Tropical cyclone

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Image:Cyclone Catarina from the ISS on March 26 2004.JPG

"Hurricane" and "Typhoon" redirect here. For other uses, see Hurricane (disambiguation) and Typhoon (disambiguation).

A tropical cyclone is a storm system fueled by the heat released when moist air rises and the water vapor in it condenses. The term describes the storm's origin in the tropics and its cyclonic nature, which means that its circulation is counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. Tropical cyclones are distinguished from other cyclonic windstorms such as nor'easters, European windstorms, and polar lows by the heat mechanism that fuels them, which makes them "warm core" storm systems. Depending on their location and strength, there are various terms by which tropical cyclones are known, such as hurricane, typhoon, tropical storm, cyclonic storm, and tropical depression.

Tropical cyclones can produce extremely strong winds, tornadoes, torrential rain, high waves, and storm surges. They are born and sustained over large bodies of warm water and lose their strength over land, which is why coastal regions can receive much damage 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 25 miles/40 km inland. Although their effects on human populations can be devastating, tropical cyclones also can have the beneficial effect of relieving drought conditions. They carry heat away from the tropics, an important mechanism of the global atmospheric circulation that maintains equilibrium in the Earth's troposphere.

[edit] Classifications, terminology, and naming

[edit] Intensity classifications

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.

<span id="Tropical depression" />A tropical depression is an organized system of clouds and thunderstorms with a defined surface circulation and maximum sustained winds of less than 17 m/s (33 kt, 38 mph, or 62 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. It is already a low-pressure system, however, hence the name "depression."<ref name = "NOAA preparedness"/> The practice of the Philippines is to name tropical depressions from their own naming convention when the depressions are within the Philippines' sphere of influence.

<span id="Tropical storm" />A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds of at least 17 and not more than 32 m/s (34–63 kt, 39–73 mph, or 62–117 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).<ref name = "NOAA preparedness"/>

<span id="Hurricane" />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 m/s (64 kt, 74 mph, or 118 km/h).<ref name = "NOAA preparedness"/> 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 10–50 mi (16–80 km) wide in which the strongest thunderstorms and winds circulate around the storm's center. Maximum sustained winds in the strongest tropical cyclones have been measured at more than 85 m/s (165 kt, 190 mph, 305 km/h).

[edit] Terminology differences by region

Image:Typhoon Odessa 1985.jpg

Depending on their current basin and intensity, tropical cyclones may be referred to using one of many different terms, and each basin uses a separate system of terminology, making comparison difficult. This, however, is only common in the Pacific Ocean, where hurricanes from the Central North Pacific sometimes cross into the Northwest Pacific and are called typhoons. On rare occasions, the reverse will occur. If a tropical storm in the Northwestern Pacific reaches sustained winds of hurricane-strength on the Beaufort scale, it is referred to as a typhoon. It is also of note that typhoons with sustained winds greater than 239 kilometres per hour (150 mph) are called super typhoons by Joint Typhoon Warning Center.

The unique terms used in the Southwestern Indian Ocean include the following:

• "Moderate tropical storm" means a tropical disturbance in which the maximum of the average wind speed is 34 to 47 knots (63 to 88 km/h).
• "Severe tropical storm" means a tropical disturbance in which the maximum of the average wind speed is 48 to 63 knots (89 to 117 km/h).
• "Tropical cyclone" means a tropical disturbance in which the maximum of the average wind speed is 64 to 89 knots (118 to 165 km/h).
• "Intense tropical cyclone" means a tropical disturbance in which the maximum of the average wind speed is 90 to 115 knots (166 to 212 km/h).
• "Very intense tropical cyclone" means a tropical disturbance in which the maximum of the average wind speed is greater than 115 knots (greater than 212 km/h).<ref>Tropical Cyclone Programme (2006). Tropical Cyclone Operational Plan for the South-West Pacific Ocean. World Meteorological Organization. Retrieved on 2006-09-08.</ref>

There are many regional names for tropical cyclones, including baguio (and when spelled in the vernacular, bagyo) in the Philippines<ref name="Baguio AMS">American Meteorological Society. AMS Glossary: B. Glossary of Meteorology. Allen Press. Retrieved on 2006-12-01.</ref> and Taino in Haiti, but they are not used in operational warnings by the various tropical cyclone warning centers.

[edit] Categories and ranking

Main article: Tropical cyclone scales

<span id="Major hurricane" />Hurricanes are ranked according to their maximum winds using the Saffir-Simpson Hurricane Scale. A Category 1 storm has the lowest maximum winds (74-95 mph, 119-153 km/h) while a Category 5 hurricane has the highest (> 155 mph, 249 km/h). The U.S. National Hurricane Center classifies hurricanes of Category 3 and above as major hurricanes.

