). Eventually the outer eyewall replaces the inner one completely and the storm can be the same intensity as it was previously or, in some cases, even stronger. While the most obvious motion of clouds is toward the center, tropical cyclones also develop an outward flow of clouds. These originate from air that has released its moisture and is expelled at high altitude through a chimney effect of the storm engine. This outflow produces high, thin cirrus clouds that spiral away from the center. The high cirrus clouds may be the first signs of an approaching hurricane.

Categories and ranking

Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds, a Category 5 hurricane has the highest. The rankings are not absolute in terms of effects. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. In fact, tropical systems of less than hurricane strength can produce significant damage and human casualties, especially from flooding and landslides. The National Hurricane Center classifies hurricanes of Category 3 and above as Major Hurricanes. The Joint Typhoon Warning Center classifies typhoons with wind speeds of at least 150 mi/h (67 m/s or 241 km/h, equivalent to a strong Category 4 storm) as Super Typhoons. The definition of sustained winds recommended by the World Meteorological Organization (WMO) and used by most weather agencies is that of a 10-minute average. The U.S. weather service defines sustained winds based on 1-minute average speed measured about 10 meters (33 ft) above the surface.

Other storm systems

An extratropical cyclone is a storm that derives energy from horizontal temperature differences, which are typical in higher latitudes. A tropical cyclone can become extratropical as it moves toward higher latitudes if its energy source changes from heat released by condensation to differences in temperature between air masses. From space, extratropical storms have a characteristic "comma-shaped" cloud pattern. Extratropical cyclones can also be dangerous because their low-pressure centers cause powerful winds. In the United Kingdom and Europe, some severe northeast Atlantic cyclonic depressions are referred to as "hurricanes," even though they rarely originate in the tropics. These European windstorms can generate hurricane-force winds but are not given individual names. However, two powerful extratropical cyclones that ravaged France, Germany, and the United Kingdom in December 1999, "Lothar" and "Martin", were named due to their unexpected power (equivalent to a category 1 or 2 hurricane). In British Shipping Forecasts, winds of force 12 on the Beaufort scale are described as "hurricane force." There is also a polar counterpart to the tropical cyclone, called a polar low.

Movement and track

Large-scale winds

Although tropical cyclones are large systems generating enormous energy, their movements over the earth's surface are often compared to that of leaves carried along by a stream. That is, large-scale winds—the streams in the earth's atmosphere—are responsible for moving and steering tropical cyclones. The path of motion is referred to as a tropical cyclone's track. The major force affecting the track of tropical systems in all areas are winds circulating around high-pressure areas. Over the North Atlantic Ocean, tropical systems are steered generally westward by the east-to-west winds on the south side of the Bermuda High, a persistent high-pressure area over the North Atlantic. Also, in the area of the North Atlantic where hurricanes form, trade winds, which are prevailing westward-moving wind currents, steer tropical waves (precursors to tropical depressions and cyclones) westward from off the African coast toward the Caribbean and North America.

Coriolis effect

The earth's rotation also imparts an acceleration (termed the Coriolis Acceleration or Coriolis Effect). This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents (i.e. in the north, the northern part of the cyclone has winds to the west, and the Coriolis force pulls them slightly north. The southern part is pulled south, but since it is closer to the equator, the Coriolis force is a bit weaker there). Thus, tropical cyclones in the Northern Hemisphere, which commonly move west in the beginning, normally turn north (and are then usually blown east), and cyclones in the Southern Hemisphere are deflected south, if no strong pressure systems are counteracting the Coriolis Acceleration. The Coriolis acceleration also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds. (Much of that is due to the conservation of angular momentum - air is drawn in from an area much larger than the cyclone such that the tiny angular velocity of that air will be magnified greatly when the distance to the storm center shrinks.)

Interaction with high and low pressure systems

Finally, when a tropical cyclone moves into higher latitude, its general track around a high-pressure area can be deflected significantly by winds moving toward a low-pressure area. Such a track direction change is termed recurve. A hurricane moving from the Atlantic toward the Gulf of Mexico, for example, will recurve to the north and then northeast if it encounters winds blowing northeastward toward a low-pressure system passing over North America. Many tropical cyclones along the U.S. East Coast and in the Gulf of Mexico are eventually forced toward the northeast by low-pressure areas which move from west to east over North America.

Track 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. With their understanding of the forces that act on tropical cyclones, and a wealth of data from earth-orbiting satellites and other sensors, scientists have increased the accuracy of track forecasts over recent decades. High-speed computers and sophisticated simulation software allow forecasters to produce computer models that forecast tropical cyclone tracks based on the future position and strength of high- and low-pressure systems. But while track forecasts have become more accurate than 20 years ago, scientists say they are less skillful at predicting the intensity of tropical cyclones. They attribute the lack of improvement in intensity forecasting to the complexity of tropical systems and an incomplete understanding of factors that affect their development.

Landfall

Officially, "landfall" is when a storm's center (the center of the eye, not its edge) reaches land. Naturally, storm conditions may be experienced on the coast and inland well before landfall. In fact, for a storm moving inland, the landfall area experiences half the storm before the actual landfall. For emergency preparedness, actions should be timed from when a certain wind speed will reach land, not from when landfall will occur.

Unusual landfall areas

The following areas rarely have a recorded landfall of a tropical cyclone: Europe: Because of the high latitudes, the European mainland have only a handful recorded landfalls made by hurricanes and or tropical storms. Notable examples are Hurricane Debbie of 1961 and Hurricane Vince of 2005. Azores: Like Europe, the Azores have a some recorded landfalls of hurricanes and tropical storms. Canary Islands: Until Tropical Storm Delta of 2005, the Canary Islands were rarely affected by any tropical storm or hurricanes. West African Coast: No recorded landfall of a tropical storm or hurricane although some come close but bypass the area. Cape Verde Islands: Some records of landfall made by a tropical storm or hurricane, most notably 1982's Tropical Storm Beryl that killed 115 people. Venezuela: Rarely a tropical storm or hurricane makes landfall in this country. Notable examples are 1993's Tropical Storm Bret and Hurricane Joan of 1988. California: Rarely a tropical storm or hurricane have ever affected California. Notable storms were a tropical storm in 1939 and a hurricane in 1858. New Zealand: On rare circumstances, a cyclone or two have made landfall in that country.

