Tropical cyclone scales
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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.
Within all basins tropical cyclones are named when the sustained winds hit 35 kn (40 mph; 65 km/h)
Atlantic and East Pacific
|Five||≥70 m/s, ≥137 knots
≥157 mph, ≥252 km/h
|Four||58–70 m/s, 113–136 knots
130–156 mph, 209–251 km/h
|Three||50–58 m/s, 96–112 knots
111–129 mph, 178–208 km/h
|Two||43–49 m/s, 83–95 knots
96–110 mph, 154–177 km/h
|One||33–42 m/s, 64–82 knots
74–95 mph, 119–153 km/h
|18–32 m/s, 34–63 knots
39–73 mph, 63–118 km/h
|≤17 m/s, ≤33 knots
≤38 mph, ≤62 km/h
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. The Saffir-Simpson Scale is based on 1-minute maximum sustained wind speeds. In these oceanic basins, tropical cyclones with maximum sustained winds below 34 kn (39 mph; 63 km/h) 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 reach 35 kn (40 mph; 65 km/h), 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 kn (74 mph; 119 km/h) 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 kn (96 mph; 154 km/h) 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 kn (110 mph; 178 km/h) 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 kn (131 mph; 211 km/h), it will be ranked as a Category 4 hurricane. Storms with winds that surpass 136 kn (157 mph; 252 km/h) 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.
|Severe Tropical Storm||48–63 knots
|Tropical Storm||34–47 knots
|Tropical Depression||≤33 knots
Any tropical cyclone that develops within the Northern Hemisphere between 180° and 100°E is monitored by the Japan Meteorological Agency's Regional Specialized Meteorological Center in Tokyo, Japan, on behalf of the WMO/ESCAP's Typhoon Committee. Other warning centres such as the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), the China Meteorological Agency (CMA) and the Hong Kong Observatory (HKO) also monitor tropical cyclones developing within the basin.
- Official classifications
All classifications from RSMC Tokyo are based on 10-minute maximum sustained wind speed. The lowest category is a tropical depression having wind speed under 33 kn (38 mph; 61 km/h), while a tropical storm has wind speed between 34 kn (39 mph; 63 km/h) and 47 kn (54 mph; 87 km/h). A severe tropical storm has wind speed between 48 kn (55 mph; 89 km/h) and 63 kn (72 mph; 117 km/h), while the highest category is a typhoon having wind speed above 64 kn (74 mph; 119 km/h).
For own purposes, RSMC Tokyo unofficially divides the typhoon category into three categories, with a strong typhoon having wind speed between 64 kn (74 mph; 119 km/h) and 84 kn (97 mph; 156 km/h). A very strong typhoon has wind speed between 85 kn (98 mph; 157 km/h) and 104 kn (120 mph; 193 km/h), while a violent typhoon has wind speed over 105 kn (121 mph; 194 km/h).
- Other classifications
Some of the members of the typhoon committee use scales varying from the Typhoon Committees one. Both the CMA and the HKO use the Typhoon Committees scale but divide their classifications of typhoons into severe and super when wind speed reaches 81 kn (93 mph; 150 km/h) or 100 kn (120 mph; 190 km/h)., while the CMA also adopts a lower limit of Beaufort Force 6 for a tropical depression.
Both the Joint Typhoon Warning Center (JTWC) and the PAGASA merge the severe tropical storm into the tropical storm category, yet the JTWC has the super typhoon category when wind speed reaches 130 kn (150 mph; 240 km/h). In addition, the Central Weather Bureau (CWB) has its own scale in Chinese but uses the Typhoon Committee scale in English.
Although most agencies use 10-minute maximum sustained wind speed, the JTWC uses 1-minute maximum sustained wind speed and the CMA uses 2-minute maximum sustained wind speed. Also during late-2014, the PAGASA has said that they will use the super typhoon category officially on 2015 with wind speeds more than 120 kn (140 mph; 220 km/h).
North Indian Ocean
|Super Cyclonic Storm||>120 kt
|Cyclonic Storm||34–47 kt
|Deep Depression||28–33 kt
Any tropical cyclone that develops within the Northern Hemisphere between 100°E and 45°E is monitored by the India Meteorological Department's Regional Specialized Meteorological Center in New Delhi, India. Other warning centres such as RSMC La Reunion, the Bangladesh Meteorological Department and the United States Joint Typhoon Warning Center, also monitor tropical cyclones developing within the basin. RSMC New Delhi uses a 3-minute averaging period to determine the sustained windspeeds of a tropical cyclone.