<span id="Super typhoon" />The U.S. Joint Typhoon Warning Center classifies West Pacific typhoons as tropical cyclones with winds greater than 73 mph (118 km/h). Typhoons with wind speeds of at least 150 mph (67 m/s or 241 km/h), equivalent to a strong Category 4 hurricane, are dubbed Super Typhoons.

The Australian Bureau of Meteorology uses a 1 to 5 scale called "tropical cyclone severity categories." Unlike the Saffir-Simpson Hurricane Scale, severity categories are based on estimated maximum wind gusts. A Category 1 storm features gusts less than 125 km/h (78 mph) while gusts in a Category 5 cyclone are at least 280 km/h (174 mph). Category 3, 4, and 5 storms are classified as "severe."<ref name="BOM intensity">Emergency Management Australia and Bureau of Meteorology (2002). Tropical Cyclone Severity Categories. Surviving Cyclones. Bureau of Meteorology. Retrieved on 2006-12-03.</ref>

Most countries use the maximum 10-minute wind average suggested by the World Meteorological Organization, which was once the standard in the United States. Meteorologists in the U.S. now use maximum 1-minute maximum sustained winds 10 meters above the ground to determine tropical cyclone strength.<ref name="NHC glossary">National Hurricane Center (2005). Glossary of NHC/TPC Terms. National Oceanic and Atmospheric Administration. Retrieved on 2006-11-29.</ref> Maximum wind speeds are typically about 12% lower with the 10-minute method than with the 1-minute method.<ref name="NWSM Defs">Tropical Cyclone Weather Services Program (June 1, 2006). Tropical cyclone definitions (PDF). National Weather Service. Retrieved on 2006-11-30.</ref><ref name="FEMA glossary">Federal Emergency Management Agency (2004). Hurricane Glossary of Terms. Retrieved on 2006-03-24. Accessed through the Wayback Machine.</ref>

The rankings are not absolute in terms of damage and other effects because the rankings are based only on windspeed. Lower-category storms can inflict greater damage than higher-category storms, depending on factors such as local terrain and total rainfall. For instance, a Category 2 hurricane that strikes a major urban area will likely do more damage than a large Category 5 hurricane that strikes a mostly rural region. In fact, tropical systems of minimal strength can produce significant damage and human casualties from flooding and landslides, particularly if they are slow-moving or very large in size. An example of a small, slow moving system producing great damage was Tropical Storm Allison, which caused $5.5 billion in damage (2001 USD) without ever reaching hurricane intensity.<ref name="AllisonTCR">Stewart, Stacy R. (February 8, 2002). Tropical Cyclone Report: Tropical Storm Allison: 5-17 June 2001. National Hurricane Center. Retrieved on 2006-11-30.</ref>

[edit] Origin of storm terms

• The word typhoon has two possible origins:
  • From the Chinese 大風 (daaih fūng (Cantonese); dà fēng (Mandarin)) which means "great wind."<ref name = "EO lib names">Earth Observatory. Hurricanes: The Greatest Storms on Earth. NASA. Retrieved on 2006-07-19.</ref> (The Chinese term as 颱風 táifēng, and 台風 taifū 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).^{[citation needed]}
  • From Urdu, Persian or Arabic ţūfān (طوفان) < Greek tuphōn (Τυφών).^{[citation needed]}
  • Portuguese tufão is also related to typhoon. See Typhon for more information.
• The word hurricane is derived from the name of a native Caribbean Amerindian storm god, Huracan, via Spanish huracán.<ref name = "AOML FAQ B4">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What is the origin of the word "hurricane"?. NOAA. Retrieved on 2006-07-25.</ref>
• The word cyclone was coined by a Captain Henry Piddington, who used it to refer to the storm that blew a freighter in circles in Mauritius in February of 1845.<ref name="Whipple 53">Whipple, Addison (1982). Storm. Alexandria, VA: Time Life Books, 53. ISBN 0-8094-4312-0.</ref> Tropical cyclones are then circular wind storms that form in the tropics.

[edit] Types of storms they can evolve towards or from

See also: Cyclone

In addition to tropical cyclones, there are two other classes of cyclones within the spectrum of cyclone types. These kinds of cyclones, known as extratropical cyclones and subtropical cyclones, can be stages a tropical cyclone passes through during its formation or dissipation.

[edit] Extratropical cyclone

Main article: Extratropical cyclone

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;<ref name = "AOML FAQ A7">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What is an extra-tropical cyclone?. NOAA. Retrieved on 2006-07-25.</ref> Additionally, although not as frequently, an extratropical cyclone can transform into a subtropical storm, and from there into a tropical cyclone. From space, extratropical storms have a characteristic "comma-shaped" cloud pattern. Extratropical cyclones can also be dangerous when their low-pressure centers cause powerful winds and very high seas.

