View source for Tropical cyclone intensity scales

{{Short description|Scales of the intensity of tropical cyclones}}
{{Good article}}
{{Use mdy dates|date=September 2017}}
{{Tropicalcyclone}}

[[Tropical cyclone]]s are ranked on one of five '''tropical cyclone intensity scales''', according to their [[maximum sustained wind]]s and which [[tropical cyclone basins]] they are located in. Only a few classifications are used officially by the meteorological agencies monitoring the tropical cyclones, but other scales also exist, such as [[accumulated cyclone energy]], the Power Dissipation Index, the Integrated Kinetic Energy Index, and the [[Hurricane Severity Index]].

Tropical cyclones that develop in the [[Northern Hemisphere]] are classified by the warning centres on one of three intensity scales. Tropical cyclones or [[subtropical cyclone]]s 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]], and 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.

The [[India Meteorological Department]]'s scale uses seven 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 [[Tropical cyclone basins|South-West Indian Ocean]] is defined by [[Météo-France]] for use in various French territories, including [[New Caledonia]] and [[French Polynesia]].

The definition of [[maximum sustained wind|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&nbsp;[[metre|m]] (33&nbsp;[[foot (length)|ft]]) above the sea surface. However, the Saffir–Simpson hurricane scale is based on wind speed measurements averaged over a 1-minute period, at {{convert|10|m|ft|abbr=on}}. The scale used by [[Regional Specialized Meteorological Centre]] (RSMC)  New Delhi applies a 3-minute averaging period, and the Australian scale is based on both 3-second [[wind gust]]s and maximum sustained winds averaged over a 10-minute interval. These differences make direct comparisons between basins difficult.

Within all basins tropical cyclones are named when the sustained winds reach at least {{convert|35|kn|mph km/h|abbr=on}}.

==Background==
{{Unreferenced section|date=September 2017}}

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 Organization]]'s [[Regional Specialized Meteorological Center]]s 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.

==Atlantic, Eastern and Central Pacific==
{{Saffir–Simpson scale}}
{{Main|Saffir–Simpson scale}}

Tropical cyclones that occur within the Northern Hemisphere to the east of the [[180th meridian|anti-meridian]], are officially monitored by either the [[National Hurricane Center]] or the [[Central Pacific Hurricane Center]].{{RA IV Hurricane Operational Plan}} 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.<ref name="RA IV HOP"/> The region also defines a subtropical cyclone as a non-frontal low pressure disturbance, that has the characteristics of both tropical and extratropical cyclones.<ref name="RA IV HOP"/> 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 {{convert|34|kn|mph km/h|abbr=on}}.<ref name="RA IV HOP"/>

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);<ref name="OFCM NHOP">{{cite report|publisher=Office of the Federal Coordinator for Meteorological Services and Supporting Research|url=http://www.ofcm.gov/publications/nhop/FCM-P12-2017.pdf|archive-url=https://web.archive.org/web/20170503174529/http://www.ofcm.gov/publications/nhop/FCM-P12-2017.pdf|url-status=dead|archive-date=May 3, 2017|title=National Hurricane Operations Plan|date=May 2017|access-date=October 14, 2018}} </ref> {{Citation needed span|text=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.|date=May 2023}}

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)<ref name="RA IV HOP"/> 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 {{convert|34–63|kn|mph km/h|abbr=on}}, then it will be called either a tropical or subtropical storm and [[Tropical cyclone naming|assigned a name]]<ref name="RA IV HOP"/> (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 ''[[2018 Atlantic hurricane season#Tropical Storm Kirk|TS 12 (KIRK)]]'').

