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Tropical systems are officially ranked on one of several tropical cyclone scales according to their maximum sustained winds and in what oceanic basin they are located. 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 Hurricane Severity Index.

Should a tropical cyclone form in the North Atlantic Ocean or the North-eastern Pacific Ocean, it will be classified using one of the categories in the Saffir-Simpson Hurricane Scale. In the Western Pacific, tropical cyclones will be ranked using the Japan Meteorological Agency's scale. The Regional Specialized Meteorological Centre (RSMC) in New Delhi, India also uses a different scale to assess the maximum sustained winds of a tropical cyclone. In the Southern Hemisphere, the Météo-France forecast center on La Reunion uses a scale that covers the whole of the South West Indian Ocean. Both the Australian Bureau of Meteorology and the RSMC in Nadi, Fiji use the Australian tropical cyclone intensity scale.

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.[1][2] 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.[3][4] These make direct comparisons between basins difficult.

Atlantic and East Pacific

Saffir–Simpson scale, 1-minute maximum sustained winds
Category m/s knots mph km/h
5 ≥ 70 ≥ 137 ≥ 157 ≥ 252
4 58–70 113–136 130–156 209–251
3 50–58 96–112 111–129 178–208
2 43–49 83–95 96–110 154–177
1 33–42 64–82 74–95 119–153
TS 18–32 34–63 39–73 63–118
TD ≤ 17 ≤ 33 ≤ 38 ≤ 62
Main article: Saffir-Simpson Hurricane Scale

The Saffir-Simpson Hurricane Scale is the classification system used for tropical cyclones in the Atlantic Ocean and in the Pacific Ocean east of the anti-meridian.[5] In these oceanic basins, tropical cyclones with maximum sustained winds below 34 kt (65 km/h, 39 mph) are labelled as tropical depressions by either the National Hurricane Center (if it is in the North Atlantic or North-east Pacific Basin) or the Central Pacific Hurricane Center (if located in the North Central Pacific Ocean). Should a tropical depression reach 35 kt (65 km/h, 40 mph), it will receive a name and will be classified as a tropical storm. If the tropical storm continues to intensify and reaches maximum sustained winds of 64 kt (119 km/h, 74 mph) then the tropical storm will be designated as a hurricane.[6]

The Saffir-Simpson scale counts with five different classifications for the intensity of a hurricane, with a Category 1 storm having the lowest maximum winds, whilst a Category 5 hurricane having the highest. Storms that meet the 64-knot threshold, but do not possess maximum sustained winds in excess of 83 kt (177 km/h, 96 mph) are classified as Category 1 hurricanes. A Category 1 storm will be upgraded to a Category 2 hurricane if its maximum sustained winds reach 83 knots. Tropical cyclones that possess wind speeds of at least 96 kt (178 km/h, 111 mph) are classified as Category 3 hurricanes. Category 3 also marks the point at which the NHC and CPHC classify strong storms as major hurricanes.[7] If a hurricane's maximum sustained winds reach 114 kt, (210 km/h, 131 mph), it will be ranked as a Category 4 hurricane. Storms with winds that surpass 136 kt (250 km/h, 156 mph) are of Category 5 intensity.[7] 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 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.[7]

Historically, the term great hurricane was used to describe storms that possessed winds of at least 110 kt (200 km/h, 125 mph), 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.[8]

Western Pacific

RSMC Tokyo's Tropical Cyclone Intensity Scale
Category Sustained winds
Violent typhoon ≥105 knots
≥194 km/h
Very strong typhoon 85–104 knots
157–193 km/h
Typhoon 64–84 knots
118–156 km/h
Severe tropical storm 48–63 knots
89–117 km/h
Tropical storm 34–47 knots
62–88 km/h
Tropical depression ≤33 knots
≤61 km/h