[edit] Subtropical cyclone

Main article: Subtropical cyclone

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°. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.<ref name = "AOML FAQ A6">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What is a sub-tropical cyclone?. NOAA. Retrieved on 2006-07-25.</ref> From an operational standpoint, a tropical cyclone is usually not considered to become subtropical during its extratropical transition.<ref name=PadgetDecember2000>Padgett, Gary (2001). Monthly Global Tropical Cyclone Summary for December 2000. Retrieved on 2006-03-31.</ref> At this time, subtropical cyclones are handled operationally similarly to tropical cyclones only in the northern half of the Western Hemisphere and the southwest Indian Ocean.

[edit] Major basins and related warning centers

See also: Tropical cyclogenesis

Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the north Atlantic Ocean, the eastern and western parts of the Pacific Ocean (considered separately because tropical cyclones rarely form in the central Pacific), the southwestern Pacific, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean. The western Pacific is the most active and the north Indian the least active. An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least of Category 3 intensity).<ref name="Landsea variability table">Chris Landsea. Climate Variability table - Tropical Cyclones. Atlantic Oceanographic and Meteorological Laboratory, National Oceanic and Atmospheric Administration. Retrieved on 2006-10-19.</ref>

There are six Regional Specialised Meteorological Centres (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 five Tropical Cyclone Warning Centres (TCWCs) that provide information to smaller regions.<ref name="WMO RSMC list">World Meteorological Organization (April 25, 2006). RSMCs. Tropical Cyclone Programme (TCP). Retrieved on 2006-11-05.</ref> The RSMCs and TCWCs, however, are not the only organizations that provide information about tropical cyclones to the public. The Joint Typhoon Warning Center (JTWC) issues informal advisories in all basins except the Northern Atlantic and Northeastern Pacific. The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) issues informal advisories, as well as names, for tropical cyclones that approach the Philippines in the Northwestern Pacific. The Canadian Hurricane Centre (CHC) issues advisories on hurricanes and their remnants that affect Canada.

• Northern Atlantic Ocean: This region includes the North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. Tropical cyclone formation here varies widely from year to year, ranging from one to over twenty per year with an average of around ten.<ref name = "AOML FAQ E10">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What are the average, most, and least tropical cyclones occurring in each basin?. NOAA. Retrieved on 2006-11-30.</ref> 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 also are occasionally affected. 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. Occasionally, a hurricane that evolves into an extratropical cyclone can reach western Europe, including Hurricane Gordon (2006), which spread high winds across Spain and the British Isles in September 2006.<ref>Eric. S. Blake. Tropical Cyclone Report: Hurricane Gordon. Retrieved on 2006-11-29.</ref> Tropical Storm Vince, which made landfall on the southwestern coast of Spain as a tropical depression in September 2005, is the only known system to impact Europe as a tropical cyclone.<ref>James. L. Franklin. Tropical Cyclone Report: Hurricane Vince. Retrieved on 2006-11-29.</ref>
• Northeastern Pacific Ocean: This is the second most active basin and has the highest number of storms per unit area. Storms that form here can affect western Mexico, Texas, Hawaii, northern Central America, California, Arizona, and on rare occasions, Japan. No hurricane included in the modern database has made landfall in California; however, historical records from 1858 speak of a storm that brought San Diego winds over 75 mph/65 kts (marginal hurricane force), though it is not known if the storm actually made landfall.<ref>Michael Chenoweth and Christopher Landsea. The San Diego Hurricane of 2 October 1858. Retrieved on 2006-12-01.</ref> Tropical storms in 1939 and 1976 brought gale-force winds to California.
• Northwestern Pacific Ocean: Tropical storms in this region often affect China, Japan, Hong Kong, the Philippines, and Taiwan, as well as countries in Southeast Asia such as Vietnam, South Korea, and parts of Indonesia, plus numerous Oceanian islands. This is by far the most active basin, accounting for one-third of all tropical cyclone activity. The coast of China sees the most landfalling tropical cyclones worldwide.<ref name="Weyman and Anderson-Berry">Weyman, C & L Anderson-Berry Societal Impact of Tropical Cyclones accessed April 26, 2006</ref> The Philippines receives an average of 6-7 tropical cyclone landfalls per year.<ref>Shoemaker. Characteristics of Tropical Cyclones Affecting the Philippine Islands. Retrieved on 2006-11-29.</ref>
• 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 a double peak: one in April and May, before the onset of the monsoon, and another in October and November, just after.<ref>Joint Typhoon Warning Center. 1.2: North Indian Tropical Cyclones. Retrieved on 2006-11-29.</ref> The most deadly tropical cyclones have formed in this basin, including the 1970 Bhola cyclone that killed 200,000 people. Nations affected include India, Bangladesh, Sri Lanka, Thailand, Myanmar, and Pakistan. Rarely, a tropical cyclone that forms in this basin affects the Arabian Peninsula.
• Southwestern Pacific Ocean: Tropical activity in this region largely affects Australia and Oceania. Tropical storms rarely reach the vicinity of Brisbane, Australia and into New Zealand, usually during or after extratropical transition.<ref>Mark Sinclair. How often is New Zealand hit by tropical cyclones? Retrieved on 2006-12-01.</ref>
• Southeastern Indian Ocean: Tropical activity in this region affects Australia and Indonesia. According to the Australian Bureau of Meteorology, the most frequently hit portion of Australia is between Exmouth and Broome in Western Australia.<ref name="Tropical Cyclone Climatology in Western Australia">Bureau of Meteorology. BoM - Tropical Cyclones in Western Australia. Retrieved on August 8, 2006.</ref>
• Southwestern Indian Ocean: Despite nearly a half century of historical data, research at Reunion Island into tropical cyclones has been a priority only since 1999, when Météo-France began assigning additional personnel for research purposes. <ref>World Meteorological Organization. [www.wmo.ch/web/www/TCP/RSMC-TCWC/RSMC-LaReunion.doc Tropical Cyclone RSMC / South-West Indian Ocean.] Retrieved on 2006-11-29.</ref> Cyclones forming in this area can affect Madagascar, Mozambique, Mauritius, Reunion, Comoros, Tanzania, and Kenya.<ref>World Meteorological Organization. [www.wmo.ch/web/www/TCP/RSMC-TCWC/RSMC-LaReunion.doc Tropical Cyclone RSMC / South-West Indian Ocean.] Retrieved on 2006-11-29.</ref>

