Tropical cyclone scales

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. 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. These make direct comparisons between basins difficult.

Atlantic and East Pacific
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. 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 should 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.

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. 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. 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.

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.

Western Pacific
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. 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.

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 35 knots, (40 mph, 65 km/h). A tropical depression is upgraded to a tropical storm should its sustained wind speeds exceed 35 knots, (40 mph, 65 km/h). Tropical storms also receive official names from RSMC Tokyo. Should the storm intensify further and reach sustained wind speeds of 50 knot (60 mph, 95 km/h) then it will be classified as a severe tropical storm. Once the system's maximum sustained winds reach wind speeds of 65 knots (70 mph 120 km/h), the JMA will designate the tropical cyclone as a typhoon—the highest category on its scale. From 2009 the Hong Kong observatory started to further divide typhoon into two further classifications severe typhoon and super typhoon. A severe typhoon has winds of at least 80 knot (95 mph, 150 km/h) whilst a super typhoon has winds of at least 100 knot (115 mph, 185 km/h).

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. 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.

North Indian Ocean
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. 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.

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). 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). 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). When a tropical system is classified as a cyclonic storm, it is assigned a name by the IMD.

In cases where cyclonic storms possess wind speeds greater than 48 kt, (88 km/h, 55 mph), they are classified as severe cyclonic storms. 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). 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).

Prior to 1988, cyclones were classified into 4 categories, which were depression, deep depression, cyclonic storms and severe cyclonic storms. 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. 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).

South-West Indian Ocean
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. 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.

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). 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.

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

Australia and Fiji
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. Both warning centres use the Australian tropical cyclone intensity scale, which measures tropical cyclones using a six category system. 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.

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. If it forms to the west of 160°E it is labelled as a tropical low by the Australian Bureau of Meteorology. 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.

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. 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. 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).

Comparisons across basins
The terminology for tropical cyclones differs from one region to another. Below is a summary of the classifications used by Regional Specialized Meteorological Centres worldwide:

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.

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. 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. As well as being squared ACE can also be cubed, and this version is known as the Power Dissipation Index (PDI).

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.

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 Honolulu, as well as the Joint Typhoon Warning Center, define sustained winds based on 1-minute average speed, and are also measured 10 m (33 ft) above the surface. To convert a one-minute wind speed from a tropical cyclone to a ten minute wind speed, the 1 minute speed is multiplied by 0.88.