Tropical cyclone scales

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Tropical cyclones are unofficially ranked on one of five tropical cyclone intensity scales, according to their maximum sustained winds and which tropical cyclone basin(s) they are located in. Only a few scales of classifications are used officially by the meteorological agencies monitoring the tropical cyclones, but some alternative scales also exist, such as accumulated cyclone energy, the Power Dissipation Index, the Integrated Kinetic Energy Index, and the Hurricane Severity Index.

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Tropical cyclone basins area of tropical cyclone formation

Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the north Atlantic Ocean, the eastern and western parts of the northern Pacific Ocean, 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, super typhoons, or major hurricanes.

Accumulated cyclone energy (ACE) is a measure used by various agencies including the National Oceanic and Atmospheric Administration (NOAA) and the India Meteorological Department to express the activity of individual tropical cyclones and entire tropical cyclone seasons. It uses an approximation of the wind energy used by a tropical system over its lifetime and is calculated every six hours. The ACE of a season is the sum of the ACEs for each storm and takes into account the number, strength, and duration of all the tropical storms in the season. The highest ACE calculated for a single storm is 82, for Hurricane/Typhoon Ioke in 2006.

Contents

Tropical cyclones that develop in the Northern Hemisphere are unofficially classified by the warning centres on one of three intensity scales. Tropical cyclones or subtropical cyclones that exist within the North Atlantic Ocean or the North-eastern Pacific Ocean are classified as either tropical depressions or tropical storms. Should a system intensify further and become a hurricane, then it will be classified on the Saffir–Simpson hurricane wind scale, which is based on the estimated maximum sustained winds over a 1-minute period. In the Western Pacific, the ESCAP/WMO Typhoon Committee uses four separate classifications for tropical cyclones that exist within the basin, which are based on the estimated maximum sustained winds over a 10-minute period.

Northern Hemisphere half of Earth that is north of the equator

The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's North Pole.

Atlantic Ocean Ocean between Europe, Africa and the Americas

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers. It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

Pacific Ocean Ocean between Asia and Australia in the west, the Americas in the east and Antarctica or the Southern Ocean in the south.

The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east.

The India Meteorological Department's scale uses 7 different classifications for systems within the North Indian Ocean, and are based on the systems estimated 3-minute maximum sustained winds. Tropical cyclones that develop in the Southern Hemisphere are only officially classified by the warning centres on one of two scales, which are both based on 10-minute sustained wind speeds: The Australian tropical cyclone intensity scale is used to classify systems within the Australian or South Pacific tropical cyclone basin. The scale used to classify systems in the South-West Indian Ocean is defined by Meteo France for use in various French territories, including New Caledonia and French Polynesia.

Southern Hemisphere part of Earth that lies south of the equator

The Southern Hemisphere is the half of Earth that is south of the Equator. It contains all or parts of five continents, four oceans and most of the Pacific Islands in Oceania. Its surface is 80.9% water, compared with 60.7% water in the case of the Northern Hemisphere, and it contains 32.7% of Earth's land.

New Caledonia Overseas territory of France in the southwest Pacific Ocean

New Caledonia is a special collectivity of France in the southwest Pacific Ocean, located to the south of Vanuatu, about 1,210 km (750 mi) east of Australia and 20,000 km (12,000 mi) from Metropolitan France. The archipelago, part of the Melanesia subregion, includes the main island of Grande Terre, the Loyalty Islands, the Chesterfield Islands, the Belep archipelago, the Isle of Pines, and a few remote islets. The Chesterfield Islands are in the Coral Sea. Locals refer to Grande Terre as Le Caillou.

French Polynesia French overseas country in the Southern Pacific ocean

French Polynesia is an overseas collectivity of the French Republic and the only overseas country of France. It is composed of 118 geographically dispersed islands and atolls stretching over an expanse of more than 2,000 kilometres (1,200 mi) in the South Pacific Ocean. Its total land area is 4,167 square kilometres (1,609 sq mi).

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 at a height of 10  m (33  ft). However, the Saffir–Simpson Hurricane Scale is based on wind speed measurements averaged over a 1-minute period, at 10 m (33 ft) above the surface. The scale used by RSMC New Delhi applies a 3-minute averaging period, and the Australian scale is based on both 3-second wind gusts and maximum sustained winds averaged over a 10-minute interval. These make direct comparisons between basins difficult.

The maximum sustained wind associated with a tropical cyclone is a common indicator of the intensity of the storm. Within a mature tropical cyclone, it is found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are determined via their sampling and averaging the sampled results over a period of time. Wind measuring has been standardized globally to reflect the winds at 10 metres (33 ft) above the Earth's surface, and the maximum sustained wind represents the highest average wind over either a one-minute (US) or ten-minute time span, anywhere within the tropical cyclone. Surface winds are highly variable due to friction between the atmosphere and the Earth's surface, as well as near hills and mountains over land.

World Meteorological Organization Specialised agency of the United Nations

The World Meteorological Organization (WMO) is an intergovernmental organization with a membership of 192 Member States and Territories. Its current Secretary-General is Petteri Taalas and the President of the World Meteorological Congress, its supreme body, is David Grimes. The Organization is headquartered in Geneva, Switzerland.

The metre or meter is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in vacuum in 1/299 792 458 of a second.

Within all basins tropical cyclones are named when the sustained winds hit 35 kn (40 mph; 65 km/h).

Background

Tropical cyclones are defined as being warm cored, non-frontal synoptic cyclones, that develop over tropical or subtropical waters, with organized atmospheric convection and have a definite cyclonic surface wind circulation. They are classified by the wind speeds located around the circulation centre and are ranked, by the World Meteorological Organisation's Regional Specialized Meteorological Centers on one of five tropical cyclone scales. The scale used for a particular tropical cyclone depends on what basin the system is located in; with for example the Saffir–Simpson hurricane wind scale and the Australian tropical cyclone intensity scales both used in the Western Hemisphere. All of the scales rank tropical cyclones using their maximum sustained winds, which are either observed, measured or estimated using various techniques, over a period between one and ten minutes.