[edit] Times of formation

Worldwide, tropical cyclone activity peaks in late summer when the difference between temperatures aloft and sea surface temperatures are 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.<ref name = "AOML FAQ G1">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: When is hurricane season?. NOAA. Retrieved on 2006-07-25.</ref>

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 time frame 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.<ref name = "AOML FAQ G1"/>

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.<ref name = "AOML FAQ G1"/>

[edit] When naming occurs

Main article: Lists of tropical cyclone names

Storms reaching tropical storm strength are given names for several reasons: to eliminate confusion when there are multiple systems in any individual basin at the same time, 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 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.

[edit] Naming schemes

In the North Atlantic and Northeastern Pacific regions, feminine and masculine names are alternated in alphabetic order during a given season. The gender of the season's first storm also alternates year to year. 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 North Atlantic; only "Q" and "U" are omitted in the Northeastern Pacific. This allows for 21 names in the North Atlantic and 24 names in Northeastern Pacific. 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 language 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. This was first necessary during the 2005 Atlantic season when the list was exhausted. 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 Northwestern 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. Since 1981, the numbering system had been the primary system to identify tropical cyclone among Typhoon Committee members and it is still in use. International numbers are assigned by Japan Meteorological Agency on the order that a tropical storm forms while different internal numbers may be assigned by different NMCs. The Typhoon "Songda" in September 2004 was internally called the typhoon number 18 in Japan but typhoon number 19 in China. Internationally, it is recorded as the TY Sonda (0418) with "04" taken from the year. Names are retired from the lists upon request. The most common reason is to memorialize the extensive damage caused by the storm. When names are retired, the contributing member should propose new names. A possible way to do so is through local name nomination contest, which was done in Hong Kong<ref>Hong Kong Observatory. Change of Tropical Cyclone Names : "Dolphin" and "Lionrock" to replace "Yanyan" and "Tingting". Retrieved on 2006-12-01.</ref> and China.<ref>www.tianqi.cn</ref>

The Australian Bureau of Meteorology maintains three lists of names, one for each of the Western, Northern and Eastern Australian regions. These lists are in alphabetical order and alternate gender, but are used sequentially rather than switched each year. There are also Fiji region and Papua New Guinea region names agreed upon WMO RA V Tropical Cyclone Committee members.

The RA I Tropical Cyclone Committee for the South-West Indian Ocean creates the lists of names for the Southwestern Indian Ocean. The committee adopted two separate lists of names for the 2006-07 and 2007-08 tropical cyclone seasons at its October 2005 meeting in Gaborone, Botswana. Nominations for the lists were submitted by Mauritius, Malawi, Mozambique, Namibia, Seychelles, South Africa, Swaziland, Zimbabwe, Tanzania, Botswana, Comoros, Lesotho, and Madagascar. If a tropical disturbance reaches "moderate tropical storm" status west of 55 degrees east longitude, then the Sub-regional Tropical Cyclone Advisory Centre in Madagascar assigns the appropriate name to the storm. If a tropical disturbance reaches "moderate tropical storm" status between 55 and 90 degrees east longitude, then the Sub-regional Tropical Cyclone Advisory Centre in Mauritius assigns the appropriate name to the storm.<ref>World Meteorological Organization. Tropical Cyclone Operational Plan For the South-West Indian Ocean. Accessed September 8, 2006</ref>

[edit] 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 southwestern Indian Ocean, Météo-France in Réunion 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 include Cyclone Adeline-Juliet in early 2005 and Cyclone Bertie-Alvin in late 2005.