Any tropical cyclone that forms to the west of 180° and east of 100°E in the Northern Hemisphere is officially monitored by the Regional Specialized Meteorological Center in Tokyo, Japan.[9] The Japan Meteorological Agency, which runs RSMC Tokyo, uses four different categories to measure the wind speed produced by a tropical cyclone. These classifications are based on the maximum sustained winds produced by the storm averaged over a 10-minute interval.[9]

A tropical depression is the lowest category that the Japan Meteorological Agency uses and is the term used for a tropical system that has wind speeds not exceeding 33 knots (61 km/h, 38 mph).[9] A tropical depression is upgraded to a tropical storm should its sustained wind speeds exceed 34 knots (62 km/h, 39 mph). Tropical storms also receive official names from RSMC Tokyo.[9] Should the storm intensify further and reach sustained wind speeds of 48 knot (89 km/h, 55 mph) then it will be classified as a severe tropical storm.[9]

Once the system's maximum sustained winds reach wind speeds of 64 knots (118 km/h, 74 mph), the JMA will designate the tropical cyclone as a typhoon—the highest category on its scale.[9] The JMA also unofficially divides typhoons into three further classifications—strong (強い, 64–84 knots), very strong (非常に強い, 85–104 knots), and intensive (猛烈な, ≥105 knots).[10]

The United States' Joint Typhoon Warning Center (JTWC) unofficially classifies typhoons with wind speeds of at least 130 knots (67 m/s; 150 mph; 241 km/h)—the equivalent of a strong Category 4 storm in the Saffir-Simpson scale—as super typhoons.[11] However, the maximum sustained wind speed measurements that the JTWC uses are based on a 1-minute averaging period, akin to the U.S.' National Hurricane Center and Central Pacific Hurricane Center. As a result, the JTWC's wind reports are higher than JMA's measurements, as the latter are based on a 10-minute averaging interval.[12]

From 2009 the Hong Kong Observatory started to further divide typhoon into two further classifications severe typhoon and super typhoon.[13] A severe typhoon has winds of at least 81 knot (95 mph, 150 km/h) whilst a super typhoon has winds of at least 100 knot (115 mph, 185 km/h).[13] In Hong Kong, the Observatory's 'tropical cyclone signals', actually an assessment of the threat or effects of a tropical cyclone, are universally used to describe the power of a typhoon, viz T1 (a warning that a cyclone is within 800 km), T3 (strong wind), T8 (gale or storm force winds expected or blowing; subdivided by wind direction), T9 and T10 (more powerful winds; rare). When the T3, and especially the T8, signal is 'hoisted', much public activity, including the operation of schools and businesses, ceases, under regulatory force.[14]

China uses a very similar scale except for the following:

  • 2-minute sustained winds are used.[15]
  • The sustained winds of Tropical Depression is defined as being equivalent to a Beaufort force 6–7, i.e. a lower limit is set.[16]
  • Typhoon is further divided into three categories since the sudden introduction of the extended Beaufort scale on May 15, 2006. The sustained winds of Typhoon, Severe Typhoon, and Super Typhoon are defined as being equivalent to a Beaufort force 12–13, 14–15, and 16–17 respectively.[17][18]

Taiwan uses the JMA's scale in English but another scale in Chinese. The Philippines uses the JMA's scale in international shipping warnings but merges the category "Severe Tropical Storm" with "Tropical Storm" in public advisories.

North Indian Ocean

India Meteorological Department
Tropical Cyclone Intensity Scale
Category Sustained 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 Storm 34–47 kt
63–88 km/h
Deep Depression 28–33 kt
51–62 km/h
Depression 17–27 kt
31–50 km/h

Any tropical cyclone that forms between longitude 45°E and 100°E in the Northern Hemisphere is monitored by the India Meteorological Department (IMD), who run the Regional Specialized Meteorological Center in New Delhi, India.[19] Since 1998, RSMC New Delhi has used six different categories to measure the wind speed of a tropical cyclone based on the maximum sustained winds over a 3-minute averaging period.[19]