Atmospheric convection

Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.

Atlantic, Eastern and Central Pacific

Saffir–Simpson scale
CategoryWind speeds
(for 1-minute maximum sustained winds)
m/s knots (kn) mph km/h
Five ≥ 70 m/s   ≥ 137 kn   ≥ 157 mph   ≥ 252 km/h  
Four  58–70 m/s    113–136 kn    130–156 mph    209–251 km/h  
Three  50–58 m/s    96–112 kn    111–129 mph    178–208 km/h  
Two  43–49 m/s    83–95 kn    96–110 mph    154–177 km/h  
One  33–42 m/s    64–82 kn    74–95 mph    119–153 km/h  
Related classifications
(for 1-minute maximum sustained winds)
Tropical storm  18–32 m/s    34–63 kn    39–73 mph    63–118 km/h  
Tropical depression  ≤ 17 m/s    ≤ 33 kn    ≤ 38 mph    ≤ 62 km/h  

Tropical cyclones that occur within the Northern Hemisphere to the east of the anti-meridian, are officially monitored by either the National Hurricane Center or the Central Pacific Hurricane Center. [1] Within the region a tropical cyclone is defined to be a warm cored, non-frontal synoptic disturbance, that develops over tropical or subtropical waters, with organized atmospheric convection and a closed well defined circulation centre. [1] The region also defines a subtropical cyclone as a non-frontal low pressure disturbance, that has the characteristics of both tropical and extratropical cyclones. [1] Once either of these classifications are met, then advisories are initiated and the warning centers will classify the system as either a tropical or subtropical depression, if the one-minute sustained winds estimated or measured as less than 34 kn (38 mph; 62 km/h). [1]

180th meridian is the meridian which is 180° east or west of the Prime Meridian with which it forms a great circle

The 180th meridian or antimeridian is the meridian 180° both east and west of the Prime Meridian, with which it forms a great circle dividing the earth into the Western and Eastern Hemispheres. It is common to both east longitude and west longitude. It mostly passes through the open waters of the Pacific Ocean, but passes across land in Russia, Fiji and Antarctica. This meridian is used as the basis for the International Date Line, but the latter deviates from it to maintain date consistency within the territories of Russia, the United States, Kiribati, Fiji and New Zealand.

National Hurricane Center division of the United States National Weather Service

The National Hurricane Center (NHC) is the division of the United States' National Weather Service responsible for tracking and predicting tropical weather systems between the Prime Meridian and the 140th meridian west poleward to the 30th parallel north in the northeast Pacific Ocean and the 31st parallel north in the northern Atlantic Ocean. The agency, which is co-located with the Miami branch of the National Weather Service, is situated on the campus of Florida International University in University Park, Florida.

Central Pacific Hurricane Center

The Central Pacific Hurricane Center (CPHC) of the United States National Weather Service is the official body responsible for tracking and issuing tropical cyclone warnings, watches, advisories, discussions, and statements for the Central Pacific region: from the equator northward, 140°W–180°W, most significantly for Hawai‘i. It is the Regional Specialized Meteorological Center (RSMC) for tropical cyclones in this region, and in this capacity is known as RSMC Honolulu.

Also, it will be assigned a tropical cyclone number (or TC number for short) comprising an officially spelled-out number (from ONE to THIRTY or less; these numbers are not recycled until next year) followed by (except for North Atlantic systems) a hyphen and a suffix letter ("-E" for East Pacific, "-C" for Central Pacific); [2] a two-digit (plus any suffix) abbreviation (like TD 08 for North Atlantic depression EIGHT, TD 21E for East Pacific depression TWENTYONE-E, or TD 03C for Central Pacific depression THREE-C) is also generated for bulletin and other automated purposes.

However, if a tropical disturbance is capable of producing tropical storm or hurricane conditions on land within 48 hours, then advisories will be initiated and it will be classified as a potential tropical cyclone (PTC) [1] with a two-digit PTC number (for example, PTC-09 or PTC-15E) that otherwise looks identical to a TC number. Should the system intensify further or already have one-minute sustained winds of 34–63 kn (39–73 mph; 63–118 km/h), then it will be called either a tropical or subtropical storm and assigned a name [1] (which replaces the spelled-out TC number; the two-digit number is still kept for purposes like the Automated Tropical Cyclone Forecasting System, as in 2018's TS 12 (KIRK) ).

Should the tropical system further intensify and have winds estimated or measured, as greater than 64 kn (74 mph; 119 km/h), then it will be called a hurricane and classified on the Saffir–Simpson hurricane wind scale. [1] The lowest classification on the SSHWS is a Category 1 hurricane, which has winds of between 64–82 kn (74–95 mph, 119–153 km/h). [1] [3] Should the hurricane intensify further then it will be rated as a Category 2 hurricane, if it has winds of between 83–95 kn (96–110 mph, 154–177 km/h). [1] [3] When a system becomes a Category 3 hurricane with winds of between 96–112 kn (111–129 mph, 178–208 km/h), it is considered to be a major hurricane by the warning centers. [3] A Category 4 hurricane has winds of 113–136 kn (130–156 mph, 209–251 km/h), while a Category 5 hurricane has winds of at least 137 kn (157 mph, 252 km/h). [1] [3] A post tropical cyclone is a system that has weakened, into a remnant low or has dissipated and formal advisories are usually discontinued at this stage. [1] However, advisories may continue if the post tropical cyclone, poses a significant threat to life and property. [1] They may also continue if the remnants of the system have a chance of regeneration and producing tropical storm or hurricane-force winds over land within 48 hours. [1]

The SSHS was originally created using both wind speed and storm surge, but since the relationship between wind speed and storm surge is not necessarily definite, the scale was changed to the "Saffir–Simpson Hurricane Wind Scale" (SSHWS), based entirely on wind speed.