2. A tropical storm crosses from the Atlantic into the Pacific, or vice versa

   It was the policy of National Hurricane Center (NHC) to rename a tropical storm which crossed from Atlantic into Pacific, or vice versa. Examples include Hurricane Cesar-Douglas in 1996 and Hurricane Joan-Miriam in 1988.<ref name = "AOML FAQ E15">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What tropical storms and hurricanes have moved from the Atlantic to the Northeast Pacific or vice versa?. NOAA. Retrieved on 2006-07-25.</ref> Should a tropical cyclone during its passage over Mexico or Central America maintain its area of low pressure without dissipation into the adjacent tropical cyclone basin, it retains its name. However, a new name is given if the original surface circulation dissipates.<ref name=NHCLarry>NHC Tropical Storm Larry Discussion Number 16 accessed March 31, 2006</ref> Up to now, there has been no tropical cyclone retaining its name during the passage from the Northeast Pacific to the Atlantic basin, or vice versa, since the policy change in 2001.

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. One example is Tropical Depression 10-Tropical Depression 12 (which became Hurricane Katrina) from 2005.

4. Human errors

   Sometimes, there may be human faults leading to the renaming of a tropical cyclone. This is especially true if the system is poorly organized or if it passes from the area of responsibility of one forecaster to another. Examples include Tropical Storm Ken-Lola in 1989<ref name=JTWCKenLola>JTWC Ken-Lola Report accessed March 30, 2006</ref> and Tropical Storm Upana-Chanchu in 2000<ref name=Padgett>Padgett, G. Monthly Global Tropical Cyclone Summary for July 2000 accessed March 30, 2000</ref>

[edit] 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. If a second storm struck on the same saint's day later, it would be referred to as segundo (Spanish for "the second"), as with Hurricane San Felipe Segundo.

The practice of giving storms people's names was introduced by Clement Lindley Wragge, an Anglo-Australian meteorologist at the end of the 19th century. He used female names, the names of politicians who had offended him, and names from history and mythology.<ref name = "AOML FAQ B1">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: Why are tropical cyclones named?. NOAA. Retrieved on 2006-07-25.</ref><ref name="BOM Question 13">Bureau of Meteorology FAQ Question 13 accessed March 31, 2006</ref> During World War II, tropical cyclones were given feminine names, mainly for the convenience of forecasters and in a somewhat ad hoc manner. In addition, George R. Stewart's 1941 novel Storm helped to popularize the concept of giving names to tropical cyclones.<ref name = "AOML FAQ J4">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What fictional books, plays, and movies have been written involving tropical cyclones?. NOAA. Retrieved on 2006-07-25.</ref>

From 1950 through 1952, names from the Joint Army/Navy Phonetic Alphabet were used for storms in the North Atlantic.<ref>Chris Landsea. Subject: B1) Why are tropical cyclones named? Retrieved on 2006-11-30.</ref> The modern naming convention was 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, the practice of systematically naming tropical storms and hurricanes was initiated in 1953 by the United States National Hurricane Center. Naming is now maintained by the World Meteorological Organization (WMO).

In keeping with the common English language practice of referring to named inanimate objects such as boats, trains, etc., using the female pronoun "she," names used were exclusively feminine.^{[citation needed]} The first storm of the year was assigned a name beginning with the letter "A," the second with the letter "B," etc. Because tropical storms and hurricanes are often destructive, some considered this practice sexist. The WMO 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, because these are the three predominant languages of the region that the storms typically affect. In the southern hemisphere, male names were given to cyclones starting in 1975.<ref name="BOM Question 13"/>

[edit] Mechanics of tropical cyclones

Structurally, a tropical cyclone is a large, rotating system of clouds, wind, and thunderstorms. Its primary energy source is the release of the heat of condensation from water vapor condensing at high altitudes, the heat being 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 rotation and gravity of the Earth.<ref name = "NOAA preparedness">National Weather Service (September 2006). Hurricanes... Unleashing Nature's Fury: A Preparedness Guide (PDF). NOAA. Retrieved on 2006-12-02.</ref> 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. Condensation leads to higher wind speeds, as a tiny fraction of the released energy is converted into mechanical energy;<ref name = "AOML FAQ C5c">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: Why don't we try to destroy tropical cyclones by nuking them?. NOAA. Retrieved on 2006-07-25.</ref> 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.<ref name="NOAA Question of the Month">NOAA Question of the Month for August 2000 accessed March 31, 2006</ref> This gives rise to factors that provide the system with 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 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.