Dissipation

A tropical cyclone can cease to have tropical characteristics in several ways:
- It moves over land, thus depriving it of the warm water it needs to power itself, and quickly loses strength. Most strong storms become disorganized areas of low pressure within a day or two of landfall. There is, however, a chance they could regenerate if they manage to get back over open warm water. If a storm is over mountains for even a short time, it can rapidly lose strength. This is, however, the cause of many storm fatalities, as the dying storm unleashes torrential rainfall, and in mountainous areas, this can lead to deadly mudslides. The storm loses strength slower over flatter or marshy areas than over mountainous terrain which disrupts the surface circulation of the storm more.
- It remains in the same area of ocean for too long, sucking up all the warm water. Without warm surface water, the storm cannot survive.
- It experiences wind shear, causing the convection to lose direction and the heat engine to break down.
- It can be weak enough to be consumed by another area of low pressure, disrupting it and joining to become a large area of non-cyclonic thunderstorms. (Such, however, can re-strengthen the non-tropical system as a whole.)
- It enters colder waters. This does not necessarily mean the death of the storm, but the storm will lose its tropical characteristics. These storms are extratropical cyclones.
- An outer eye wall forms (typically around 50 miles from the center of the storm), choking off the convection toward the inner eye wall. Such weakening is generally temporary unless it meets other conditions above. Even after a tropical cyclone is said to be extratropical or dissipated, it can still have tropical storm force (or occasionally hurricane force) winds and drop several inches of rainfall. When a tropical cyclone reaches higher latitudes or passes over land, it may merge with weather fronts or develop into a frontal cyclone, also called extratropical cyclone. In the Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach Europe as a European windstorm.

Artificial dissipation

In the 1960s and 1970s, the United States government attempted to weaken hurricanes in its Project Stormfury by seeding selected storms with silver iodide. It was thought that the seeding would disrupt the storm's eyewall, causing it to collapse and thus reduce the winds. The winds of Hurricane Debbie dropped as much as 30 percent, but then regained their strength after each of two seeding forays. In an earlier episode, disaster struck when a hurricane east of Jacksonville, Florida, was seeded, promptly changed its course, and smashed into Savannah, Georgia. 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 percent chance of making landfall within 48 hours. This placed severe restrictions on the project, and when the Navy pulled out in 1972, it all but killed any further attempts at hurricane seeding in the Atlantic. It was later discovered that eyewall disruption happens naturally as part of eyewall replacement cycles, and so the success of the program was impossible to gauge. Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans; covering the ocean in a substance that inhibits evaporation; or blasting the cyclone apart with nuclear weapons. These approaches all suffer from the same flaw: tropical cyclones are simply too large for any of them to be practical [http://www.aoml.noaa.gov/hrd/tcfaq/C5f.html]. However, it has been suggested by some that we can change the course of a storm during its early stages of formation, (detailed by an article, Controlling Hurricanes, Scientific American, 2005), such as using satellite to alter the environmental conditions or, more realistically, spreading degradable film of oil over the ocean, which prevent water vapour from fueling the storm.

Monitoring, observation and tracking

Intense tropical cyclones pose a particular observation challenge. As they are a dangerous oceanic phenomenon, weather stations are rarely available on the site of the storm itself. Surface level observations are generally available only if the storm is passing over an island or a coastal area, or it has overtaken an unfortunate ship. Even in these cases, real-time measurement taking is generally possible only in the periphery of the cyclone, where conditions are less catastrophic. It is however possible to take in-situ measurements, in real-time, by sending specially equipped reconnaissance flights into the cyclone. In the Atlantic basin, these flights are regularly flown by US government hurricane hunters [http://www.hurricanehunters.com/]. The aircraft used are WC-130 Hercules and WP-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. This demonstrated a new way to probe the storms at low altitudes that human pilots seldom dare[http://www.sunherald.com/mld/sunherald/12699210.htm]. Tropical cyclones far from land are tracked by weather satellites using visible light and infrared bands. These satellite images are received regularly on half hour intervals. As the hurricane approaches land, the cyclone can also be imaged remotely by a nationwide system of Doppler radar. Land-based Doppler radars play a crucial role during landfall because they give forecasters the ability to see the storms location and intensity minute by minute. Recently, university researchers have begun to deploy mobile weather stations fortified to withstand hurricane-force winds. The two largest programs are the Florida Coastal Monitoring Program [http://www.ce.ufl.edu/~fcmp] and the Wind Engineering Mobile Instrumented Tower Experiment [http://www.atmo.ttu.edu/WEMITE/wemite.html]. During landfall, the NOAA Hurricane Research Division compares and quality controls reconnaissance aircraft data—which include flight-level, GPS sonde and stepped frequency microwave radiometer wind speed estimates—to wind speed data transmitted in real-time from weather stations erected near or at the coast. The National Hurricane Center uses these data to evaluate conditions at landfall and to verify its forecasts.

Naming of tropical cyclones

Storms reaching tropical storm strength (winds exceeding 17 metres per second, 38 mph, or 62 km/h) are given names, to assist in recording insurance claims, to assist in warning people of the coming storm, and to further indicate that these are important storms that should not be ignored. These names are taken from lists which vary from region to region and are drafted a few years ahead of time. The lists are decided upon, depending on the regions, either by committees of the World Meteorological Organization (called primarily to discuss many other issues), or by national weather services involved in the forecasting of the storms. Each year, the names of particularly destructive storms (if there were any) are "retired" and new names are chosen to take their place.