A depression is the lowest category that RSMC New Delhi uses to designate tropical systems, and systems designated as depressions have wind speeds of under 27 kt (51 km/h, 31 mph).[20] A depression is classified as a deep depression when it has maximum sustained winds between 27 kt (51 km/h, 31 mph) and 33 kt (61 km/h, 38 mph).[20] Should a deep depression intensify further, it will be classified as a cyclonic storm if its sustained winds reach 34 kt (62 km/h, 39 mph).[20] When a tropical system is classified as a cyclonic storm, it is assigned a name by the IMD.[19]

In cases where cyclonic storms possess wind speeds greater than 48 kt, (88 km/h, 55 mph), they are classified as severe cyclonic storms.[19] A severe cyclonic storm is labelled as a very severe cyclonic storm when it reaches wind speeds greater than 64 kt, (118 km/h, 74 mph).[19] Finally, a super cyclonic storm is the highest category that the India Meteorological Department uses in its scale, and is used to refer to tropical cyclones that have maximum sustained winds exceeding 120 kt, (222 km/h, 138 mph).[20]

Prior to 1988, cyclones were classified into 4 categories, which were depression, deep depression, cyclonic storms and severe cyclonic storms.[21] However in 1988 the IMD started to rate cyclones with wind speeds of more than 64 kt, (118 km/h, 74 mph) as severe cyclonic storms.[21] The IMD then made another change in 1998 to introduce a category for super cyclonic storms, which are cyclonic storms with wind speeds of more than 120 kt, (222 km/h, 138 mph).[21]

South-Western Indian Ocean

Southwest Indian Ocean
Tropical Cyclone Intensity Scale
Category Sustained winds
Very Intense
Tropical Cyclone 		>115 kt
>212 km/h
Intense
Tropical Cyclone 		90–115 kt
166–212 km/h
Tropical Cyclone 64–89 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 forms within the Southern Indian Ocean to the west of 90°E is monitored by Météo-France who run the Regional Specialized Meteorological Center in La Reunion.[22] RSMC La Reunion uses seven different categories to measure the wind speed of a tropical cyclone. It is based on a 10-minute average maximum sustained winds, rather than 1-minute maximum sustained winds, which is what the Saffir-Simpson Hurricane Scale uses.[22]

A tropical disturbance is the lowest category on the South-west Indian Ocean Tropical Cyclone scale, and has wind speeds of 28 knots (50 km/h, 32 mph).[22] A tropical disturbance is designated as a tropical depression when the disturbance reaches wind speeds above 28 knots (32 mph, 50 km/h). 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.[23]

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.[23] A severe tropical storm is designated as a tropical cyclone when it reaches wind speeds of 64 knots (118 km/h, 74 mph).[22] 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.[22] 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).[23]

Australia and Fiji

Australian tropical cyclone intensity scale
Category Sustained
winds		 Gusts
Five >107 kn (198 km/h; 123 mph) >151 kn (280 km/h; 174 mph)
Four 86–107 kn (159–198 km/h; 99–123 mph) 122–151 kn (226–280 km/h; 140–174 mph)
Three 64–85 kn (119–157 km/h; 74–98 mph) 90–121 kn (167–224 km/h; 104–139 mph)
Two 48–63 kn (89–117 km/h; 55–72 mph) 68–89 kn (126–165 km/h; 78–102 mph)
One 34–47 kn (63–87 km/h; 39–54 mph) 49–67 kn (91–124 km/h; 56–77 mph)

Any tropical cyclone that forms to the east of 90°E in the Southern Hemisphere is monitored by either the Australian Bureau of Meteorology and/or the Regional Specialized Meteorological Center in Nadi, Fiji.[4] Both warning centres use the Australian tropical cyclone intensity scale, which measures tropical cyclones using a six category system.[4] It is based on estimated maximum wind gusts, which are a further 30-40% stronger than the 10-minute average sustained winds. This is different from the Saffir-Simpson Hurricane Scale, which uses 1-minute maximum sustained winds.[1]