Should the tropical system further intensify and have winds estimated or measured, as greater than {{convert|64|kn|mph km/h|abbr=on}}, then it will be called a hurricane and classified on the [[Saffir–Simpson scale|Saffir–Simpson hurricane wind scale]].<ref name="RA IV HOP"/> The lowest classification on the SSHWS is a Category 1 hurricane, which has winds of between {{convert|64 and 82|kn|mph km/h|abbr=on}}.<ref name="RA IV HOP"/><ref name="NHC SSHWS">{{cite web|publisher=National Hurricane Center|title=Saffir–Simpson hurricane scale Information|date=May 24, 2013|url=http://www.nhc.noaa.gov/aboutsshws.php|access-date=November 8, 2015}}</ref> Should the hurricane intensify further then it will be rated as a Category 2 hurricane, if it has winds of between {{convert|83 and 95|kn|mph km/h|abbr=on}}.<ref name="RA IV HOP"/><ref name="NHC SSHWS" /> When a system becomes a Category 3 hurricane with winds of between {{convert|96 and 112|kn|mph km/h|abbr=on}}, it is considered to be a major hurricane by the warning centers.<ref name="NHC SSHWS" /> A Category 4 hurricane has winds of {{convert|113 to 136|kn|mph km/h|abbr=on}}, while a Category 5 hurricane has winds of at least {{convert|137|kn|mph km/h|abbr=on}}.<ref name="RA IV HOP"/><ref name="NHC SSHWS" /> 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.<ref name="RA IV HOP"/> However, advisories may continue if the post tropical cyclone poses a significant threat to life and property.<ref name="RA IV HOP"/> 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&nbsp;hours.<ref name="RA IV HOP"/>
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<ref name="AOML FAQ F1">{{cite web|author=[[Atlantic Oceanographic and Meteorological Laboratory]], Hurricane Research Division|title=Frequently Asked Questions: What regions around the globe have tropical cyclones and who is responsible for forecasting there?|publisher=[[NOAA]]|date=July 12, 2007|access-date=December 24, 2008|url=http://www.aoml.noaa.gov/hrd/tcfaq/F1.html}}</ref

<ref name="NHC glossary">{{cite web|author=[[National Hurricane Center]]|url=http://www.nhc.noaa.gov/aboutgloss.shtml|year=2005 |title=Glossary of NHC/TPC Terms |access-date=December 24, 2008 |publisher=[[National Oceanic and Atmospheric Administration]]}}</ref> -->

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.<ref name="NHC SSHWS" /><ref>{{cite web |url= http://www.weather.gov/infoservicechanges/sshws.pdf |title= Experimental Saffir–Simpson Hurricane Wind Scale |year= 2009 |publisher= National Hurricane Center |access-date= August 17, 2009 |archive-url= https://web.archive.org/web/20090806142811/http://www.weather.gov/infoservicechanges/sshws.pdf |archive-date= August 6, 2009 |url-status= dead }}</ref>

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 [[Saffir–Simpson hurricane scale#Category 2|Category&nbsp;2]] hurricane that strikes a major urban area will likely do more damage than a large Category&nbsp;5 hurricane that strikes a mostly rural region. In fact, tropical systems of less than hurricane strength, as in the case of [[Tropical Storm Allison]], can produce significant damage and human casualties, especially from flooding and landslides.<ref>{{Cite web |date=2005 |title=Hurricane Basics |url=https://repository.library.noaa.gov/view/noaa/12828 |access-date=December 21, 2023 |website=repository.library.noaa.gov |publisher=Tropical Prediction Center/National Hurricane Center (U.S.)}}</ref>

Historically, the term ''great hurricane'' was used to describe storms that possessed winds of at least {{convert|110|kn|mph km/h}}, large radii (over 160&nbsp;km / 100&nbsp;[[statute mile|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.<ref name="Great Hurricane">{{cite tech report|url=https://repository.library.noaa.gov/view/noaa/35476|title=Florida Hurricanes and Tropical Storms: 1871–1993: An Historical Survey|author1=Fred Doehring|author2=Iver W. Duedall|author3=John M. Williams|year=1994|publisher=Florida Sea Grant College Program|pages=53–54|access-date=June 18, 2024|number=TP-71|isbn=0-912747-08-0}}</ref>

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 {{convert|115|kn|mph km/h|round=5|abbr=on}}.<ref name="SSHWS PIS">{{cite web|author=Tew, Mark|title=Public Information Statement: Minor Modification of Saffir–Simpson Hurricane Wind Scale Thresholds Effective May 15, 2012 |date=March 1, 2012 |access-date=November 7, 2015 |publisher=United States National Weather Service |url=http://www.nhc.noaa.gov/news/20120301_pis_sshws.php |archive-url=https://web.archive.org/web/20151122174409/http://www.nhc.noaa.gov/news/20120301_pis_sshws.php |archive-date=November 22, 2015 |url-status=live}}</ref>
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==Western Pacific==
{{Japan Meteorological Agency's Tropical Cyclone Intensity Scale}}
Tropical cyclones that occur within the Northern Hemisphere between the [[180th meridian|anti-meridian]] and [[100th meridian east|100°E]] are officially monitored by the [[Japan Meteorological Agency]] (JMA, RSMC Tokyo).<ref name="TCOM">{{cite report|author=Typhoon Committee|title=Typhoon Committee Operational Manual 2015|access-date=November 13, 2015|url=https://www.wmo.int/pages/prog/www/tcp/documents/TCP-23EDITION2015.pdf|year=2015|publisher=World Meteorological Organization|archive-url=https://web.archive.org/web/20201110193753/https://www.wmo.int/pages/prog/www/tcp/documents/TCP-23EDITION2015.pdf|archive-date=November 10, 2020|url-status=dead}}</ref> 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.<ref name="TCOM" /> The lowest classification used by the Typhoon Committee is a tropical depression, which has 10-minute sustained winds of less than {{convert|34|kn|m/s mph km/h|abbr=on}}.<ref name="TCOM" /> Should the tropical depression intensify further it is named and classified as a tropical storm, which has winds speeds between {{convert|34|-|47|kn|m/s mph km/h|abbr=on}}.<ref name="TCOM" /> Should the system continue to intensify further then it will be classified as a severe tropical storm, which has winds speeds between {{convert|48|-|63|kn|m/s mph km/h|abbr=on}}.<ref name="TCOM" /> The highest classification on the Typhoon Committee's scale is a typhoon, which has winds speeds greater than {{convert|64|kn|m/s mph km/h|abbr=on}}.<ref name="TCOM" />