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.

Deep convection as a driving force is what primarily distinguishes tropical cyclones from other meteorological phenomena.<ref name="BOM Question 6">Bureau of Meteorology FAQ Question 6 accessed March 31, 2006</ref> Because this is strongest in a tropical climate, this 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.<ref name="BOM Question 6"/> To continue to drive its heat engine, a tropical cyclone must remain over warm water, which provides the needed atmospheric moisture. The evaporation of this moisture is accelerated by the high winds and reduced atmospheric pressure in the storm, resulting in a positive feedback loop. As a result, when a tropical cyclone passes over land, its strength diminishes rapidly.<ref name = "AOML FAQ C2">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: Doesn't the friction over land kill tropical cyclones?. NOAA. Retrieved on 2006-07-25.</ref>

Image:GulfMexTemps 2005Hurricanes.gif The passage of a tropical cyclone over the ocean can cause the upper ocean to cool substantially, which can influence subsequent cyclone development. Cooling is primarily caused by upwelling of cold water from below due to the wind stresses the tropical cyclone itself induces upon the upper layers of the ocean. Additional cooling may come from cold water from falling raindrops. 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.<ref name="NASA Cooling">Passing of Hurricanes Cools Entire Gulf accessed April 26, 2006</ref>

Scientists at the National Center for Atmospheric Research estimate that a tropical cyclone releases heat energy at the rate of 50 to 200 trillion joules per day.<ref name="NOAA Question of the Month"/> For comparison, this rate of energy release is equivalent to exploding a 10-megaton nuclear bomb every 20 minutes<ref name="UCAR">University Corporation for Atmospheric Research Hurricanes: Keeping an eye on weather's biggest bullies accessed March 31, 2006</ref> or 200 times the world-wide electrical generating capacity per day.<ref name="NOAA Question of the Month"/>

While the most obvious motion of clouds is toward the center, 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.<ref name = "NOAA preparedness"/> 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 tropical cyclone.<ref name = "AOML FAQ H5">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What's it like to go through a hurricane on the ground? What are the early warning signs of an approaching tropical cyclone?. NOAA. Retrieved on 2006-07-26.</ref>

[edit] Physical structure

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 the shield of cirrus clouds produced by the eyewall thunderstorms. Typically, these are the highest and coldest clouds in the cyclone.
• 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). The eye is normally circular in shape, and may range in size from 3 km to 320 km (2 miles to 200 miles) in diameter. 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 the stadium effect. In weaker cyclones, the CDO covers the circulation center, resulting in no visible eye.
• Eyewall: A band around the eye of greatest wind speed, where clouds reach highest and precipitation is heaviest. The heaviest wind damage occurs where a hurricane's eyewall passes over land. 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. Even if the cyclone is weaker at the end of the eyewall replacement cycle, the fact that it has just undergone one and will not undergo another one soon will allow it to strengthen further, if other conditions allow it to do so.
• Rainbands: Bands of showers and thunderstorms that spiral cyclonically toward the storm center. High wind gusts and heavy downpours often occur in individual rainbands, with relatively calm weather between bands. Tornadoes often form in the rainbands of landfalling tropical cyclones. Annular hurricanes are distinctive for their lack of rainbands.
• Outflow: While all surface low pressure areas require divergence aloft to continue deepening, the divergence over tropical cyclones is in all directions away from the center. The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic rotation, due to the corilois force. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to requiring a relative lack of vertical wind shear to maintain the warm core at the center of the storm.

[edit] Formation

[edit] Factors in formation

Main article: Tropical cyclogenesis

The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood. Six factors appear to be generally necessary, although tropical cyclones may occasionally form without meeting all of these conditions:

1. Water temperatures of at least 26.5 °C (80°F)<ref name = "AOML FAQ A15">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: How do tropical cyclones form?. NOAA. Retrieved on 2006-07-26.</ref> down to a depth of at least 50 m (150 feet). Waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.<ref name = "AOML FAQ A16">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: Why do tropical cyclones require 80°F (26.5°C) ocean temperatures to form?. NOAA. Retrieved on 2006-07-25.</ref>
2. Rapid cooling with height. This allows the release of latent heat, which is the source of energy in a tropical cyclone.<ref name = "AOML FAQ A15"/>
3. High humidity, especially in the lower-to-mid troposphere. When there is a great deal of moisture in the atmosphere, conditions are more favourable for disturbances to develop.<ref name = "AOML FAQ A15"/>
4. Low wind shear. When wind shear is high, the convection in a cyclone or disturbance will be disrupted, preventing formation of the feedback loop.<ref name = "AOML FAQ A15"/>
5. Distance from the equator. This allows the Coriolis force to deflect winds blowing towards the low pressure center, causing a circulation. The minimum distance is about 500 km (310 miles) or 5 degrees from the equator.<ref name = "AOML FAQ A15"/>
6. A pre-existing system of disturbed weather. The system must have some sort of circulation as well as a low pressure center.<ref name = "AOML FAQ A15"/>

