Tropical cyclone intensity scales

s 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 and used by most weather agencies is that of a 10-minute average at a height of 10 m 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 New Delhi applies a 3-minute averaging period, and the Australian scale is based on both 3-second wind gusts and maximum sustained winds averaged over a 10-minute interval. These differences make direct comparisons between basins difficult.
Within all basins tropical cyclones are named when the sustained winds reach at least.

Background

Tropical cyclones are defined as being warm cored, non-frontal synoptic cyclones, that develop over tropical or subtropical waters, with organized atmospheric convection and have a definite cyclonic surface wind circulation. They are classified by the wind speeds located around the circulation centre and are ranked, by the World Meteorological 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. The region also defines a subtropical cyclone as a non-frontal low pressure disturbance, that has the characteristics of both tropical and extratropical cyclones. 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.
Also, it will be assigned a tropical cyclone number comprising an officially spelled-out number followed by a hyphen and a suffix letter ;
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 with a two-digit PTC number 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.
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", 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 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. 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. The lowest classification used by the Typhoon Committee is a tropical depression, which has 10-minute sustained winds of less than. Should the tropical depression intensify further it is named and classified as a tropical storm, which has winds speeds between. Should the system continue to intensify further then it will be classified as a severe tropical storm, which has winds speeds between. The highest classification on the Typhoon Committee's scale is a typhoon, which has winds speeds greater than.
The China Meteorological Administration, the Hong Kong Observatory, Macao Meteorological and Geophysical Bureau, 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 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.
In addition to the national meteorological services of each nation, the United States' Joint Typhoon Warning Center monitors the basin, and issues warnings on significant tropical cyclones for the United States Government, assigning them two-digit TC numbers. 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 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 to its TC number ; however, in cases when the JTWC upgrades a depression to tropical storm without the JMA following suit, the spelled-out number is parenthesized and appended to the TC number as placeholder name, as in TS 16W , 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.