The [[China Meteorological Administration]], the [[Hong Kong Observatory]] (HKO), Macao [[Meteorological and Geophysical Bureau]] (SMG), [[PAGASA]] and the JMA, all divide the typhoon category further for domestic purposes.<ref name="TCOM" /> The JMA divides the typhoon category into three categories, with a 10-minute maximum wind speed below {{convert|84|kn|m/s mph km/h|abbr=on}} assigned for the (strong) typhoon category. A very strong typhoon has wind speeds between {{convert|85|-|104|kn|m/s mph km/h|abbr=on}}, while a violent typhoon has wind speeds of {{convert|105|kn|m/s mph km/h|abbr=on}} or greater.<ref name="TCOM" /> The HKO, SMG and the CMA also divide the typhoon category into three categories, with both assigning a maximum wind speed of {{convert|80|kn|m/s mph km/h|abbr=on}} to the typhoon category. A severe typhoon has wind speeds of {{convert|85|-|104|kn|m/s mph km/h|abbr=on}}, while a super typhoon has winds of {{convert|100|kn|m/s mph km/h|abbr=on}}.<ref name="TCOM" /><ref>{{cite web|url=http://www.weather.gov.hk/wxinfo/news/2009/20090318_appendix1e.pdf|title=Classifications of Tropical cyclones|date=March 18, 2009|publisher=Hong Kong Observatory|access-date=July 27, 2012|archive-url=https://web.archive.org/web/20120330002011/http://www.weather.gov.hk/wxinfo/news/2009/20090318_appendix1e.pdf|archive-date=March 30, 2012|url-status=dead}}</ref> In May 2015, following the damage caused by [[Typhoon Haiyan]] in 2013, PAGASA introduced the term Super Typhoon and used it for systems with winds greater than {{convert|120|kn|m/s mph km/h|abbr=on}},<ref>{{cite web|last=Cervantes|first=Ding|title=Pagasa bares 5 new storm categories|url=http://www.abs-cbnnews.com/nation/regions/05/15/15/pagasa-bares-5-new-storm-categories|access-date=May 20, 2015|date=May 16, 2015|publisher=ABS-CBN}}</ref> but later adjusted to at least {{convert|99.9|kn|m/s mph km/h|abbr=on}} on March 23, 2022.<ref name="ABS-CBN News">{{cite news | url=https://news.abs-cbn.com/amp/news/03/23/22/pagasa-redefines-super-typhoon-tweaks-wind-signals | title=PAGASA redefines 'super typhoon', revises wind signals | publisher=[[ABS-CBN News]] | date=March 23, 2022 | access-date=March 28, 2022}}</ref><ref name="TCWS2022verRappler">{{Cite web |last=Arceo |first=Acor |date=March 23, 2022 |title=PAGASA changes super typhoon definition, wind signals |url=https://www.rappler.com/nation/weather/pagasa-changes-super-typhoon-definition-tropical-cyclone-wind-signals-march-2022/ |archive-url=https://archive.today/20220823150431/https://www.rappler.com/nation/weather/pagasa-changes-super-typhoon-definition-tropical-cyclone-wind-signals-march-2022/ |archive-date=August 23, 2022 |access-date=August 23, 2022 |website=Rappler}}</ref><ref name="TCWS2022verPDI">{{Cite web |last=Sarao |first=Zacarian |date=March 24, 2022 |title=Pagasa revises definition of 'super typhoon,' signal system |url=https://newsinfo.inquirer.net/1572737/pagasa-revises-definition-of-super-typhoon-signal-system |archive-url=https://archive.today/20220823150444/https://newsinfo.inquirer.net/1572737/pagasa-revises-definition-of-super-typhoon-signal-system |archive-date=August 23, 2022 |access-date=August 23, 2022 |website=Inquirer.net |publisher=Philippine Daily Inquirer}}</ref><ref name="2022TCWS">{{Cite web |date=March 23, 2022 |title=Press Release: DOST-PAGASA modifies Tropical Cyclone Wind Signal (TCWS) system |url=https://bagong.pagasa.dost.gov.ph/press-release/108 |archive-url=https://archive.today/20220823131633/https://bagong.pagasa.dost.gov.ph/press-release/108 |archive-date=August 23, 2022 |access-date=August 23, 2022 |website=Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA)}}</ref><ref name="newTCWSvid">{{Cite web |date=March 23, 2022 |title=Modified Tropical Cyclone Warning Signal (TCWS) System (23 March 2022) |url=https://www.youtube.com/watch?v=ke2drZ-2YfU |access-date=August 23, 2022 |website=[[YouTube]] |publisher=Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) |language=fil}}</ref> In 2018, following devastating damage caused by [[Typhoon Hato]] to Macau, SMG introduced the super typhoon ({{langx|pt|Super tufão}}) category together with the severe typhoon ({{langx|pt|Tufão severo}}) category as that of HKO.<ref>{{cite web|url=https://bo.io.gov.mo/bo/i/2018/16/ordem61_cn.asp|title=《第61/2018號行政命令》|trans-title=Executive Order No. 61/2018|author=Government Printing Bureau, MSAR Government|date=2018-04-16|access-date=2024-09-07|language=zh}} ([https://bo.io.gov.mo/bo/i/2018/16/ordem61.asp#61 Portuguese version])</ref>

