are ranked on one of five tropical cyclone intensity scales, according to their maximum sustained winds 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 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, 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 South-West Indian Ocean is defined by Météo-France for use in various French territories, including New Caledonia and French Polynesia.
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 metre (33 ft) above the sea surface. However, the Saffir–Simpson hurricane scale is based on wind speed measurements averaged over a 1-minute period, at . 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 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 .
Background
Atlantic, Eastern and Central Pacific
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);
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) 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 , then it will be called either a tropical or subtropical storm and assigned a name (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 , then it will be called a hurricane and classified on the Saffir–Simpson hurricane wind scale. The lowest classification on the SSHWS is a Category 1 hurricane, which has winds of between . Should the hurricane intensify further then it will be rated as a Category 2 hurricane, if it has winds of between . When a system becomes a Category 3 hurricane with winds of between , it is considered to be a major hurricane by the warning centers. A Category 4 hurricane has winds of , while a Category 5 hurricane has winds of at least . 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. However, advisories may continue if the post tropical cyclone poses a significant threat to life and property. 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.
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, as in the case of Tropical Storm Allison, 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 , large radii (over 160 km / 100 statute mile) and that caused large amounts of destruction. This term fell into disuse after the introduction of the Saffir–Simpson scale in the early 1970s.
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 .
Western Pacific
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. The JMA divides the typhoon category into three categories, with a 10-minute maximum wind speed below assigned for the (strong) typhoon category. A very strong typhoon has wind speeds between , while a violent typhoon has wind speeds of or greater. The HKO, SMG and the CMA also divide the typhoon category into three categories, with both assigning a maximum wind speed of to the typhoon category. A severe typhoon has wind speeds of , while a super typhoon has winds of . 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 , but later adjusted to at least on March 23, 2022. In 2018, following devastating damage caused by Typhoon Hato to Macau, SMG introduced the super typhoon () category together with the severe typhoon () category as that of HKO. ( Portuguese version)
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, assigning them two-digit TC numbers (with suffix "W"). 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. These classifications are Tropical Depression, Tropical Storm, Typhoon, and Super Typhoon. The United States' Joint Typhoon Warning Center (JTWC) unofficially classifies typhoons with wind speeds of at least —the equivalent of a strong Category 4 storm on the Saffir–Simpson scale—as super typhoons. 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 (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 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.
In addition, the Taiwan Central Weather Administration has its own scale in Chinese but uses the Typhoon Committee scale in English.
North Indian Ocean
Historically, a system has been classified as a depression if its surface pressure is lower than its surroundings. Other classifications historically used include: cyclonic storm where the winds did not exceed Beaufort scale on the Beaufort scale and a Cyclone where the winds are either force 11 and 12 on the Beaufort scale. Between 1924 and 1988, tropical cyclones were classified into four categories: depression, deep depression, cyclonic storms and severe cyclonic storms. 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 . 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. During 2015, another modification to the scale took place, with the IMD calling a system with 3-minute maximum sustained wind speeds between : an extremely severe cyclonic storm.
The American Joint Typhoon Warning Center also monitors the basin, and issues warnings on significant tropical cyclones on behalf of the United States, 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).
South-West Indian Ocean
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 .
A system is designated as a tropical depression or a subtropical depression when it reaches wind speeds above . If a tropical depression reaches wind speeds of then it will be classified as a moderate tropical storm and assigned a name by either the Sub Regional Center in Mauritius or Madagascar. Since the 2024–25 cyclone season, a subtropical system will be classified as a subtropical storm if it reaches wind speeds of .
If the named tropical system intensifies further and reaches winds speeds of , 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 . If a tropical cyclone intensify further and reaches wind speeds of , 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 .
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 tropical 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; 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 or placeholders, like typhoons and North Indian Ocean cyclones above).
Australia and Fiji
For systems below tropical cyclone strength there are various terms used, including Tropical Disturbance, Tropical Low and Tropical Depression. 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. 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 near the centre. 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. 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; 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 or placeholders parenthesized, as for typhoons and Indian Ocean cyclones above).
Alternative scales
Accumulated cyclone energy (ACE)
Hurricane Severity Index (HSI)
See also
- Chicago Mercantile Exchange Hurricane Index
- Rapid intensification
- Tropical cyclone naming
External links
- Regional specialized meteorological centres
- US National Hurricane Center – North Atlantic, Eastern Pacific
- Central Pacific Hurricane Center – Central Pacific
- Japan Meteorological Agency – NW Pacific
- India Meteorological Department – Bay of Bengal and the Arabian Sea
- Météo-France – La Reunion – South Indian Ocean from 30°E to 90°E
- Fiji Meteorological Service – South Pacific west of 160°E, north of 25° S
- Tropical cyclone warning centres
- Indonesian Meteorological Department – South Indian Ocean from 90°E to 125°E, north of 10°S
- Papua New Guinea's National Weather Service – South Pacific Ocean from 125°E to 160°E, north of 10°S
- Australian Bureau of Meteorology . – South Indian Ocean & South Pacific Ocean from 90°E to 160°E, south of 10°S
- Meteorological Service of New Zealand Limited – South Pacific west of 160°E, south of 25°S
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