When a tropical cyclone that has wind speeds below 35 knots (65 km/h, 40 mph) forms east of 160°E it is labelled as either a tropical disturbance or a tropical depression by RSMC Nadi.[4] If it forms to the west of 160°E it is labelled as a tropical low by the Australian Bureau of Meteorology.[4] However if it forms to the north of 10°S and between 90°E to 125°E the low is labelled as a tropical depression by the Tropical Cyclone Warning Center in Jakarta, Indonesia.[24]

If a tropical depression should reach 35 knots (65 km/h, 40 mph), it will be named by the TCWC or RSMC and be classified as a tropical cyclone.[25] Should the cyclone intensify further reaching maximum sustained winds of 65 knots (145 km/h, 75 mph) then the cyclone will be designated as a category three severe tropical cyclone.[25] A severe tropical cyclone will be classified as a category five severe tropical cyclone should the cyclone's maximum sustained wind speed be greater than 110 knots (200 km/h, 130 mph) and gusts be above 150 knots (280 km/h, 175 mph).[25]

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:

Template:Tropical cyclone classification

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; it works on a scale that ranges from one to six, with six having the highest destructive potential.[26]

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.[27] 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.[27] The numbers are usually divided by 10,000 to make them more manageable. The unit of ACE is 104 kt2, and for use as an index the unit is assumed.[27] As well as being squared ACE can also be cubed, and this version is known as the Power Dissipation Index (PDI).[28]

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.[29] 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.[29] Points are awarded on a sliding scale, with the majority of points reserved for hurricane force and greater wind fields.[29]

Wind speed conversions

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, RSMC Miami and RSMC Honolulu, as well as the Joint Typhoon Warning Center, define sustained winds based on 1-minute average speed (also measured 10 m (33 ft) above the surface).[1][2]

Studies have shown that the two definitions are correlated, with the cyclone's maximum one-minute wind speed conventionally about 14% higher than its best ten-minute one. (To convert from a one-minute wind speed to a ten-minute wind speed, the one-minute speed is multiplied by 0.88. In the other direction, the ten-minute wind speed is multiplied by 1.14 to produce the one-minute wind speed.) This relationship is approximate, as the conversion factor varies with different land or sea surfaces and atmospheric stability.[30]