Naming schemes

The WMO's Regional Association IV Hurricane Committee selects the names for Atlantic Basin and central and eastern Pacific storms. In the Atlantic and Eastern North Pacific regions, feminine and masculine names are assigned alternately in alphabetic order during a given season. The "gender" of the season's first storm also alternates year to year: the first storm of an odd-numbered year gets feminine name, while the first storm of an even-numbered year gets a masculine name. Six lists of names are prepared in advance, and each list is used once every six years. Five letters — "Q," "U," "X," "Y" and "Z" — are omitted in the Atlantic; only "Q" and "U" are omitted in the Eastern Pacific, so the format accommodates 21 or 24 named storms in a hurricane season. Names of storms may be retired by request of affected countries if they have caused extensive damage. The affected countries then decide on a replacement name of the same gender (and if possible, the same ethnicity) as the name being retired. If there are more than 21 named storms in an Atlantic season or 24 named storms in an Eastern Pacific season, the rest are named as letters from the Greek alphabet: the 22nd storm is called "Alpha," the 23rd "Beta," and so on. This was first necessary during the 2005 season when the names Alpha, Beta, Gamma, Delta, and Epsilon were all used. There is no precedent for a storm named with a Greek Letter causing enough damage to justify retirement; how this situation would be handled is unknown. In the Central North Pacific region, the name lists are maintained by the Central Pacific Hurricane Center in Honolulu, Hawaii. Four lists of Hawaiian names are selected and used in sequential order without regard to year. In the Western North Pacific, name lists are maintained by the WMO Typhoon Committee. Five lists of names are used, with each of the 14 nations on the Typhoon Committee submitting two names to each list. Names are used in the order of the countries' English names, sequentially without regard to year. Japan Meteorological Agency uses a secondary naming system in Western North Pacific that numbers a typhoon on the order it formed, resetting on December 31 of every year. The Typhoon Songda in September 2004 is internally called the typhoon number 18 and is recorded as the typhoon 0418 with 04 taken from the year. The Australian Bureau of Meteorology maintains three lists of names, one for each of the Western, Northern and Eastern Australian regions. There are also Fiji region and Papua New Guinea region names. The Seychelles Meteorological Service maintains a list for the Southwest Indian Ocean.

History of tropical cyclone naming

For several hundred years after Europeans arrived in the West Indies, hurricanes there were named after the saint's day on which the storm struck. The practice of giving storms people's names was introduced by Clement Wragge, an Anglo-Australian meteorologist at the end of the 19th century. He used feminine names and the names of politicians who had offended him. During World War II, tropical cyclones were given feminine names, mainly for the convenience of the forecasters and in a somewhat ad hoc manner. For a few years afterwards, names from the Joint Army/Navy Phonetic Alphabet were used. The modern naming convention came about in response to the need for unambiguous radio communications with ships and aircraft. As transportation traffic increased and meteorological observations improved in number and quality, several typhoons, hurricanes or cyclones might have to be tracked at any given time. To help in their identification, beginning in 1953 the practice of systematically naming tropical storms and hurricanes was initiated by the United States National Hurricane Center, and is now maintained by the WMO. In keeping with the common English language practice of referring to inanimate objects such as boats, trains, etc., using the female pronoun "she," names used were exclusively feminine. The first storm of the year was assigned a name beginning with the letter "A", the second with the letter "B", etc. However, since tropical storms and hurricanes are primarily destructive, some considered this practice sexist. The National Weather Service responded to these concerns in 1979 with the introduction of masculine names to the nomenclature. It was also in 1979 that the practice of preparing a list of names before the season began. The names are usually of English, French or Spanish origin in the Atlantic basin, since these are the three predominant languages of the region where the storms typically form.

Renaming of tropical cyclones

In most cases, a tropical cyclone retains its name throughout its life. However, a tropical cyclone may be renamed in several occasions. 1. A tropical storm enters the southwestern Indian Ocean from the east In the south Indian Ocean, RSMC la Reunion names a tropical storm once it crosses 90°E from the east, even though it has been named. In this case, the Joint Typhoon Warning Center (JTWC) will put two names together with a hyphen. Examples: Oscar-Itseng(2004), Adeline-Juliet(2005) 2. A tropical storm crosses from the Atlantic into the Pacific, or vice versa, before 2001 It was the policy of National Hurricane Center (NHC) to rename a tropical storm which crossed from Atlantic into Pacific, or vice versa. Examples: Cesar-Douglas(1996), Joan-Miriam(1988) In 2001, when Iris moved across Central America, NHC mentioned that Iris would retain its name if it regenerated in the Pacific. However, the Pacific tropical depression developed from the remnants of Iris was called Fifteen-E instead. The depression later became tropical storm Manuel. NHC explained that the Iris had dissipated as a tropical cyclone prior to entering the eastern North Pacific basin, the new depression was properly named Fifteen-E, rather than Iris. In 2003, when Larry was about to move across Mexico, NHC attempted to provide greater clarity: :Should Larry remain a tropical cyclone during its passage over Mexico into the Pacific, it would retain its name. However, a new name would be given if the surface circulation dissipates and then regenerates in the Pacific. Up to now, there has been no tropical cyclone retaining its name during the passage from Atlantic to Pacific, or vice versa. 3. Uncertainties of the continuation When the remnants of a tropical cyclone redevelop, the redeveloping system will be treated as a new tropical cyclone if there are uncertainties of the continuation, even though the original system may contribute to the forming of the new system. Example: TD10/TD12 (eventually developed into Hurricane Katrina) (2005) 4. Human faults Sometimes, there may be human faults leading to the renaming of a tropical cyclone. Example: Ken-Lola(1989)

Effects

Ken-Lola. Katrina was the most costly tropical cyclone in United States history.]] A mature tropical cyclone can release heat at a rate upwards of 6x10¹⁴ watts [http://www.noaa.gov/questions/question_082900.html]. Tropical cyclones on the open sea cause large waves, heavy rain, and high winds, disrupting international shipping and sometimes sinking ships. However, the most devastating effects of a tropical cyclone occur when they cross coastlines, making landfall. A tropical cyclone moving over land can do direct damage in four ways.
- High winds - Hurricane strength winds can damage or destroy vehicles, buildings, bridges, etc. High winds also turn loose debris into flying projectiles, making the outdoor environment even more dangerous.
- Storm surge - Tropical cyclones cause an increase in sea level, which can flood coastal communities. This is the worst effect, as cyclones claim 80% of their victims when they first strike shore.
- Heavy rain - The thunderstorm activity in a tropical cyclone causes intense rainfall. Rivers and streams flood, roads become impassable, and landslides can occur.
- Tornado activity - The broad rotation of a hurricane often spawns tornadoes. While these tornadoes are normally not as strong as their non-tropical counterparts, they can still cause tremendous damage. Tornado Often, the secondary effects of a tropical cyclone are equally damaging. They include:
- Disease - The wet environment in the aftermath of a tropical cyclone, combined with the destruction of sanitation facilities and a warm tropical climate, can induce epidemics of disease which claim lives long after the storm passes. One of the most common post-hurricane injuries is stepping on a nail in storm debris, leading to a risk of tetanus or other infection. Infections of cuts and bruises can be greatly amplified by wading in sewage-polluted water.
- Power outages - Tropical cyclones often knock out power to tens or hundreds of thousands of people (or occasionally millions if a large urban area is affected), prohibiting vital communication and hampering rescue efforts.
- Transportation difficulties - Tropical cyclones often destroy key bridges, overpasses, and roads, complicating efforts to transport food, clean water, and medicine to the areas that need it.