In addition to the national meteorological services of each nation, the United States' [[Joint Typhoon Warning Center]] (JTWC) monitors the basin, and issues warnings on significant tropical cyclones for the United States Government,<ref name="JTWC Products">{{cite web|title=Products and Services Notice|location=Pearl Harbor, Hawaii|publisher=Joint Typhoon Warning Center|url=http://www.usno.navy.mil/JTWC/products-and-services-notice|access-date=October 14, 2018|archive-date=June 9, 2017|archive-url=https://web.archive.org/web/20170609092158/http://www.usno.navy.mil/JTWC/products-and-services-notice|url-status=dead}}</ref> assigning them two-digit TC numbers (with suffix "W").<ref name="OFCM NHOP" /> 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.<ref name="JTWC FAQs">{{cite web|title=Frequently Asked Questions|location=Pearl Harbor, Hawaii|publisher=Joint Typhoon Warning Center|url=http://www.usno.navy.mil/JTWC/frequently-asked-questions-1/frequently-asked-questions|access-date=October 14, 2018|archive-date=October 4, 2013|archive-url=https://web.archive.org/web/20131004091412/http://www.usno.navy.mil/JTWC/frequently-asked-questions-1/frequently-asked-questions/|url-status=dead}}</ref> These classifications are Tropical Depression, Tropical Storm, Typhoon, and Super Typhoon.<ref name="JTWC FAQs" /> The United States' Joint Typhoon Warning Center (JTWC) unofficially classifies typhoons with wind speeds of at least {{convert|130|kn|mph km/h}}—the equivalent of a strong Category&nbsp;4 storm on the [[Saffir–Simpson scale]]—as ''super typhoons''.<ref>{{cite web|url=http://www.usno.navy.mil/JTWC/frequently-asked-questions-1#labels|author=[[Joint Typhoon Warning Center]]|title=What are the description labels used with tropical cyclones by JTWC?|publisher=[[Joint Typhoon Warning Center]] – Frequently Asked Questions (FAQ)|date=2008-03-31|access-date=2008-12-22|archive-date=September 1, 2013|archive-url=https://web.archive.org/web/20130901174657/http://www.usno.navy.mil/JTWC/frequently-asked-questions-1#labels|url-status=dead}}</ref> Also, when a tropical depression is upgraded to tropical storm and named by the JMA, the JTWC appends the international name (parenthesized) to its TC number<ref name="OFCM NHOP" /> (i.e., 2018 tropical depression ''TWENTY-W'', abbr. ''TD 20W'', became [[Tropical Storm Bebinca (2018)|Tropical Storm Bebinca]], but was referred to as ''TS 20W (BEBINCA)'' in JTWC advisories); however, in cases when the JTWC upgrades a depression to tropical storm without the 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.<ref name="JTWC FAQs" />

