Tropical cyclone intensity scales

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 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. ==Atlantic, Eastern and Central Pacific==

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. 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. 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) Should the hurricane intensify further then it will be rated as a Category 2 hurricane, if it has winds of between . --> 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 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. 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==

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). 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. 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. 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 classifications are Tropical Depression, Tropical Storm, Typhoon, and Super Typhoon. 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 ==North Indian Ocean==

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). 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. 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. 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==

Any tropical cyclone that develops within the Southern Hemisphere between Africa and 90°E is monitored by Météo-France's La Réunion tropical cyclone centre (MFR, RSMC La Réunion). 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 . 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 . 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==

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. 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. A Category 1 tropical cyclone is estimated to have 10-minute sustained wind speeds of , while a Category 2 tropical cyclone is estimated to have 10-minute sustained wind speeds of . When a system becomes a Category 3 tropical cyclone it is reclassified as a Severe tropical cyclone and has wind speeds of . A Category 4 severe tropical cyclone has winds of , while the maximum rating is a Category 5 severe tropical cyclone, which has winds of at least . 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==

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. 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. 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. 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. HSI was developed by ImpactWeather (now StormGeo) meteorologists as a proprietary method of hurricane severity classification. HSI misclassifies hurricanes in some cases because it does not account for rainfall. HSI is based on a lookup table rather than an equation. It does not take into account the translational speed of the storm. ==See also==