Beneficial effects of tropical cyclones

Although cyclones take an enormous toll in lives and personal property, they may bring much-needed precipitation to otherwise dry regions. Hurricane Camille averted drought conditions and ended water deficits along much of its path. Hurricane Floyd did the same thing in New Jersey in 1999. The destruction caused by Camille on the Gulf coast spurred redevelopment as well, greatly increasing local property values. On the other hand, disaster response officials point out that redevelopment encourages more people to live in clearly dangerous areas subject to future deadly storms (as shown by the effects of Hurricane Katrina). Of course, many former residents and businesses do relocate to inland areas away from the threat of future hurricanes as well. Hurricanes also help to maintain global heat balance by moving warm, moist tropical air to the mid-latitudes and polar regions.

Long term trends in cyclone activity

While the number of storms in the Atlantic has increased since 1995, there seems to be no signs of a global trend; the global number of tropical cyclones remains about 90 ± 10. [http://wind.mit.edu/~emanuel/anthro2.htm]. Atlantic storms are certainly becoming more destructive financially, since five of the ten most expensive storms in United States history have occurred since 1990. This can to a large extent be attributed to the number of people living in susceptible coastal area, and massive development in the region since the last surge in Atlantic hurricane activity in the 1960s. Often in part because of the threat of hurricanes, many coastal regions

Hurricane (disambiguation)

Weather Phenomena
- A hurricane is a type of storm, otherwise called a typhoon or tropical cyclone.
- On the Beaufort scale, hurricane is the name assigned to the highest wind-speed category. Sports teams
- The National Hockey League franchise in North Carolina is the Carolina Hurricanes.
- Athletic teams from the University of Miami are known as the Hurricanes, or simply 'Canes.
- The Super 14 rugby union franchise from Wellington, New Zealand is known as 'the Hurricanes'. Airplanes
- The Hawker Hurricane was a propeller-driven fighter aircraft used by the RAF. Places
- Hurricane is the name of several places in the United States:
- Hurricane, Utah
- Hurricane, West Virginia
- Hurricane, Alabama Events
- Operation Hurricane was the name given by the British to their first nuclear test, in 1952.
- The Hurricane Festival is a German rock music festival. Literature
- The Hurricane is a novel by Charles Nordhoff and James Norman Hall, about a Pacific Ocean hurricane. People
- Rubin "Hurricane" Carter is a former boxer whose conviction for murder was overturned.
- The Hurricane is the stage name of a professional wrestler, Gregory Helms. Films
- The Hurricane (1937), directed by John Ford starring Dorothy Lamour, is based on the novel by Nordhoff and Hall.
- Hurricane (1979) is also based on the novel.
- The Hurricane (1999) is about Rubin "Hurricane" Carter. Music
- Hurricane is the name of a Bob Dylan song about Rubin "Hurricane" Carter.
- Hurricane is also the name of a song by Something Corporate
- Hurricane is also the name of a song by Ani Difranco
- [http://hyperrust.org/Music/?s168 Like A Hurricane] is the name of a song by Neil Young
- Hurricane #1 were a band consisting of Andy Bell, Alex Lowe, Will Pepper and Gareth Farmer. Food & Drink Hurricane (drink) is an alcoholic drink that was originated in New Orleans, Louisiana

Typhoon (disambiguation)

Typhoon can refer to:
- tropical cyclones
- Typhon, a monster of Greek mythology
- Typhoon, a novel by Joseph Conrad.
- Typhoon class submarine
- the Eurofighter Typhoon, a multirole combat aircraft.
- the Hawker Typhoon, a British World War II aircraft
- the GMC Typhoon, a high-performance sport-utility automobile
- German codename for the military offensive directed against Moscow, starting on 30 September 1941, see Battle of Moscow
- Fred Ottman, Typhoon, a professional wrestler

Meteorology

visible at the top of the image.]] Meteorology is the scientific study of the atmosphere that focuses on weather processes and forecasting. Meteorological phenomena are observable weather events which illuminate and are explained by the science of meteorology. Those events are bound by the variables that exist in Earth's atmosphere. They are temperature, pressure, water vapor, and the gradients and interactions of each variable, and how they change in time. The majority of Earth's observed weather is located in the troposphere. Meteorology, climatology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences.

History of meteorology

Early achievements in meteorology

atmospheric sciences
- 350 BCE The term meteorology comes from Aristotle's Meteorology. Although the term meteorology is used today to describe a subdiscipline of the atmospheric sciences, Aristotle's work is more general. The work touches upon much of what is known as the earth sciences. In his own words:

...all the affections we may call common to air and water, and the kinds and parts of the earth and the affections of its parts.

One of the most impressive achievements in Meteorology is his description of what is now known as the hydrologic cycle:

Now the sun, moving as it does, sets up processes of change and becoming and decay, and by its agency the finest and sweetest water is every day carried up and is dissolved into vapour and rises to the upper region, where it is condensed again by the cold and so returns to the earth.

- 1607 Galileo Galilei constructs a thermoscope. Not only did this device measure temperature, but it represented a paradigm shift. Up to this point, heat and cold were believed to be qualities of Aristotle's elements (fire, water, air, and earth). Note: There is some controversy about who actually built this first thermoscope. There is some evidence for this device being independently built at several different times. This is the era of the first recorded meteorological observations. As there was no standard measurement, they were of little use until the work of Daniel Gabriel Fahrenheit and Anders Celsius in the 18th century.
- 1643 Evangelista Torricelli, a contemporary and one-time assistant of Galileo, creates the first man-made sustained vacuum in 1643, and in the process creates the first barometer. Changes in height of mercury in this Toricelli Tube lead to his discovery that atmospheric pressure changes over time.
- 1648 Blaise Pascal discovers that atmospheric pressure decreases with height, and deduces that there is a vacuum above the atmosphere.
- 1667 Robert Hooke builds an anemometer to measure windspeed.
- 1686 Edmund Halley maps the trade winds, deduces that atmospheric changes are driven by solar heat, and confirms the discoveries of Pascal about atmospheric pressure.
- 1735 George Hadley is the first to take the rotation of the Earth into account to explain the behavior of the trade winds. Although the mechanism Hadley described was incorrect, predicting trade winds half as strong as the actual winds, the circulating cells that Hadley described later become known as Hadley cells. Hadley cell
- 1743-1784 Benjamin Franklin observes that weather systems in North America move from west to east, demonstrates that lightning is electricity, publishes the first scientific chart of the Gulf Stream, links a volcanic eruption to weather, and speculates about the effect of deforestation on climate.
- 1780 Horace de Saussure constructs a hair hygrometer to measure humidity.
- 1802-1803 Luke Howard writes On the Modification of Clouds in which he assigns cloud types Latin names. Synoptic-scale weather observations were still hindered by the difficulty of establishing certain weather characteristics such as clouds or wind. These were solved when Luke Howard and Francis Beaufort introduced their systems for classifying clouds (1802) and wind speeds (1806), respectively. The real turning point however was the invention of the telegraph in 1843 that allowed exchange of weather information with unprecedented speed.