In addition, the Taiwan [[Central Weather Administration]] has its own scale in Chinese but uses the Typhoon Committee scale in English.<ref>{{cite web|url=http://www.cwb.gov.tw/V7/knowledge/planning/typhoon.htm#eq05|title=特輯|last=交通部中央氣象局|date=February 1, 2008|website=www.cwb.gov.tw|access-date=July 31, 2012|archive-date=September 23, 2015|archive-url=https://web.archive.org/web/20150923212246/http://www.cwb.gov.tw/V7/knowledge/planning/typhoon.htm#eq05|url-status=dead}}</ref>

==North Indian Ocean==
{{India Meteorological Department's Tropical Cyclone Intensity Scale}}
Any tropical cyclone that develops within the North Indian Ocean between [[100th meridian east|100°E]] and [[45th meridian east|45°E]] is monitored by the [[India Meteorological Department]] (IMD, RSMC New Delhi).<ref name="NIO TCOP2">{{cite report|date=June 8, 2015|title=Tropical Cyclone Operational Plan for the Bay of Bengal and the Arabian Sea 2015|access-date=March 29, 2015|url=https://www.wmo.int/pages/prog/www/tcp/documents/TCP-21Edition2015_final.pdf|publisher=World Meteorological Organization|pages=11–12|type=Report No. TCP-21|author=WMO/ESCAP Panel on Tropical Cyclones}}</ref> 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.<ref name="NIO TCOP2" /> The lowest official classification used in the North Indian Ocean is a depression, which has 3-minute sustained wind speeds of between {{convert|17 and 27|kn|mph km/h|abbr=on}}.<ref name="NIO TCOP2" /> Should the depression intensify further then it will become a deep depression, which has winds between {{convert|28 and 33|kn|mph km/h|abbr=on}}.<ref name="NIO TCOP2" /> If the system intensifies further, it will be classified as a cyclonic storm and be assigned a name by the IMD, if it should develop gale-force wind speeds of between {{convert|34 and 47|kn|mph km/h|abbr=on}}.<ref name="NIO TCOP2" /> Severe cyclonic storms have wind speeds between {{convert|48 and 63|kn|mph km/h|abbr=on}}, while very severe cyclonic storms have hurricane-force winds of {{convert|64–89|kn|mph km/h|abbr=on}}. Extremely severe cyclonic storms have hurricane-force winds of {{convert|90|–|119|kn|mph km/h|abbr=on}}.<ref name="NIO TCOP2" /> The highest classification used in the North Indian Ocean is a super cyclonic storm, which has hurricane-force winds of at least {{convert|120|kn|mph km/h|abbr=on}}.<ref name="NIO TCOP2" />

Historically, a system has been classified as a depression if its surface pressure is lower than its surroundings.<ref name="IMD BT Report">{{cite report|url=http://www.rsmcnewdelhi.imd.gov.in/images/pdf/archive/best-track/bestrack.pdf|title=Best track data of tropical cyclonic disturbances over the north Indian Ocean|date=July 14, 2009|publisher=India Meteorological Department|access-date=October 31, 2015}}</ref> Other classifications historically used include: cyclonic storm where the winds did not exceed [[Beaufort scale|force 10]] on the Beaufort scale and a Cyclone where the winds are either force 11 and 12 on the Beaufort scale.<ref name="IMD BT Report" /> Between 1924 and 1988, tropical cyclones were classified into four categories: depression, deep depression, cyclonic storms and severe cyclonic storms.<ref name="IMD BT Report" /> 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 {{convert|64|kn|mph km/h|abbr=on}}.<ref name="IMD BT Report" /> 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.<ref name="IMD BT Report" /> During 2015, another modification to the scale took place, with the IMD calling a system with 3-minute maximum sustained wind speeds between {{convert|90|and|119|kn|mph km/h|abbr=on}}: an extremely severe cyclonic storm.<ref>{{cite conference|title=Final report on the Third Joint Session of Panel on Tropical Cyclones & Typhoon Committee February 9–13, 2015|url-status=live |location=Bangkok, Thailand |page=10 |url=https://www.wmo.int/pages/prog/www/tcp/documents/3JS_Reportdraft_FINAL.pdf |archive-url=https://web.archive.org/web/20160419055649/https://www.wmo.int/pages/prog/www/tcp/documents/3JS_Reportdraft_FINAL.pdf |archive-date=April 19, 2016}}</ref>