Although increasing echelons of the scale correspond to stronger winds, 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, population density 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 less than hurricane strength can produce significant damage and human casualties, especially from flooding and landslides.

Historically, the term great hurricane was used to describe storms that possessed winds of at least 110  kn (125 mph; 200 km/h), large radii (over 160  km / 100  mi) and that caused large amounts of destruction. This term fell into disuse after the introduction of the Saffir–Simpson scale in the early 1970s. [4]

A minor change to the scale was made ahead of the 2012 hurricane season, with the wind speeds for Categories 3–5 tweaked to eliminate the rounding errors that had occurred during previous seasons, when a hurricane had wind speeds of 115 kn (130 mph; 215 km/h). [5]

Western Pacific

ESCAP/WMO Typhoon Committee's Tropical Cyclone Intensity Scale
CategorySustained winds
Typhoon≥64  knots
≥118 km/h
Severe Tropical Storm48–63  knots
89–117 km/h
Tropical Storm34–47  knots
62–88 km/h
Tropical Depression≤33  knots
≤61 km/h

Tropical cyclones that occur within the Northern Hemisphere between the anti-meridian and 100°E, are officially monitored by the Japan Meteorological Agency (JMA, RSMC Tokyo). [6] Within the region a tropical cyclone is defined to be a non-frontal synoptic scale cyclone originating over tropical or sub-tropical waters, with organized convection and a definite cyclonic surface wind circulation. [6] The lowest classification used by the Typhoon Committee is a tropical depression, which has 10-minute sustained winds of less than 34 kn (17 m/s; 39 mph; 63 km/h). [6] Should the tropical depression intensify further it is named and classified as a tropical storm, which has winds speeds between 34–47 kn (17–24 m/s; 39–54 mph; 63–87 km/h). [6] Should the system continue to intensify further then it will be classified as a severe tropical storm, which has winds speeds between 48–63 kn (25–32 m/s; 55–72 mph; 89–117 km/h). [6] The highest classification on the Typhoon Committee's scale is a typhoon, which has winds speeds greater than 64 kn (33 m/s; 74 mph; 119 km/h). [6]

The China Meteorological Administration, the Hong Kong Observatory (HKO), PAGASA and the JMA, all divide the typhoon category further for domestic purposes. [6] The JMA divides the typhoon category into three categories, with a 10-minute maximum wind speed below 84 kn (43 m/s; 97 mph; 156 km/h) assigned for the (strong) typhoon category. A very strong typhoon has wind speeds between 85–104 kn (44–54 m/s; 98–120 mph; 157–193 km/h), while a violent typhoon has wind speeds of 105 kn (54 m/s; 121 mph; 194 km/h) or greater. [6] The HKO and the CMA also divide the typhoon category into three categories, with both assigning a maximum wind speed of 80 kn (41 m/s; 92 mph; 150 km/h) to the typhoon category. A severe typhoon has wind speeds of 85–104 kn (44–54 m/s; 98–120 mph; 157–193 km/h), while a super typhoon has winds of 100 kn (51 m/s; 120 mph; 190 km/h). [6] [7] In May 2015, PAGASA introduced the term Super Typhoon and used it for systems with winds greater than 120 kn (62 m/s; 140 mph; 220 km/h). [8]

In addition to the national meteorological services of each nation, the United States Joint Typhoon Warning Center monitors the basin, and issues warnings on significant tropical cyclones for the United States Government, [9] assigning them two-digit TC numbers (with suffix "W"). [2] These warnings use a 1-minute sustained wind speed and can be compared to the Saffir–Simpson hurricane wind scale; however, the JTWC uses their own scale for intensity classifications in this basin. [10] These classifications are Tropical Depression, Tropical Storm, Typhoon, and Super Typhoon. [10] Also, when a tropical depression is upgraded to tropical storm and named by the JMA, JTWC appends the international name (parenthesized) to its TC number [2] (i.e., 2018 tropical depression TWENTY-W, abbr. TD 20W, became tropical storm Bebinca, but was referred to as TS 20W (BEBINCA) in JTWC advisories); however, in cases when JTWC upgrades a depression to tropical storm without JMA following suit (due to the differences between JTWC and JMA wind-speed scales), the spelled-out number (without the suffix) is parenthesized and appended to the TC number as placeholder name, as in TS 16W (SIXTEEN), until JMA upgrades and names it, on which case the name replaces the placeholder. [10]

In addition, the Taiwan Central Weather Bureau has its own scale in Chinese but uses the Typhoon Committee scale in English. [11]

North Indian Ocean

India Meteorological Department
Tropical Cyclone Intensity Scale
CategorySustained winds
(3-min average)
Super Cyclonic Storm≥120  kt
≥221 km/h
Extremely Severe
Cyclonic Storm
90–119 kt
166–220 km/h
Very Severe
Cyclonic Storm
64–89 kt
118–165 km/h
Severe Cyclonic
Storm
48–63 kt
89–117 km/h
Cyclonic Storm34–47 kt
63–88 km/h
Deep Depression28–33 kt
51–62 km/h
Depression17–27 kt
31–50 km/h