The Coriolis Effect

Understanding the kinematics of how exactly the rotation of the Earth affects airflow was partial at first. Late in the 19th century the full extent of the large scale interaction of pressure gradient force and deflecting force that in the end causes air masses to move along isobars was understood. Early in the 20th century this deflecting force was named the Coriolis Effect after Gaspard-Gustave Coriolis, who had published in 1835 on the energy yield of machines with rotating parts, such as waterwheels. In 1856, William Ferrel proposed the existence of a circulation cell in the mid-latitudes with air being deflected by the coriolis force to create the prevailing westerly winds.

Numerical weather prediction

cell Early in the 20th century, advances in the understanding of atmospheric physics led to the foundation of modern numerical weather prediction. In 1922, Lewis Fry Richardson published `Weather prediction by numerical process` which described how small terms in the fluid dynamics equations governing atmospheric flow could be neglected to allow numerical solutions to be found. However, the sheer number of calculations required was too large to be completed before the advent of computers. At this time in Norway a group of meteorologists led by Vilhelm Bjerknes developed the model that explains the generation, intensification and ultimate decay (the life cycle) of midlatitude cyclones, introducing the idea of fronts, that is, sharply defined boundaries between air masses. The group included Carl-Gustaf Rossby (who was the first to explain the large scale atmospheric flow in terms of fluid dynamics), Tor Bergeron (who first determined the mechanism by which rain forms) and Jacob Bjerknes. Starting in the 1950s, numerical experiments with computers became feasible. The first weather forecasts derived this way used barotropic (that means, single-vertical-level) models, and could successfully predict the large-scale movement of midlatitude Rossby waves, that is, the pattern of atmospheric lows and highs. In the 1960s, the chaotic nature of the atmosphere was first understood by Edward Lorenz, founding the field of chaos theory. These advances have led to the current use of ensemble forecasting in most major forecasting centers, to take into account uncertainty arising due to the chaotic nature of the atmosphere.

Satellite observation

In 1960, the launch of TIROS-1, the first successful weather satellite marked the beginning of the age where weather information is available globally. Weather satellites along with more general-purpose Earth-observing satellites circling the earth at various altitudes have become an indispensable tool for studying a wide range of phenomena from forest fires to El Niño. In recent years, climate models have been developed that feature a resolution comparable to older weather prediction models. These climate models are used to investigate long-term climate shifts, such as what effects might be caused by human emission of greenhouse gases.

Weather forecasting

greenhouse gas Meteorologists and weather presenters use several methods to predict what the weather will be like in the future. Most of these methods wrer used several decades ago (before the 70's) when computers simply did not exist or were unable to perform fast enough for Numerical Weather Prediction. There are used nowadays to estimate the effectiveness of weather forecasting : compared to persistance or to climatology ... Analogs were a nightmare : one could not find a reasonable match among the infinity of possibilities offered by the atmosphere. Frontology is used now as a mean to describe objects resulting from NWP. Cold fronts exist, but warm fronts are not fronts as such : the symbol of warm air struggling against cold air does not represent the reality.
- Persistence method The persistence method assumes that conditions will not change. Often summarised as "Tomorrow equals today". This method works best over long periods of time.
- Trends method The trends method involves determining the speed and direction of fronts, high and low pressure centers, and areas of clouds and precipitation.
- Climatology method The climatology method involves using historical weather data collected over long periods of time (years) to predict conditions on a given date.
- Analog method A complex method that involves finding an "analog" or very similar weather conditions from historical data.
- Numerical forecasting method The numerical weather prediction or NWP method uses computers to take into account a large number of variables and creates of computer model of the atmosphere. This is the most successful and widely used method.

Meteorology and climatology

With the development of powerful new supercomputers like the Earth Simulator in Japan, mathematical modeling of the atmosphere can reach unprecedented accuracy. This is not only due to the enhanced spatial and temporal resolution of the grids employed, but also because these more powerful machines can model the Earth as an integrated climate system, where atmosphere, ocean, vegetation, and man-made influences depend on each other realistically. The goal in global meteorological modeling can be termed Earth System Modeling, with a growing number of models of various processes coupled to each other. Predictions for global effects like Global Warming and El Niño are expected to benefit substantially from these advancements. Regional models are attracting more interest as the resolution of global models increases. With regional weather disasters such as the Elbe flooding in 2002 and the European heat wave in 2003, decision makers expect from these models accurate assessments about the possible increase of these natural hazards in a specific region. Countermeasures such as dikes or intentional flooding might be effective in preventing or at least attenuating natural hazards. For models at all scales, increased model resolution means less reliance on parameterizations, which are empirically derived expressions for processes that cannot be resolved on the model grid. For example, in mesoscale models individual clouds can now be resolved, removing the need for formulations that average over a grid box. In global modeling, atmospheric waves such as gravity waves with short temporal and spatial scales can be represented without resorting to often overly simplified parameterizations.