See also

References

  1. ^ a b c Tropical Cyclone Weather Services Program (2006-06-01). "Tropical cyclone definitions" (PDF). National Weather Service. Retrieved 2006-11-30. Cite error: The named reference "NWSM Defs" was defined multiple times with different content (see the help page).
  2. ^ a b Federal Emergency Management Agency (2004). "Hurricane Glossary of Terms". Archived from the original on 2005-12-14. Retrieved 2006-03-24.
  3. ^ "Tropical Cyclone Operational Plan for the North Indian Ocean" (PDF). World Meteorological Organization. 2008. Retrieved 2009-07-19.
  4. ^ a b c d e "Tropical Cyclone Operational plan for the South Pacific & South-east Indian Ocean" (PDF). World Meteorological Organization. 2008. Retrieved 2008-07-03.
  5. ^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division (2007-07-12). "Frequently Asked Questions: What regions around the globe have tropical cyclones and who is responsible for forecasting there?". NOAA. Retrieved 2008-12-24.
  6. ^ National Hurricane Center (2005). "Glossary of NHC/TPC Terms". National Oceanic and Atmospheric Administration. Retrieved 2008-12-24.
  7. ^ a b c National Hurricane Center (2006-06-02). "Saffir-Simpson Hurricane Scale Information". National Oceanic and Atmospheric Administration. Retrieved 2007-02-25.
  8. ^ Fred Doehring, Iver W. Duedall, John M. Williams (1994). "Florida Hurricanes and Tropical Storms: 1871–1993: An Historical Survey" (PDF). Florida Institute of Technology. pp. 53–54. Retrieved 2008-12-26.{{cite web}}: CS1 maint: multiple names: authors list (link)
  9. ^ a b c d e f Typhoon Committee (2011). "Typhoon Committee Operational Manual" (PDF). World Meteorological Organization. Retrieved 2011-10-16.
  10. ^ "台風の大きさと強さ". Japan Meteorological Agency. Retrieved 2011-08-17.
  11. ^ Joint Typhoon Warning Center (2008-03-31). "What are the description labels used with tropical cyclones by JTWC?". Joint Typhoon Warning Center. Retrieved 2008-12-22. {{cite web}}: Text "Frequently Asked Questions (FAQ)" ignored (help)
  12. ^ "How are JTWC forecasts different than forecasts issued by tropical cyclone warning centres (TCWCs) of other countries?". Joint Typhoon Warning Center. 2008-03-31. Retrieved 2008-12-26.
  13. ^ a b "Classifications of Tropical cyclones" (PDF). Hong Kong Observatory. 2009-03-18. Retrieved 2009-04-27.
  14. ^ Hong Kong's Tropical Cyclone Warning Signals
  15. ^ "中国台风网". Typhoon.gov.cn. Retrieved 2008-12-23.
  16. ^ [1][dead link]
  17. ^ "昨日实行新标准"珍珠"属强台风_新闻中心_新浪网". News.sina.com.cn. Retrieved 2008-12-23.
  18. ^ "中国气象影视信息网-气象术语". Weathercn.com. Retrieved 2008-12-23.
  19. ^ a b c d e "Tropical Cyclone Operational plan for the Bay of Bengal and the Arabian Sea" (PDF). World Meteorological Organization. 2008. Retrieved 2008-12-23.
  20. ^ a b c d "IMD FAQ:How are low pressure system classified in India? What are the differences between low, depression and cyclone?". India Meteorological Department. 2008-06-24. Retrieved 2008-12-23.
  21. ^ a b c "IMD Best track data 1990 to 2008" (PDF). India Meteorological Department. 2009-07-14. Retrieved 2009-07-19.
  22. ^ a b c d e "Tropical Cyclone Operational Plan for the South West Indian Ocean 2006" (PDF). World Meteorological Organization. 2006. Retrieved 2008-07-03.
  23. ^ a b c "Tableau de définition des cyclones". Météo-France. 2008. Retrieved 2009-01-14.Template:Fr icon
  24. ^ "Extreme Weather Warning 20-04-08 00z". Indonesian Meteorological and Geophysical Agency. 2008. Retrieved 2008-04-20.
  25. ^ a b c "Frequently Asked Question 3 - How is a severe tropical cyclone different from a non-severe cyclone?". Australian Bureau of Meteorology. 2009. Retrieved 2009-01-14.
  26. ^ "Integrated Kinetic Energy". Atlantic Oceanographic and Meteorological Laboratory. National Oceanic and Atmospheric Administration. 2009-02-07. Retrieved 2009-01-18.
  27. ^ a b c Tropical Cyclone Weather Services Program (06-01-2009). "Background Information: The North Atlantic Hurricane Season" (PDF). National Oceanic and Atmospheric Administration. Retrieved 2008-01-16. {{cite web}}: Check date values in: |date= (help)
  28. ^ Kerry Emanuel (2005-08-04). "Increasing destructiveness of tropical cyclones over the past 30 years" (PDF). Nature. 436. doi:10.1038/nature03906. PMID 16056221. Retrieved 2010-02-15.
  29. ^ a b c "Background Information: The North Atlantic Hurricane Season". American Meteorological Society. 2008-12-19. Retrieved 2009-01-16.
  30. ^ "Intensity Observation and forecast errors". United States Naval Research Laboratory. 2009-07-04. Retrieved 2009-07-04.
Regional Specialized Meteorological Centres
Tropical Cyclone Warning Centres

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