Any tropical cyclone that develops within the North Indian Ocean between 100°E and 45°E is monitored by the India Meteorological Department (IMD, RSMC New Delhi). [12] Within the region a tropical cyclone is defined as being a non frontal synoptic scale cyclone, that originates over tropical or subtropical waters with organized convection and a definite cyclonic surface wind circulation. [12] The lowest official classification used in the North Indian Ocean is a Depression, which has 3-minute sustained wind speeds of between 17–27 kn (20–31 mph; 31–49 km/h). [12] Should the depression intensify further then it will become a Deep Depression, which has winds between 28–33 kn (32–38 mph; 50–61 km/h). [12] The system will be classified as a cyclonic storm and assigned a name by the IMD, if it should develop gale-force wind speeds of between 34–47 kn (39–54 mph; 62–88 km/h). [12] Severe Cyclonic Storms have storm force wind speeds of between 48–63 kn (55–72 mph; 89–117 km/h), while Very Severe Cyclonic Storms have hurricane-force winds of 64–89 kt (73–102 mph; 118–166 km/h). Extremely Severe Cyclonic Storms have hurricane-force winds of 90–119 kn (166–221 km/h, 104–137 mph). [12] The highest classification used in the North Indian Ocean is a Super Cyclonic Storm, which have hurricane-force winds of above 120 kn (138 mph; 222 km/h). [12]

Historically, a system has been classified as a depression if it is an area where the barometric pressure is low compared with its surroundings. [13] Other classifications historically used include: cyclonic storm where the winds did not exceed force 10 on the Beaufort scale and a Cyclone where the winds are either force 11 and 12 on the Beaufort scale. [13] Between 1924 and 1988, tropical cyclones were classified into four categories: depression, deep depression, cyclonic storms and severe cyclonic storms. [13] However, a change was made during 1988 to introduce the category "severe cyclonic storm with core of hurricane winds" for tropical cyclones, with wind speeds of more than 64 kn (74 mph; 119 km/h). [13] During 1999 the categories Very Severe Cyclonic Storm and Super Cyclonic Storm were introduced, while the severe cyclonic storm with a core of hurricane winds category was eliminated. [13] During 2015 another modification to the intensity scale took place, with the IMD calling a system with 3-minute maximum sustained wind speeds between 90–119 kn (166–221 km/h, 104–137 mph): an extremely severe cyclonic storm. [14]

The United States Joint Typhoon Warning Center also monitors the basin, and issues warnings on significant tropical cyclones on behalf of the United States Government, [9] also assigning them TC numbers as in all other basins above (albeit in an unofficial manner for this and subsequent basins; cyclones originating in the Arabian Sea are assigned suffix "A" while those in the Bay of Bengal get suffix "B"). These warnings use a 1-minute sustained wind speed and can be compared to the Saffir–Simpson hurricane wind scale, however, regardless of intensity in this basin the JTWC labels all systems as tropical cyclones with TC numbers (optionally appended with intl. names/placeholders in parentheses, as done for typhoons above). [10]

South-West Indian Ocean

Southwest Indian Ocean
Tropical Cyclone Intensity Scale
CategorySustained winds
Very Intense
Tropical Cyclone
>115 kt
>212 km/h
Intense
Tropical Cyclone
90–115 kt
166–212 km/h
Tropical Cyclone64–89 kt
118–165 km/h
Severe
Tropical Storm
48–63 kt
89–117 km/h
Moderate
Tropical Storm
34–47 kt
63–88 km/h
Tropical
Depression
28–33 kt
51–62 km/h
Tropical
Disturbance
<28 kt
<50 km/h

Any tropical cyclone that develops within the Southern Hemisphere between Africa and 90°E is monitored by Meteo France's La Reunion tropical cyclone centre (MFR, RSMC La Reunion). [15] Within the region a tropical disturbance is defined to be a non-frontal synoptic scale low pressure area, originating over tropical or sub-tropical waters with organized convection and definite cyclonic surface wind circulation with the average wind speed estimated to be not exceeding 27 knots (50 km/h)).

A tropical disturbance is MFR's generic term for a non-frontal area of low pressure that has organized convection and definite cyclonic surface wind circulation. [15] The system should be estimated to have wind speeds of less than 28 knots (50 km/h, 32 mph). [15]

A system is designated as a tropical depression or a subtropical depression when it reaches wind speeds above 28 knots (50 km/h, 32 mph). Should a tropical depression reach wind speeds of 35 knots (65 km/h, 40 mph) then it will be classified as a moderate tropical storm and assigned a name by either the Sub Regional Center in Mauritius or Madagascar. No matter how strong a subtropical system is in this basin, it is always designated as a subtropical depression. [16]

Should the named storm intensify further and reach winds speeds of 48 knots (89 km/h, 55 mph), then it will be classified as a severe tropical storm. [16] A severe tropical storm is designated as a tropical cyclone when it reaches wind speeds of 64 knots (118 km/h, 74 mph). [15] Should a tropical cyclone intensify further and reach wind speeds of 90 knots (166 km/h, 103 mph), it will be classified as an intense tropical cyclone. [15] A very intense tropical cyclone is the highest category on the South-West Indian Ocean Tropical Cyclone scale, and has winds of over 115 knots (212 km/h, 132 mph). [16]

At the tenth RA I tropical cyclone committee held during 1991, it was recommended that the intensity classifications be changed ahead of the 1993–94 tropical cyclone season. [17] Specifically it was decided that the classifications: Weak Tropical Depression, Moderate Tropical Depression and Severe Tropical Depression would be changed to Tropical Depression, Moderate Tropical Storm and Severe Tropical Storm. [17] This change was implemented ahead of the 1993–94 tropical cyclone season. [17]

The United States Joint Typhoon Warning Center also monitors the basin, and issues warnings on significant tropical cyclones on behalf of the United States Government; [9] these systems are unofficially assigned TC numbers with suffix "S" (which spans the whole South Indian Ocean, including both BMKG and BoM areas of responsibility west of 135°E). These warnings use a 1-minute sustained wind speed and can be compared to the Saffir–Simpson hurricane wind scale, however, regardless of intensity in this basin the JTWC labels all systems as tropical cyclones with TC numbers (plus any parenthesized names/placeholders, like typhoons and North Indian Ocean cyclones above). [10]