Meteorological topics and phenomena

Institutions of meteorology/atmospheric science

- [http://www.met.wau.nl Wageningen University Meteorology Department, The Netherlands]
- American Geophysical Union
- American Meteorological Society
- Chatham-Kent Meteorology Commission
- Hong Kong Observatory
- National Center for Atmospheric Research
- National Oceanic and Atmospheric Administration
- National Severe Storms Laboratory
- [http://www.ncdc.noaa.gov/ National Climatic Data Center]
- [http://www.nws.noaa.gov/ National Weather Service]
- National Centers for Environmental Prediction
- National Hurricane Center
- Storm Prediction Center
- [http://www.nwas.org/ National Weather Association]
- National Weather Center
- [http://www.msc.ec.gc.ca/ Meteorological Service of Canada]
- World Meteorological Organization
- Global Atmosphere Watch
- European Centre for Medium-Range Weather Forecasts
- Royal Meteorological Society
- [http://www.met.rdg.ac.uk Department of Meteorology, University of Reading, UK]
- Met Office
- [http://www.met.ie/ Met Eireann]
- [http://www.kmi.be/ Royal Meteorology Institute Belgium]
- [http://www.iabm.org/ International Association of Broadcast Meteorology]
- [http://www.mi.uni-hamburg.de/ Meteorological Institute Hamburg]
- [http://atm.ucdavis.edu Atmospheric Science Program at UC Davis]
- [http://weatheroffice.ec.gc.ca/canada_e.html Environment Canada Weather Office]
- [http://www.bom.gov.au Australian Bureau of Meteorology]
- [http://www.metservice.com New Zealand MetService]
- [http://www.meteoswiss.ch/en/index.shtml Meteo Suisse]
- [http://www.npmoc.navy.mil Naval Maritime Forecast Center]/[http://www.npmoc.navy.mil/jtwc.html Joint Typhoon Warning Center]
- [http://www.smg.gov.mo/ Direcção dos Serviços Meteorológicos e Geofísicos]
- Japan Meteorological Agency

Weather-related links

Please see Weather forecasting for actual weather forecast sites
- [http://australiasevereweather.com/photography/ Australia Severe Weather] has a comprehensive photo collection of weather phenomena.
- [http://amsglossary.allenpress.com/glossary Glossary of Meteorology from the American Meteorological Society], an excellent reference of nomenclature, equations, and concepts for the more advanced reader.
- [http://www.storm2k.org/ Storm2k.org] An excellent interactive resource of current weather and weather topics from professionals and weather fans alike.
- [http://www.allmetsat.com/ Allmetsat] Satellite images, observations and forecasts, tropical cyclone tracking.
- [http://ww2010.atmos.uiuc.edu World Weather 2010] The World Weather 2010 Project at the UIUC.
- [http://www.stormlab.net Storm Laboratory] place for weather software and storm chasing info. Category:Atmospheric sciences Category:Applied and interdisciplinary physics ko:기상학 ms:Meteorologi ja:気象学

Low pressure area

A low pressure area, or a low for short, is a region where the atmospheric pressure is lowest with relation to the surrounding area. Tropical storms, extratropical cyclones, subpolar cyclones, and subarctic cyclones are called low-pressure cells in English-speaking nations such as the United States and Canada. Lows are frequently associated with stronger winds and atmospheric lift. This lift will generally produce cloud cover, due to adiabatic cooling, once the air becomes saturated. Thus, low pressure typically brings cloudy or overcast skies, which may minimize diurnal temperature extremes in both summer and winter, due to the significant cloud cover. This is due to less incoming shortwave solar radiation and lower temperatures, since the clouds reflect sunlight. At night, the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons. Climatologically, low pressure forms at the Intertropical convergence zone (ITCZ), as part of the Hadley cell circulation. Many of the world's rainforests are associated with these climatological low pressure systems. Frontal lows are temperate zone phenomena, and develop along the polar front as a result of the interaction between cold and warm surface air masses. Thermal lows also form over areas such as Death Valley as the result of intense ground heating; they are much smaller in geographic extent than either convergence lows or frontal lows. Surface low pressure systems will tend to be smaller in area and have stronger surface winds than a given high pressure system, due to the addition of surface friction to the pressure gradient force and coriolis effect that drive the circulation. In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in an isolated low pressure area called a thermal low.

Cyclone

:This article is about the meteorological phenomenon. For other uses of the term see Cyclone (disambiguation). Cyclone (disambiguation).]] In meteorology, a cyclone is the rotation of a volume of air about an area of low atmospheric pressure. Cyclones are responsible for a wide variety of different meteorological phenomena such as tropical cyclones and tornadoes. Because of this, most weather forecasters avoid using the term cyclone without a qualifying term.

Structure

The center of a cyclone is a low-pressure region. Pressure gradient force, from high- to low-pressure regions, causes high winds around these regions. Wind flow around a large cyclone is almost invariantly anticlockwise, in the northern hemisphere, and clockwise, in the southern hemisphere, due to the Coriolis effect (viewed from above). Large anticyclonic storms are extremely rare on Earth, though Jupiter's Great Red Spot storm is anticyclonic.

Types of Cyclones

Tropical cyclones

Tropical cyclones (also known as tropical storms, hurricanes and typhoons) are cyclones which form over warm (generally tropical) ocean waters and draw their energy from evaporation and condensation. They are characterized by a strong area of low pressure at the surface and an area of higher pressure aloft. Tropical cyclones are associated with strong thunderstorms, high winds, and flooding.

Extratropical cyclones

Extratropical cyclones (or low-pressure cells) lie somewhere in between tropical cyclones and mid-latitude cyclones, drawing a portion of their energy through the evaporation and condensation of ocean water, and some through horizontal temperature gradients in the atmosphere. They move out of the tropical regions towards the polar regions, bringing precipitation in the form of rain or drizzle. They often form quickly along cold fronts that have stagnated after moving into an area where warm, moist air exists. The warm, moist air is less dense, therefore it overruns the more dense cold air at and behind the cold front. A cyclonic motion is imparted to the ascending air, naturally, forming a shallow cyclone. Extratropical cyclones are also formed from tropical cyclones when they move into non-tropical regions and lose tropical characteristics.

Subtropical cyclones

A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wide band of latitude, from the equator to 50°.

Mid-latitude cyclones

Mid-latitude cyclones are generated by the temperature difference between warm and cold air masses, with warm water at high latitudes generally providing that differential. These storms have a cold core, unlike tropical and extratropical cyclones. Similar storms may appear at very high latitudes: these very cold storms are called subarctic (or subantarctic) cyclones.