[edit] Locations of formation

Most tropical cyclones form in a worldwide band of thunderstorm activity called by several names: the Intertropical Discontinuity (ITD), the Intertropical Convergence Zone (ITCZ), or the monsoon trough.

Most of these systems 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, tropical cyclones rarely form or move within about 5 degrees of the equator, where the Coriolis effect is weakest.<ref name=BOMmap>Bureau of Meteorology Worldwide Tropical Cyclone Tracks 1979-88</ref> However, it is possible for tropical cyclones to form within this boundary as did Typhoon Vamei in 2001 and Cyclone Agni in 2004.

[edit] Movement and track

[edit] Steering 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 analogized by Dr. Neil Frank, former director of the National Hurricane Center, to "leaves carried along by a stream."<ref name = "AOML FAQ G6">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What determines the movement of tropical cyclones?. NOAA. Retrieved on 2006-07-25.</ref>

Tropical systems, while generally located equatorword 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.<ref name = "AOML FAQ G6"/> 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.<ref name = "AOML FAQ A4">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: What is an easterly wave?. NOAA. Retrieved on 2006-07-25.</ref> These waves are the precursors to many tropical cyclones within this region.<ref name="MWR Avila 1995">Avila, Lixion, Richard Pasch (March 1995). "Atlantic tropical systems of 1993" (PDF). Monthly Weather Review 123 (3): 887-896. Retrieved on 2006-07-25.</ref> In the Indian Ocean and Western Pacific (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.<ref name="MILLER7">Millersville University. ESCI 344 – Tropical Meteorology Lesson 7 – Tropical Cyclones: Climatology. Retrieved on 2006-11-26.</ref>

[edit] Coriolis effect

The Earth's rotation imparts an acceleration known as the Coriolis Acceleration or Coriolis Effect. This acceleration causes cyclonic systems to turn towards the poles in the absence of strong steering currents.<ref>Steven K. Baum. The Glossary: Cn-Cz. Retrieved on 2006-11-29.</ref> The poleward portion of a tropical cyclone has winds blowing towards the west, and the Coriolis accelleration pulls them slightly more poleward. The winds blowing towards the east on the equatorward portion of the cyclone are pulled slightly towards the equator. But because the Coriolis accelleration is increasingly weak as you move toward the equator, the net drag on the cyclone is poleward. Thus, tropical cyclones in the Northern Hemisphere normally turn north (before being blown east), and tropical cyclones in the Southern Hemisphere normally turn south (before being blown east), if no strong pressure systems counteract the Coriolis acceleration.

The Coriolis acceleration also initiates cyclonic rotation, but it is not the driving force that brings this rotation to high speeds. These speeds instead result from the conservation of angular momentum. This means that air is drawn in from an area much larger than the cyclone such that the tiny rotational speed (originally imparted by the Coriolis acceleration) is magnified greatly as the air is drawn into the low pressure center.<ref>University of Tennessee. Astronomy 162 Lectures: Conservation of Angular Momentum. Retrieved on 2006-11-29.</ref>

[edit] Interaction with the mid-latitude westerlies

When a tropical cyclone moves into higher latitudes to the north of 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. A typhoon moving through the Pacific Ocean towards Asia, for example, will recurve to the north and then northeast offshore of Japan if the typhoon encounters winds blowing northeastward toward a low-pressure system passing over China or Siberia. Many tropical cyclones are eventually forced toward the northeast by low-pressure areas, which move from west to east when they are north of the subtropical ridge.

[edit] Landfall

Officially, "landfall" is when a storm's center (the center of its circulation, not its edge) crosses the coastline. Storm conditions may be experienced on the coast and inland hours before landfall. For a storm moving inland, the landfall area experiences half the storm by the time of actual landfall. 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.<ref name="NHC glossary"/> For a list of notable and unusual landfalling tropical cyclones, see list of notable tropical cyclones.