Australia and Fiji

Australian tropical cyclone
intensity scale
CategorySustained
winds
Gusts
Five>107 kt
>198 km/h
>151 kt
>280 km/h
Four86–107 kt
158–198 km/h
122–151 kt
226–280 km/h
Three64–85 kt
118–157 km/h
90–121 kt
167–225 km/h
Two48–63 kt
89–117 km/h
68–89 kt
126–166 km/h
One34–47 kt
63–88 km/h
49–67 kt
91–125 km/h

Tropical cyclones that occur within the Southern Hemisphere to the east of 90°E are officially monitored by one or more tropical cyclone warning centres. [18] These are run by the Fiji Meteorological Service, New Zealand's MetService, Indonesia's Badan Meteorologi, Klimatologi, dan Geofisika, Papua New Guinea's National Weather Service and the Australian Bureau of Meteorology. [18] Within the region a tropical cyclone is defined as being a non-frontal low pressure system of synoptic scale that develops over warm waters, with a definite organized wind circulation and 10-minute sustained wind speeds of 34 kn (63 km/h; 39 mph) or greater near the centre. [18] Once this definition has been met then all of the centres name the system and start to use the Australian tropical cyclone intensity scale, which measures tropical cyclones using a five category system based on 10-minute maximum sustained winds. [18] [19] A Category 1 tropical cyclone is estimated to have 10-minute sustained wind speeds of 34–47 kn (39–54 mph; 63–87 km/h), while a Category 2 tropical cyclone is estimated to have 10-minute sustained wind speeds of 48–63 kn (55–72 mph; 89–117 km/h). [19] [20] When a system becomes a Category 3 tropical cyclone it is reclassified as a Severe tropical cyclone and has wind speeds of 64–85 kn (74–98 mph; 119–157 km/h). [19] [20] A Category 4 severe tropical cyclone has winds of 86–110 kn (99–130 mph; 157–200 km/h), while the maximum rating is a Category 5 severe tropical cyclone, which has winds of at least 108 kn (124 mph; 200 km/h). [19] [20]

For systems below tropical cyclone strength there are various terms used, including Tropical Disturbance, Tropical Low and Tropical Depression. [18] A tropical disturbance is defined as being a non – frontal system of synoptic scale originating over the tropics, with persistent enhanced convection and/or some indication of a circulation. [18] A tropical depression or tropical low is a disturbance with a defined circulation, where the central position can be estimated, and the maximum 10-minute average wind speed is less than 34 kn (39 mph; 63 km/h) near the centre. [18] The FMS numbers these systems when they have a potential to develop into a tropical cyclone or persist to cause significant impact to life and property, within its area of responsibility and have been analysed for the previous 24 hours. [18] The Australian tropical cyclone intensity scale was introduced by the BoM, ahead of the 1989–90 cyclone season.

The United States Joint Typhoon Warning Center also monitors the basin, and issues warnings on significant tropical cyclones on behalf of the United States Government; [9] these systems are unofficially assigned TC numbers with either suffix "S" (if originating west of 135°E; spans the whole South Indian Ocean, including MFR's area of responsibility) or suffix "P" (if east of 135°E; spans the whole South Pacific Ocean, merging BoM, PNG-NWS, FMS, and MSNZ AORs together). These warnings use a 1-minute sustained wind speed and can be compared to the Saffir–Simpson hurricane wind scale, however, regardless of intensity in these basins the JTWC labels all systems as tropical cyclones with TC numbers (plus any names/placeholders parenthesized, as for typhoons and Indian Ocean cyclones above). [10]

Alternative scales

There are other scales that are not officially used by any of the Regional Specialized Meteorological Centres or the Tropical Cyclone Warning Centres. However they are used by other organizations, such as the National Oceanic and Atmospheric Administration. An example of such scale is the Integrated Kinetic Energy index, which measures the destructive potential of the storm surge on the coast; it works on a scale that ranges from one to six, with six having the highest destructive potential. [21]

Accumulated cyclone energy (ACE) is used by the National Oceanic and Atmospheric Administration and other agencies to express the activity of individual tropical cyclones that are above tropical storm strength and entire tropical cyclone seasons. [22] It is calculated by taking the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots or higher) at six-hour intervals. [22] The numbers are usually divided by 10,000 to make them more manageable. The unit of ACE is 104 kn2, and for use as an index the unit is assumed. [22] As well as being squared for ACE, wind speed can also be cubed, which is referred to as the Power Dissipation Index (PDI). [23]

The Hurricane Severity Index (HSI) is another scale used and rates the severity of all types of tropical and subtropical cyclones based on both the intensity and the size of their wind fields. [24] The HSI is a 0 to 50 point scale, allotting up to 25 points for a tropical cyclone's intensity and up to 25 points for wind field size. [24] Points are awarded on a sliding scale, with the majority of points reserved for hurricane force and greater wind fields. [24]

Comparisons across basins

The terminology for tropical cyclones differs from one region to another. Below is a summary of the classifications used by the official warning centres worldwide. NHC, CPHC and JTWC use 1-minute sustained winds, the IMD uses 3-minute sustained winds (not shown below) while all other warning centers use 10-minute sustained winds.