Polar low

Polar lows are similar in behavior and size to tropical cyclones, although generally much shorter lived. Polar lows are typically several hundred kilometres in diameter, generally have strong winds (although generally not at hurricane intensity) and last one to two days on average. Unlike most typical cyclones they develop extremely rapidly reaching peak intensity within 24 hours. They generally form under cold upper-level lows when cold arctic air flows over a warm body of water. On satellite imagery Polar Lows appear very similar to hurricanes with an eye and convective bands wrapping the storm in the counter-clockwise fashion. Research aircraft data suggests that these 'arctic hurricanes' may be warm-cored systems. Polar Lows are currently difficult to predict due to scarcity of data. Most predictions in this area are more subjective than the prediction of tropical cyclones.

Arctic cyclone

Arctic cyclones are vast areas of low pressure in polar regions that have a weak cyclonic rotation.

Mesocyclones

A mesocyclone is an area of vertical atmospheric rotation, typically 2-6 miles across. They are often found in the right-rear flank of supercell thunderstorms, and are visible as a hook echo on Doppler weather radar. The presence of a mesocyclone can only be truly verified by radar, although visual clues such as curved inflow bands may be present. Mesocyclones form when strong changes of wind speed and/or direction with height (wind shear) sets the lower part of the atmosphere spinning horizontally. The updraft of a thunderstorm can then draw this area of spinning air from horizontal to vertical.

Tornadoes

In North America, tornadoes are sometimes described as cyclones because they involve powerful winds around a low-pressure vortex. However, they differ from other cyclones by their very local nature; most cyclones are massive storms, while tornados are comparatively small but extremely powerful. Tornadoes occur on too local a scale for the Coriolis effect to determine the direction of rotation; for this reason tornado winds sometimes flow anticyclonically, or opposite the direction dictated by the Coriolis effect.

Martian cyclones

On April 27, 1999, a rare cyclone 1,100 miles in diameter was detected by the Hubble Space Telescope in the northern polar region of Mars. It consisted of three cloud bands wrapped around a massive 200 mile diameter eye, and contained features similar to storms that have been detected in the poles of Earth. It was only observed briefly, as it seemed to be dissipating when it was imaged six hours later, and was not seen on later imaging passes. [http://www.news.cornell.edu/Chronicle/99/5.27.99/Mars_cyclone.html] (Dust devils have also been observed on Mars.)

External links

- [http://www.physicalgeography.net/fundamentals/7s.html Fundamental of Physical Geography: The Mid-Latitude Cyclone] - Dr. Michael Pidwirny, University of British Columbia, Okanagan
- [http://nsidc.org/arcticmet/glossary/cyclogenesis.html Glossary Definition: Cyclogenesis] - The National Snow and Ice Data Center
- [http://nsidc.org/arcticmet/glossary/cyclolysis.html Glossary Definition: Cyclolysis] - The National Snow and Ice Data Center
- [http://stormwiki.unk.edu/index.php/Cyclonic_rotation Glossary Definition: Cyclonic rotation] - StormWiki
- [http://www.weatheronline.co.uk/feature/wf261103.htm Weather Facts: The Polar Low] - Weather Online UK Category:Meteorology Category:Tropical cyclone meteorology Category:Weather Category:Weather hazards ja:サイクロン

Equator

The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. The equator divides the planet into a Northern Hemisphere and the Southern Hemisphere. The latitude of the equator is, by definition, 0°. The length of Earth's equator is about 40,075.0 km, or 24,901.5 miles. The equator is one of the five main circles of latitude based on the relationship of the Earth's rotation and plane of orbit around the sun. Additionally, the equator is the only line of latitude which is also a great circle The Sun, in its seasonal movement through the sky, passes directly over the equator twice each year on the Vernal and Autumnal Equinoxes, which occur in March and September (respectively). At the equator, the rays of the sun are perpendicular to the surface of the earth on these dates. Places near the equator experience the quickest rates of sunrise and sunset in the world, taking minutes. Such places also have a relatively constant amount of day/night time on every day throughout the year compared with more northerly or southerly places.

Equatorial climate

In many tropical regions people identify two seasons, wet and dry, but most places very close to the equator are wet throughout the year, although seasons can vary depending on a variety of factors including elevation and proximity to an ocean. ocean The surface of the Earth at the equator is mainly ocean. The highest point on the Equator is 4,690 m, at 77° 59' 31" W on the south slopes of Volcán Cayambe (summit 5,790 m) in Ecuador. This is a short distance above the snow line, and is the only point on the Equator where snow lies on the ground (Google Earth satellite data and photos).

Equatorial countries

The equator traverses the land and/or water of 13 countries in total:
- São Tomé and Príncipe - passing through Ilhéu das Rolas, an islet in this archipelago
- Gabon
- Republic of the Congo
- Democratic Republic of Congo
- Uganda
- Kenya
- Somalia
- Maldives - misses every island, passing between Gaafu Dhaalu Atoll and Gnaviyani Atoll
- Indonesia
- Sumatra - also small islands Tanah Masa to the West and Lingga to the East
- Borneo - Kalimantan
- Sulawesi
- Halmahera - also small islands Kayoa to the West and Gebe to the East
- Kawe, a small island near Waigeo - and other islets throughout Indonesia
- Kiribati - misses every island
- Gilbert Islands - passing between Aranuka and Nonouti Atolls
- Line Islands - passing between Kiritimati Island and Malden Island, though neither is very close to the equator
- Ecuador
- Galapagos Islands - passing through Isabela Island.
- Mainland Ecuador
- Colombia
- Brazil

Meters per second

Metre per second (U.S. spelling: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector), defined by distance in metres divided by time in seconds. The symbol is m/s, or equivalently, m s^-1.

Conversions

1 metre per second is equivalent to:
- 3.2808 feet per second
- 2.2369 miles per hour
- 3.6 km/h

Mph

The initials MPH can stand for more than one thing:
- Miles per hour, a measurement of speed
- Master of Public Health, a post-graduate degree in the U.S.. See Master's degree.