[edit] 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 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 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 its structure. Many storm fatalities occur in mountainous terrain, as the dying storm unleashes torrential rainfall which can lead to deadly floods and mudslides.
• It remains in the same area of ocean for too long, mixing the upper 100 feet (30 meters) of water, which draws up colder water due to upwelling, and becomes too cool to support the storm. Without warm surface water, the storm cannot survive.<ref>Encyclopaedia Brittanica. Life of a cyclone > Dissipation. Retrieved on 2006-11-30.</ref>
• It moves over waters significantly below 26°C. This will cause the storm will 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.<ref>Jonathan Edwards. Tropical Cyclone Formation. Retrieved on 2006-11-30.</ref>
• It experiences wind shear, causing the convection and heat engine to move away from the center which normally ceases development of a tropical cyclone. This process can take 3-5 days.
• 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.<ref>SECTION 3. TROPICAL CYCLONE INTENSITY TERMINOLOGY. Retrieved on 2006-11-30.</ref> 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. In the Pacific ocean and Atlantic ocean, such tropical-derived cyclones of higher latitudes can be violent and may occasionally remain at hurricane-force wind speeds when they reach the west coast of North America or Europe, where they are known as European windstorms. The extratropical remnants of Hurricane Iris in 1995 became such a windstorm.<ref>Edward N. Rappaport. Preliminary Report: Hurricane Iris. Retrieved on 2006-11-29.</ref>
• An outer eye wall forms (typically around 50-100 miles from the center of the storm), choking off the convection within the inner eye wall. Such weakening is generally temporary unless it meets other conditions above.
• It 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.

[edit] 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 cause supercooled water in the outer rainbands to freeze, causing the inner eyewall to collapse and thus reducing 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 in 1947, disaster struck when a hurricane east of Jacksonville, Florida promptly changed its course after being seeded, and smashed into Savannah, Georgia.<ref name="Whipple 151">Whipple, Addison (1982). Storm. Alexandria, VA: Time Life Books, 151. ISBN 0-8094-4312-0.</ref> 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, 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.<ref name = "AOML FAQ C5a">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: Why don't we try to destroy tropical cyclones by seeding them with silver iodide?. NOAA. Retrieved on 2006-07-25.</ref>

Other approaches have been suggested over time, including cooling the water under a tropical cyclone by towing icebergs into the tropical oceans, dropping large quantities of ice into the eye at very early stages so that latent heat is absorbed by ice at the entrance (storm cell perimeter bottom) instead of heat energy being converted to kinetic energy at high altitudes vertically above, covering the ocean in a substance that inhibits evaporation, or blasting the cyclone apart with nuclear weapons. Project Cirrus even involved throwing dry ice on a cyclone.<ref>Scotti, R. A. (2003)"Sudden Sea" p. 47 Little, Brown, and Company ISBN 0-316-73911-1</ref> These approaches all suffer from the same flaw: tropical cyclones are simply too large for any of them to be practical.<ref name = "AOML FAQ C5f">Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division. Frequently Asked Questions: Why don't we try to destroy tropical cyclones by (fill in the blank)?. NOAA. Retrieved on 2006-07-25.</ref>

[edit] Observation and forecasting

[edit] Observation

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 if it has overtaken an unfortunate ship. Even in these cases, real-time measurements are 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 United States government hurricane hunters.<ref name="Hurricane Hunters">Hurricane Hunters homepage accessed March 30, 2006</ref> 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.<ref name="SunHerald">Bowman, L. "Drones defy heart of storm." South Mississippi Sun-Herald accessed March 30, 2006</ref>

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 because it shows a storm's location and intensity minute by minute.

Recently, academic researchers have begun to deploy mobile weather stations fortified to withstand hurricane-force winds. The two largest programs are the Florida Coastal Monitoring Program<ref name=FCMP>Florida Coastal Monitoring Program project overview accessed March 30, 2006</ref> and the Wind Engineering Mobile Instrumented Tower Experiment.<ref name=WEMITE>WEMITE homepage accessed March 30, 2006</ref> During landfall, the NOAA Hurricane Research Division compares and verifies data from reconnaissance aircraft, including wind speed data taken at flight level and from GPS dropwindsondes and stepped-frequency microwave radiometers, to wind speed data transmitted in real time from weather stations erected near or at the coast. The National Hurricane Center uses the data to evaluate conditions at landfall and to verify forecasts.

[edit] 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. 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. Combining forecast models with increased 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.<ref>National Hurricane Center. National Hurricane Center Forecast Verification. Retreived on 2006-11-30.</ref> However, scientists say they are less skillful at predicting the intensity of tropical cyclones.<ref>National Hurricane Center. NHC Official Annual Average Intensity Errors Atlantic Basin Tropical Cyclones. Retrieved on 2006-11-30.</ref> 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.

[edit] Effects

See also: Tropical cyclone rainfall climatology

A mature tropical cyclone can release heat at a rate upwards of 6x10¹⁴ watts.<ref name="NOAA Question of the Month"/> Tropical cycl