Tropical cyclone classifications
Beaufort
scale
1-minute sustained winds
(NHC/CPHC/JTWC)
10-minute sustained winds
(WMO/JMA/MF/BOM/FMS)
NE Pacific &
N Atlantic
NHC/CPHC [25]
NW Pacific
JTWC
NW Pacific
JMA
N Indian Ocean
IMD [12]
SW Indian Ocean
MF
Australia & S Pacific
BOM/FMS [18]
0–7<32 knots (37 mph; 59 km/h)<28 knots (32 mph; 52 km/h)Tropical DepressionTropical DepressionTropical DepressionDepressionZone of Disturbed WeatherTropical Disturbance
Tropical Depression
Tropical Low
733 knots (38 mph; 61 km/h)28–29 knots (32–33 mph; 52–54 km/h)Deep DepressionTropical Disturbance
834–37 knots (39–43 mph; 63–69 km/h)30–33 knots (35–38 mph; 56–61 km/h)Tropical StormTropical StormTropical Depression
9–1038–54 knots (44–62 mph; 70–100 km/h)34–47 knots (39–54 mph; 63–87 km/h)Tropical StormCyclonic StormModerate Tropical StormCategory 1
tropical cyclone
1155–63 knots (63–72 mph; 102–117 km/h)48–55 knots (55–63 mph; 89–102 km/h)Severe Tropical StormSevere Cyclonic StormSevere Tropical StormCategory 2
tropical cyclone
12+64–71 knots (74–82 mph; 119–131 km/h)56–63 knots (64–72 mph; 104–117 km/h)Category 1 hurricaneTyphoon
72–82 knots (83–94 mph; 133–152 km/h)64–72 knots (74–83 mph; 119–133 km/h)TyphoonVery Severe
Cyclonic Storm
Tropical CycloneCategory 3 severe
tropical cyclone
83–95 knots (96–109 mph; 154–176 km/h)73–83 knots (84–96 mph; 135–154 km/h)Category 2 hurricane
96–97 knots (110–112 mph; 178–180 km/h)84–85 knots (97–98 mph; 156–157 km/h)Category 3 major hurricane
98–112 knots (113–129 mph; 181–207 km/h)86–98 knots (99–113 mph; 159–181 km/h)Extremely Severe
Cyclonic Storm
Intense Tropical CycloneCategory 4 severe
tropical cyclone
113–122 knots (130–140 mph; 209–226 km/h)99–107 knots (114–123 mph; 183–198 km/h)Category 4 major hurricane
123–129 knots (142–148 mph; 228–239 km/h)108–113 knots (124–130 mph; 200–209 km/h)Category 5 severe
tropical cyclone
130–136 knots (150–157 mph; 241–252 km/h)114–119 knots (131–137 mph; 211–220 km/h)Super TyphoonSuper Cyclonic StormVery Intense Tropical Cyclone
>137 knots (158 mph; 254 km/h)>120 knots (140 mph; 220 km/h)Category 5 major hurricane

See also

Related Research Articles

The Saffir–Simpson hurricane wind scale (SSHWS), formerly the Saffir–Simpson hurricane scale (SSHS), classifies hurricanes – Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms – into five categories distinguished by the intensities of their sustained winds.

1992 Pacific typhoon season typhoon season in the Pacific Ocean

The 1992 Pacific typhoon season had no official bounds; it ran year-round in 1992. Despite this, most tropical cyclones tend to form in the northwestern Pacific Ocean between May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northwestern Pacific Ocean.

Below is a timeline of the 2007 North Indian Ocean cyclone season, documenting major events with regards to tropical cyclone formation, strengthening, weakening, landfall, extratropical transition, as well as dissipation. The 2007 North Indian Ocean cyclone season was an ongoing event in the annual cycle of tropical cyclone formation.

1992–93 South Pacific cyclone season cyclone season in the South Pacific ocean

The 1992–93 South Pacific cyclone season was an above-average tropical cyclone season with ten tropical cyclones occurring within the South Pacific to the east of 160°E. The season officially ran from November 1, 1992, to April 30, 1993, with the first disturbance of the season forming on December 3 and the last disturbance dissipating on April 6.

1996–97 South Pacific cyclone season cyclone season in the South Pacific ocean

The 1996–97 South Pacific cyclone season was one of the most active and longest South Pacific tropical cyclone seasons on record, with 12 tropical cyclones occurring within the South Pacific basin between 160°E and 120°W. The season officially ran from November 1, 1996 - April 30, 1997, however, the season ended later than normal with three systems monitored after the official end of the season. The strongest tropical cyclone of the season was Cyclone Gavin which had a minimum pressure of 925 hPa (27.32 inHg). After the season had ended 4 tropical cyclone names were retired from the naming lists, after the cyclones had caused significant impacts to South Pacific islands.

1990–91 Australian region cyclone season cyclone season in the Australian region

The 1990–91 Australian region cyclone season was a slightly below average cyclone season, with ten tropical cyclones occurring within the region between 90°E and 160°E. The season officially ran from November 1, 1990, to April 30, 1991, with the first disturbance of the season forming on 10 December and the last disturbance moving out of the region during 11 May. Six people were killed by Cyclone Joy when it made landfall on Australia. During the season, tropical cyclones were monitored by the Australian Bureau of Meteorology, who ran Tropical Cyclone Warning Centers (TCWC) in Perth, Darwin, and Brisbane. The United States Joint Typhoon Warning Center (JTWC) and Papua New Guinea National Weather Service also monitored systems within the basin during the season. The JTWC designated systems with a number and either a S or a P suffix depending on which side of 135E. The Bureau of Meteorology and Papua New Guinea national Weather Service both used the Australian Tropical Cyclone Intensity Scale, and estimated windspeeds over a ten-minute period, while the JTWC estimated sustained winds over a one-minute period and are comparable to the Saffir–Simpson Hurricane Scale.