Atlantic Ocean

The Atlantic Ocean is Earth's second-largest ocean, covering approximately one-fifth of its surface. The ocean's name, derived from Greek mythology, means the "Sea of Atlas". This ocean occupies an elongated, S-shaped basin extending in a north-south direction and is divided into the North Atlantic and South Atlantic by equatorial counter currents at about 8° north latitude. Bounded by the Americas on the west and Europe and Africa on the east, the Atlantic is linked to the Pacific Ocean by the Arctic Ocean on the north and the Drake Passage on the south. An artificial connection between the Atlantic and Pacific is also provided by the Panama Canal. On the east, the dividing line between the Atlantic and the Indian Ocean is the 20° east meridian. The Atlantic is separated from the Arctic Ocean by a line from Greenland to northwestern Iceland and then from northeastern Iceland to southernmost tip of Spitsbergen and then to North Cape in northern Norway. Norway on a fair day.]] Covering approximately 20% of Earth's surface, the Atlantic Ocean is second only to the Pacific in size. With its adjacent seas it occupies an area of about 106,400,000 km² (41,100,000 square miles); without them, it has an area of 82,400,000 km² (31,800,000 mi²). The land area that drains into the Atlantic is four times that of either the Pacific or Indian oceans. The volume of the Atlantic Ocean with its adjacent seas is 354,700,000 km³ (85,100,000 mi³) and without them 323,600,000 km³ (77,640,000 mi³). The average depth of the Atlantic, with its adjacent seas, is 3,332 m (10,932 ft); without them it is 3,926 m (12,881 ft). The greatest depth, 8,605 m (28,232 ft), is in the Puerto Rico Trench. The width of the Atlantic varies from 2,848 km (1,770 miles) between Brazil and Liberia to about 4,830 km (3,000 miles) between the United States and northern Africa. The Atlantic Ocean has irregular coasts indented by numerous bays, gulfs, and seas. These include the Caribbean Sea, Gulf of Mexico, Gulf of St. Lawrence, Mediterranean Sea, Black Sea, North Sea, Labrador Sea, Baltic Sea, and Norwegian-Greenland Sea. Islands in the Atlantic Ocean include Faroe Islands, Greenland, Iceland, Rockall, Great Britain, Ireland, Fernando de Noronha, the Azores, the Madeira Islands, the Canaries, the Cape Verde Islands,Sao Tome e Principe, Newfoundland, Bermuda, the West Indies, Ascension, St. Helena, Trindade, Martin Vaz, Tristan da Cunha, the Falkland Islands, and South Georgia Island. South Georgia Island

Ocean bottom

The principal feature of the bottom topography of the Atlantic Ocean is a great submarine mountain range called the Mid-Atlantic Ridge. It extends from Iceland in the north to approximately 58° south latitude, reaching a maximum width of about 1,600 km (1,000 miles). A great rift valley also extends along the ridge over most of its length. The depth of water over the ridge is less than 2,700 m (8,900 ft) in most places, and several mountain peaks rise above the water, forming islands. The South Atlantic Ocean has an additional submarine ridge, the Walvis Ridge. The Mid-Atlantic Ridge separates the Atlantic Ocean into two large troughs with depths averaging between 3,700 and 5,500 m (12,000 and 18,000 ft). Transverse ridges running between the continents and the Mid-Atlantic Ridge divide the ocean floor into numerous basins. Some of the larger basins are the Guiana, North American, Cape Verde, and Canaries basins in the North Atlantic. The largest South Atlantic basins are the Angola, Cape, Argentina, and Brazil basins. The deep ocean floor is thought to be fairly flat, although numerous seamounts and some guyots exist. Several deeps or trenches are also found on the ocean floor. The Puerto Rico Trench, in the North Atlantic, is the deepest. The Laurentian Abyss is found off the eastern coast of Canada. In the south Atlantic, the South Sandwich Trench reaches a depth of 8,428 m (27,651 ft). A third major trench, the Romanche Trench, is located near the equator and reaches a depth of about 7,454 m (24,455 ft). The shelves along the margins of the continents constitute about 11% of the bottom topography. In addition, a number of deep channels cut across the continental rise. Ocean sediments are composed of terrigenous, pelagic, and authigenic material. Terrigenous deposits consist of sand, mud, and rock particles formed by erosion, weathering, and volcanic activity on land and then washed to sea. These materials are largely found on the continental shelves and are thickest off the mouths of large rivers or off desert coasts. Pelagic deposits, which contain the remains of organisms that sink to the ocean floor, include red clays and Globigerina, pteropod, and siliceous oozes. Covering most of the ocean floor and ranging in thickness from 60 m to 3,300 m (200 ft to 11,000 ft), they are thickest in the convergence belts and in the zones of upwelling. Authigenic deposits consist of such materials as manganese nodules. They occur where sedimentation proceeds slowly or where currents sort the deposits.

Water characteristics

sediment The salinity of the surface waters in the open ocean ranges from 33 to 37 parts per thousand by mass and varies with latitude and season. Although the minimum salinity values are found just north of the equator, in general the lowest values are in the high latitudes and along coasts where large rivers flow into the ocean. Maximum salinity values occur at about 25° north latitude. Surface salinity values are influenced by evaporation, precipitation, river inflow, and melting of sea ice. Surface water temperatures, which vary with latitude, current systems, and season and reflect the latitudinal distribution of solar energy, range from less than −2 °C to 29 °C (28 °F to 84 °F). Maximum temperatures occur north of the equator, and minimum values are

Tropical cyclone

Terms for tropical cyclones

Etymology

Mechanics of a tropical cyclone

Formation

When do tropical cyclones form?

Where do tropical cyclones form?

Major basins

Unusual formation areas

Average Season

Structure and classification

Types of tropical cyclones

Categories and ranking

Other storm systems

Movement and track

Large-scale winds

Coriolis effect

Interaction with high and low pressure systems

Track forecasting

Landfall

Unusual landfall areas

Dissipation

Artificial dissipation

Monitoring, observation and tracking

Naming of tropical cyclones

Naming schemes

History of tropical cyclone naming

Renaming of tropical cyclones

Effects

Beneficial effects of tropical cyclones

Long term trends in cyclone activity

Hurricane (disambiguation)

Typhoon (disambiguation)

Meteorology

History of meteorology

Early achievements in meteorology

The Coriolis Effect

Numerical weather prediction

Satellite observation

Weather forecasting

Meteorology and climatology

Meteorological topics and phenomena

Institutions of meteorology/atmospheric science

Weather-related links

Low pressure area

See also

Cyclone

Structure

Types of Cyclones

Tropical cyclones

Extratropical cyclones

Subtropical cyclones

Mid-latitude cyclones

Polar low

Arctic cyclone

Mesocyclones

Tornadoes

Martian cyclones

See also

External links

Equator

Equatorial climate

Equatorial countries

See also

Meters per second

Conversions

See also

Mph

Atlantic Ocean

Ocean bottom

Water characteristics