The American [[Joint Typhoon Warning Center]] also monitors the basin, and issues warnings on significant tropical cyclones on behalf of the United States,<ref name="JTWC Products" /> 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 international names or placeholders in parentheses, as done for typhoons above).<ref name="JTWC FAQs" />
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==South-West Indian Ocean==
{{South-West Indian Ocean Tropical Cyclone Intensity Scale}}
Any tropical cyclone that develops within the Southern Hemisphere between Africa and [[90th meridian east|90°E]] is monitored by [[Météo-France]]'s La Réunion tropical cyclone centre (MFR, [[RSMC La Réunion]]).{{RA I Tropical cyclone operational plan}}

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. The system should be estimated to have wind speeds of less than {{convert|28|kn|km/h mph}}.<ref name="SWIO TCOP" />

A system is designated as a [[tropical depression]] or a subtropical depression when it reaches wind speeds above {{convert|28|kn|km/h mph}}. If a tropical depression reaches wind speeds of {{convert|35|kn|km/h 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]].<ref name="MFR">{{cite web|url=http://www.meteo.fr/temps/domtom/La_Reunion/TGPR/PagesFixes/GUIDE/GuideAlerteCyclonique.html#tableaudanger|publisher=[[Météo-France]]|title=Tableau de définition des cyclones|year=2008|access-date=January 14, 2009|language=fr|archive-url=https://web.archive.org/web/20090123140521/http://www.meteo.fr/temps/domtom/La_Reunion/TGPR/PagesFixes/GUIDE/GuideAlerteCyclonique.html#tableaudanger|archive-date=January 23, 2009|url-status=dead}}</ref> Since the 2024–25 cyclone season, a subtropical system will be classified as a subtropical storm if it reaches wind speeds of {{convert|35|kn|km/h mph}}.<ref name="SS">{{cite web |title=Tropical Cyclone Programme Report No. TCP-12 2023 edition |url=https://wmoomm.sharepoint.com/:b:/r/sites/wmocpdb/eve_activityarea/Tropical%20Cyclone%20Programme%20(TCP)/Operational%20Plans/01_RAI-TropicalCycloneCommittee-RA%20I%20TCC/2023_RAI_TCC-25/RAI_TCC-25_OperationalPlan_WMO-Nr1178_EN.pdf?csf=1&web=1&e=4SnO7O |publisher=World Meteorological Organisation |access-date=5 October 2024}}</ref>

If the named tropical system intensifies further and reaches winds speeds of {{convert|48|kn|km/h mph}}, then it will be classified as a severe tropical storm. A severe tropical storm is designated as a [[tropical cyclone]] when it reaches wind speeds of {{convert|64|kn|km/h mph}}.  If a tropical cyclone intensify further and reaches wind speeds of {{convert|90|kn|km/h mph}}, it will be classified as an intense tropical cyclone. A very intense tropical cyclone is the highest category on the South-West Indian Ocean Tropical Cyclone scale, and has winds of over {{convert|115|kn|km/h mph}}.<ref name="SWIO TCOP" /><ref name="MFR" />

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. 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. This change was implemented ahead of the [[1993–94 South-West Indian Ocean cyclone season|1993–94 tropical cyclone season]].<ref name="92-93 SS">{{cite book |url=http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/archives/publications/data/cSaisoncyclonique19921993.pdf |title=Cyclone Season 1992–1993 |publisher=Météo-France; RSMC La Réunion |editor=Le Goff, Guy |pages=105–106 |access-date=November 7, 2015}}</ref>