2001 India cyclone North Indian cyclone in 2001

The 2001 India cyclone was the second strongest tropical cyclone, in terms of barometric pressure, to form in the Arabian Sea on record; only Cyclone Gonu in 2007 was stronger. The storm originated from a tropical disturbance that formed east of Somalia on May 18. Over the following few days, the system gradually organized into a tropical depression. Tracking eastward, towards the coastline of southwestern India, the storm slowly intensified. Shortly before reaching shore, the system turned north and later west, away from land. After taking this turn, the storm intensified into a very severe cyclonic storm, attaining its peak intensity on May 24 with winds of 215 km/h and a barometric pressure of 932 mbar (hPa). At the time, this ranked the cyclone as the strongest known storm in the Arabian Sea.

Cyclone Gwenda Category 5 Australian region cyclone in 1999

Severe Tropical Cyclone Gwenda was tied with Cyclone Inigo as the most intense Australian tropical cyclone on record, with a barometric pressure of 900 hPa (mbar) and was the most intense storm worldwide in 1999. Forming out of a tropical disturbance over the Arafura Sea on 2 April 1999, the precursor to Gwenda tracked slowly westward and gradually became more organised. On 4 April, the system developed into a Category 1 cyclone and was named Gwenda. It began to undergo explosive intensification the following day, and in a 30-hour span ending early on 7 April, the storm's maximum 10-minute sustained wind speed increased from 75 km/h (45 mph) to 225 km/h (140 mph) and its barometric pressure decreased to 900 hPa (mbar). The Joint Typhoon Warning Center reported that the storm had peaked as a high-end Category 4 equivalent on the Saffir–Simpson hurricane scale.

1990–91 South Pacific cyclone season cyclone season in the South Pacific ocean

The 1990–91 South Pacific cyclone season was one of the least active cyclone seasons, with only three tropical cyclones occurring within the South Pacific basin which is to the east of 160°E. The season officially ran from November 1, 1990, to April 30, 1991, with the first disturbance of the season forming on November 23 and the last disturbance dissipating on May 19. This is the period of the year when most tropical cyclones form within the South Pacific Ocean. During the season there was no deaths recorded from any of the tropical cyclones while they were within the basin. However six people were killed by Cyclone Joy, when it made landfall on Australia. As a result of the impacts caused by Joy and Sina, the names were retired from the tropical cyclone naming lists.

Typhoon Roy Pacific typhoon in 1988

Typhoon Roy, known in the Philippines as Typhoon Asiang, was the second-most intense January tropical cyclone on record in the Western Pacific basin. Forming out of an area of disturbed weather on January 7, 1988, Roy quickly intensified as it moved through the Marshall Islands. By January 9, the storm intensified into a typhoon and attained its peak intensity the following day. At its peak, sustained winds reached 215 km/h (135 mph). Slight weakening took place before the storm moved through the Mariana Islands. Continuing westward, the system eventually struck the Philippines as a minimal typhoon before dissipating over the South China Sea on January 19.

Timeline of the 1990–91 South Pacific cyclone season

The 1990–91 South Pacific cyclone season was a below-average season; only two tropical cyclones occurred within the South Pacific to the east of 160°E. The season officially ran from November 1, 1990, to April 30, 1991, but the first disturbance of the season formed on November 23 and the last dissipated on May 19. This is the period of the year when most tropical cyclones form within the South Pacific Ocean. During the season, no one was killed from tropical disturbances within the South Pacific. However, six people were killed by Cyclone Joy when it made landfall on Australia. The only tropical cyclone to cause any damage while within this basin was Sina, which caused at least $18.5 million (1991 USD) worth of damage to Fiji and Tonga. As a result of the impacts of both Joy and Sina, the names were retired from the tropical cyclone naming lists.

Glossary of tropical cyclone terms

The following is a glossary of tropical cyclone terms.

Typhoon Nelson (1982)

Typhoon Nelson, known in the Philippines as Typhoon Bising, was the second tropical cyclone to strike the Philippines within a week in March 1982. Nelson originated from a tropical disturbance southeast of Guam towards the middle of March. Although the system was initially poorly organized, it developed rather quickly, and was upgraded into Tropical Storm Nelson on March 19. It tracked westward, and fluctuated in intensity for several days. On March 24, Typhoon Nelson intensified into a typhoon, and entered an episode of rapid intensification. On March 25, Nelson reached its peak intensity of 115 km/h (70 mph), but thereafter moved ashore on the Philippines, where the storm weakened significantly. On March 27, the typhoon entered the South China Sea, and the next day, briefly re-intensified before resuming a weakening trend. Nelson dissipated on March 31. Affecting the nation less than a week after Tropical Storm Mamie did, Nelson was responsible for additional flooding across much of the Philippines. Fifty-six people were killed due to the typhoon, eight of whom perished due to drownings. Over 1,200 homes were destroyed. More than 165,000 people fled to shelters, including 83,000 that were displaced from their home. Thirty fishing boats and 23 ferries were destroyed due to the system. Damage amounted to $17.2 million.

1984–85 South Pacific cyclone season cyclone season in the South Pacific ocean

The 1984–85 South Pacific cyclone season was an above-average tropical cyclone season, with nine tropical cyclones occurring within the basin between 160°E and 120°W. The season ran from November 1, 1984, to April 30, 1985, with tropical cyclones officially monitored by the Fiji Meteorological Service (FMS), Australian Bureau of Meteorology (BoM) and New Zealand's MetService. The United States Joint Typhoon Warning Center (JTWC) and other national meteorological services including Météo-France and NOAA also monitored the basin during the season. During the season there was nine tropical cyclones occurring within the basin, including three that moved into the basin from the Australian region. The BoM, MetService and RSMC Nadi all estimated sustained wind speeds over a period of 10-minutes, which are subsequently compared to the Australian tropical cyclone intensity scale, while the JTWC estimated sustained winds over a 1-minute period, which are subsequently compared to the Saffir–Simpson hurricane wind scale (SSHWS).

Cyclone Yali Category 3 South Pacific and Australian region cyclone in 1998

Severe Tropical Cyclone Yali was one of seven severe tropical cyclones to develop during the 1997–98 South Pacific cyclone season. The system that was to become Yali was first noted as a tropical disturbance, to the northeast of Vanuatu during March 17. Over the next couple of days the system moved towards the south-west and gradually developed further, before it was named Yali during March 19, after it had developed into a tropical cyclone. After it was named Yali re-curved and started moving towards the south-southeast, as the monsoonal flow to the north of the system strengthened. While the system was active, Yali affected Vanuatu and New Caledonia, before the extra-tropical remnants impacted New Zealand where a man was killed and widespread power outages and damage were reported.

Timeline of the 2013 Pacific typhoon season

This timeline documents all of the events of the 2013 Pacific typhoon season. Most of the tropical cyclones formed between May and November. The scope of this article is limited to the Pacific Ocean, north of the equator between 100°E and the International Date Line. Tropical storms that form in the entire Western Pacific basin are assigned a name by the Japan Meteorological Agency. Tropical depressions that form in this basin are given a number with a "W" suffix by the United States' Joint Typhoon Warning Center. In addition, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) assigns names to tropical cyclones that enter or form in the Philippine area of responsibility. These names, however, are not in common use outside of the Philippines.

Meteorological history of Typhoon Haiyan

Typhoon Haiyan's meteorological history began with its origins as a tropical disturbance east-southeast of Pohnpei and lasted until its degeneration as a tropical cyclone over Southern China. The thirteenth typhoon of the 2013 Pacific typhoon season, Haiyan originated from an area of low pressure several hundred kilometers east-southeast of Pohnpei in the Federated States of Micronesia on November 2. Tracking generally westward, environmental conditions favored tropical cyclogenesis and the system developed into a tropical depression the following day. After becoming a tropical storm and attaining the name Haiyan at 0000 UTC on November 4, the system began a period of rapid intensification that brought it to typhoon intensity by 1800 UTC on November 5. By November 6, the Joint Typhoon Warning Center (JTWC) assessed the system as a Category 5-equivalent super typhoon on the Saffir-Simpson hurricane wind scale; the storm passed over the island of Kayangel in Palau shortly after attaining this strength.

Tropical Storm Zelda (1991) last storm of the 1991 Pacific typhoon season

Severe Tropical Storm Zelda was the last storm of the 1991 Pacific typhoon season. A disturbance formed near the International Date Line, and strengthened into a tropical depression on November 27. The Joint Typhoon Warning Center (JTWC) reported that the depression had reached tropical storm intensity, thus naming it Zelda. The storm quickly strengthened into a Category 1 typhoon on the Saffir–Simpson hurricane wind scale near the Marshall Islands on November 29. It reached peak of 110 km/h (70 mph), with a barometric pressure of 975 mbar (28.8 inHg). The tropical storm traveled northwestward, and later northeastward. Zelda soon weakened into a tropical storm, and then a tropical depression. The JTWC discontinued warnings on December 4, while the Japanese Meteorological Agency (JMA) continued to track the storm until it crossed the International Date Line again on December 7.

Typhoon Pat (1985)

Typhoon Pat, known in the Philippines as Typhoon Luming, was a powerful typhoon that struck Japan during the summer of 1985. Pat is also one of three storms in the Western Pacific which interacted with each other. Originating from a monsoon trough towards the end of August, Pat first formed on August 24 several hundred miles east of the Philippines. It gradually intensified, and two days later, Pat was upgraded into a tropical storm. The cyclone initially moved east-northeast while continuing to deepen. However, Pat leveled off in intensity on August 27. After turning northwest, Pat attained typhoon intensity on August 28. Pat accelerated towards the north, and reached its peak intensity of 80 mph (130 km/h) on August 30. The next day, the storm crossed the southern Japanese islands and entered the Sea of Japan. Gradually weakening, Pat transitioned into an extratropical cyclone later on August 31. Early the next day, the storm moved ashore along northeastern Japan. The system dissipated on September 2 after reentering the Pacific Ocean. A total of 23 perished due to Typhoon Pat and 12 others were rendered as missing. Additionally, 79 people were injured. Furthermore, 38 houses in Japan were demolished, 110 were damaged, and over 2,000 were flooded. More than 160,000 homes lost power. A total of 165 flights were cancelled.

Typhoon Kim (1980)

Typhoon Kim, known in the Philippines as Typhoon Osang, was the second typhoon in a week to directly affect the Philippines during July 1980. Like Typhoon Joe, Kim formed from the near equatorial monsoon trough in the northwestern Pacific Ocean on July 19. The disturbance tracked quickly westward-northwest underneath a subtropical ridge, reaching tropical storm strength on the July 21 and typhoon strength on July 23. After developing an eye, Kim began to rapidly intensify, and during the afternoon of July 24, peaked in intensity as a super typhoon. Several hours later, Kim made landfall over the Philippines, but the storm had weakened considerably by this time. Throughout the Philippines, 40 people were killed, 2 via drownings, and 19,000 others were directly affected. A total of 12,000 homes were destroyed and 5,000 villages were flooded. Less than a week earlier, the same areas were affected by Joe; however, Kim was considered the more damaging of the two typhoons. Land interaction took its toll on Kim, and upon entering the South China Sea, the storm was down below typhoon intensity. Kim continued northwestward but its disrupted circulation prevented re-intensification, and it remained a tropical storm until hitting southern China July 27 to the northeast of Hong Kong, where only slight damage was reported. Later that day, Kim dissipated.

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Regional Specialized Meteorological Centres
Tropical Cyclone Warning Centres