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;<ref name="JTWC Products" /> these systems are unofficially assigned TC numbers with suffix "S" (which spans the whole South Indian Ocean, including both [[#Australia and Fiji|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 or placeholders, like typhoons and North Indian Ocean cyclones above).<ref name="JTWC FAQs" />
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==Australia and Fiji<span class="anchor" id="Australia"></span>==
{{anchor|Australia}}
{{Australian tropical cyclone intensity scale}}
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.{{RA V Tropical cyclone operational plan}} These are run by the [[Fiji Meteorological Service]], New Zealand's [[MetService]], Indonesia's [[Indonesian Agency for Meteorology, Climatology and Geophysics|Badan Meteorologi, Klimatologi, dan Geofisika]], Papua New Guinea's National Weather Service and the Australian [[Bureau of Meteorology]].<ref name="SPAC TCOP" /> 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 {{convert|34|kn|km/h mph|abbr=on}} or greater near the centre.<ref name="SPAC TCOP" /> 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.<ref name="SPAC TCOP" /><ref name="Fiji Alerts">{{cite report|publisher=Fiji Meteorological Service |url-status=dead |archive-url=https://web.archive.org/web/20151118233159/http://www.pacificdisaster.net/pdnadmin/data/original/FJ_MET_TC_2009_2010.pdf |archive-date=November 18, 2015 |title=Tropical cyclone alerts and warnings summary of procedures within Fiji: 2009–2010 season |url=http://www.pacificdisaster.net/pdnadmin/data/original/FJ_MET_TC_2009_2010.pdf |access-date=November 7, 2015}}</ref> A Category 1 tropical cyclone is estimated to have 10-minute sustained wind speeds of {{convert|34|-|47|kn|mph km/h|abbr=on}}, while a Category 2 tropical cyclone is estimated to have 10-minute sustained wind speeds of {{convert|48|-|63|kn|mph km/h|abbr=on}}.<ref name="Fiji Alerts" /><ref name="BOM FAQ">{{cite web|title=Tropical Cyclone: Frequently Asked Questions|url=http://www.bom.gov.au/cyclone/faq/|publisher=Australian Bureau of Meteorology|access-date=November 7, 2015}}</ref> When a system becomes a Category 3 tropical cyclone it is reclassified as a Severe tropical cyclone and has wind speeds of {{convert|64|-|85|kn|mph km/h|abbr=on}}.<ref name="Fiji Alerts" /><ref name="BOM FAQ" /> A Category 4 severe tropical cyclone has winds of {{convert|86–110|kn|mph km/h|abbr=on}}, while the maximum rating is a Category 5 severe tropical cyclone, which has winds of at least {{convert|108|kn|mph km/h|abbr=on}}.<ref name="Fiji Alerts" /><ref name="BOM FAQ" />

For systems below tropical cyclone strength there are various terms used, including Tropical Disturbance, Tropical Low and Tropical Depression.<ref name="SPAC TCOP" /> A tropical disturbance is defined as being a non-frontal system of synoptic scale originating over the tropics, with persistent enhanced convection or some indication of a circulation.<ref name="SPAC TCOP" /> 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 {{convert|34|kn|mph km/h|abbr=on}} near the centre.<ref name="SPAC TCOP" /> 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.<ref name="SPAC TCOP" /> 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;<ref name="JTWC Products" /> these systems are unofficially assigned TC numbers with either suffix "S" (if originating west of 135°E; spans the whole South Indian Ocean, including [[#South-West Indian Ocean|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 or placeholders parenthesized, as for typhoons and Indian Ocean cyclones above).<ref name="JTWC FAQs" />
{{Clear}}

==Alternative scales==
There are other scales that are not officially used by any of the [[Regional Specialized Meteorological Centre|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.<ref name="IKE">{{cite web|url=http://www.aoml.noaa.gov/hrd/ike/|title=Integrated Kinetic Energy|work=[[Atlantic Oceanographic and Meteorological Laboratory]]|publisher=[[National Oceanic and Atmospheric Administration]]|date=February 7, 2009|access-date=January 18, 2009}}</ref>

=== Accumulated cyclone energy (ACE) ===
[[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.<ref name="CPC">{{cite web|author=Tropical Cyclone Weather Services Program|title=Background Information: The North Atlantic Hurricane Season|url=http://www.cpc.noaa.gov/products/outlooks/background_information.shtml|format=PDF|date=June 1, 2009|access-date=January 16, 2008|publisher=[[National Oceanic and Atmospheric Administration]]|archive-url=https://web.archive.org/web/20080118063537/http://www.cpc.noaa.gov/products/outlooks/background_information.shtml|archive-date=January 18, 2008|url-status=dead}}</ref> 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.<ref name="CPC" /> The numbers are usually divided by 10,000 to make them more manageable. The unit of ACE is 10<sup>4</sup> kn<sup>2</sup>, and for use as an index the unit is assumed.<ref name="CPC" /> As well as being squared for ACE, wind speed can also be cubed, which is referred to as the Power Dissipation Index (PDI).<ref>{{cite journal|author=Kerry Emanuel|date=August 4, 2005|title=Increasing destructiveness of tropical cyclones over the past 30 years|journal=Nature|volume=436|doi=10.1038/nature03906|url=http://texmex.mit.edu/pub/emanuel/PAPERS/NATURE03906.pdf|access-date=February 15, 2010|pmid=16056221|bibcode = 2005Natur.436..686E|issue=7051|pages=686–8|s2cid=2368280}}</ref>

=== Hurricane Severity Index (HSI) ===
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.  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. Points are awarded on a sliding scale, with the majority of points reserved for hurricane force and greater wind fields. A paper describing the scale was published in 2008.<ref name="hebert-2008-05-01">{{Cite web |author=Chris Hebert (ImpactWeather Inc.), B. Weinzapfel, and M. Chambers |date=1 May 2008 |title=The Hurricane Severity Index – A destructive potential rating system for tropical cyclones |url=https://ams.confex.com/ams/28Hurricanes/techprogram/paper_139371.htm |accessdate=March 19, 2025 |work=28th Conference on Hurricanes and Tropical Meteorology |publisher=American Meteorological Society}}</ref> HSI was developed by ImpactWeather (now [[StormGeo]]) meteorologists as a proprietary method of hurricane severity classification.<ref>{{Cite web |title=How to Estimate a Hurricane's Damage Potential |url=https://stormgeo.com/insights/how-to-estimate-hurricanes-damage-potential |access-date=2024-12-02 |website=[[StormGeo]] |language=en}}</ref> HSI misclassifies hurricanes in some cases because it does not account for rainfall.<ref>{{Cite journal |last1=Rezapour |first1=Mehdi |last2=Baldock |first2=Tom E. |date=2014-12-01 |title=Classification of Hurricane Hazards: The Importance of Rainfall |journal=Weather and Forecasting |language=EN |volume=29 |issue=6 |pages=1319–1331 |bibcode=2014WtFor..29.1319R |doi=10.1175/WAF-D-14-00014.1 |issn=1520-0434 |doi-access=free}}</ref> HSI is based on a lookup table rather than an equation.<ref name="collins-2019">{{Cite book |last1=Collins |first1=Jennifer M. |url=https://books.google.com/books?id=TFWIDwAAQBAJ |title=Hurricane Risk |last2=Walsh |first2=Kevin |date=2019-02-15 |publisher=Springer |isbn=978-3-030-02402-4 |language=en}}</ref> It does not take into account the translational speed of the storm.<ref name="collins-2019" />

==See also==
{{Portal|Tropical cyclones}}
* [[Chicago Mercantile Exchange Hurricane Index]]
* [[Rapid intensification]]
* [[Tropical cyclone naming]]
* {{annotated link|Waffle House Index}}

==References==
{{Reflist|2}}

==External links==
* [https://www.youtube.com/watch?v=5lKhb5Ggd-4 Australian Bureau of Meteorology - Understanding tropical cyclone categories]

; Regional specialized meteorological centres
* [http://www.nhc.noaa.gov/ US National Hurricane Center]&nbsp;– North Atlantic, Eastern Pacific
* [http://www.prh.noaa.gov/hnl/cphc/ Central Pacific Hurricane Center] {{Webarchive|url=https://web.archive.org/web/20110923144245/http://www.prh.noaa.gov/hnl/cphc/ |date=September 23, 2011 }}&nbsp;– Central Pacific
* [http://www.jma.go.jp/en/typh/ Japan Meteorological Agency]&nbsp;– NW Pacific
* [http://www.imd.gov.in/ India Meteorological Department]&nbsp;– [[Bay of Bengal]] and the [[Arabian Sea]]
* [https://web.archive.org/web/20070330212754/http://www.meteo.fr/temps/domtom/La_Reunion/ Météo-France&nbsp;– La Reunion]&nbsp;– South Indian Ocean from 30°E to 90°E
*[http://www.met.gov.fj/ Fiji Meteorological Service]&nbsp;– South Pacific west of 160°E, north of 25° S

; Tropical cyclone warning centres
* [https://web.archive.org/web/20080626191441/http://maritim.bmg.go.id/cyclones/IDJ23200.html Indonesian Meteorological Department]&nbsp;– South Indian Ocean from 90°E to 125°E, north of 10°S
* [http://www.pngmet.gov.pg Papua New Guinea's National Weather Service]&nbsp;– South Pacific Ocean from 125°E to 160°E, north of 10°S
* [http://www.bom.gov.au/weather/cyclone/index.shtml Australian Bureau of Meteorology] {{Webarchive|url=https://web.archive.org/web/20080723222148/http://www.bom.gov.au/weather/cyclone/index.shtml |date=July 23, 2008 }}.&nbsp;– South Indian Ocean & South Pacific Ocean from 90°E to 160°E, south of 10°S
* [http://www.metservice.co.nz/public/weatherWarnings/marine-warnings.html Meteorological Service of New Zealand Limited]&nbsp;– South Pacific west of 160°E, south of 25°S

{{DEFAULTSORT:Tropical Cyclone Scales}}
[[Category:Hazard scales]]
[[Category:Scales in meteorology]]
[[Category:Tropical cyclone meteorology]]
[[Category:Wind]]