- Official classifications
The lowest category used by RSMC New Delhi, is a depression which have 3-minute sustained windspeeds of under 27 kn (31 mph; 50 km/h), while a deep depression has windspeeds of between 27 kn (31 mph; 50 km/h) and 33 kn (38 mph; 61 km/h). Should a deep depression further intensify, it will be classified as a Cyclonic Storm and assigned a name should it develop sustained windspeeds of between 34 kn (39 mph; 63 km/h) and 48 kn (55 mph; 89 km/h).
In cases where cyclonic storms possess wind speeds greater than 48 kt, (88 km/h, 55 mph), they are classified as Severe Cyclonic Storm. 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). A Super Cyclonic Storm is the highest category used to designate tropical cyclones and have 3-minute sustained windspeeds of above 120 kn (140 mph; 220 km/h)
Prior to 1988, cyclones were classified into 4 categories, which were depression, deep depression, cyclonic storms(34-47 knots) and severe cyclonic storms(48 knots or more). However in 1988 the IMD started to rate cyclones with wind speeds of more than 64 kt, (118 km/h, 74 mph) as very 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-Western Indian Ocean
|Tropical Cyclone||64–89 kt
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 (50 km/h, 32 mph). 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 based on 10-minute maximum 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).
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 RSMC Honolulu, as well as the Joint Typhoon Warning Center, define sustained winds based on 1-minute average speed (also measured 10 m (33 ft) above the surface).
Studies have shown that the two definitions are correlated, with the cyclone's maximum one-minute wind speed conventionally about 14% higher than its best ten-minute one. (To convert from a one-minute wind speed to a ten-minute wind speed, the one-minute speed is multiplied by 0.88. In the other direction, the ten-minute wind speed is multiplied by 1.14 to produce the one-minute wind speed.) This relationship is approximate, as the conversion factor varies with different land or sea surfaces and atmospheric stability.
Comparisons across basins
The terminology for tropical cyclones differs from one region to another. Below is a summary of the classifications used by the official warning centres worldwide.
|Tropical cyclone classifications |
|1-minute sustained winds||10-minute sustained winds||NE Pacific &
|N Indian Ocean
|SW Indian Ocean
|Australia & S Pacific
|0–7||<32 knots (37 mph; 59 km/h)||<28 knots (32 mph; 52 km/h)||Tropical Depression||Tropical Depression||Tropical Depression||Depression||Zone of Disturbed Weather||Tropical Disturbance
|7||33 knots (38 mph; 61 km/h)||28–29 knots (32–33 mph; 52–54 km/h)||Deep Depression||Tropical Disturbance|
|8||34–37 knots (39–43 mph; 63–69 km/h)||30–33 knots (35–38 mph; 56–61 km/h)||Tropical Storm||Tropical Storm||Tropical Depression|
|9-10||38–54 knots (44–62 mph; 70–100 km/h)||34–47 knots (39–54 mph; 63–87 km/h)||Tropical Storm||Cyclonic Storm||Moderate Tropical Storm||Category 1
|11||55–63 knots (63–72 mph; 102–117 km/h)||48–55 knots (55–63 mph; 89–102 km/h)||Severe Tropical Storm||Severe Cyclonic Storm||Severe Tropical Storm||Category 2
|12+||64–71 knots (74–82 mph; 119–131 km/h)||56–63 knots (64–72 mph; 104–117 km/h)||Category 1 hurricane||Typhoon|
|72–82 knots (83–94 mph; 133–152 km/h)||64–72 knots (74–83 mph; 119–133 km/h)||Typhoon||Very Severe
|Tropical Cyclone||Category 3 severe
|83–95 knots (96–109 mph; 154–176 km/h)||73–83 knots (84–96 mph; 135–154 km/h)||Category 2 hurricane|
|96–97 knots (110–112 mph; 178–180 km/h)||84–85 knots (97–98 mph; 156–157 km/h)||Category 3 hurricane|
|98–112 knots (113–129 mph; 181–207 km/h)||86–98 knots (99–113 mph; 159–181 km/h)||Intense Tropical Cyclone||Category 4 severe
|113–122 knots (130–140 mph; 209–226 km/h)||99–107 knots (114–123 mph; 183–198 km/h)||Category 4 hurricane|
|123–129 knots (142–148 mph; 228–239 km/h)||108–113 knots (124–130 mph; 200–209 km/h)||Category 5 severe
|130–136 knots (150–157 mph; 241–252 km/h)||114–119 knots (131–137 mph; 211–220 km/h)||Super Typhoon||Super Cyclonic Storm||Very Intense Tropical Cyclone|
|>137 knots (158 mph; 254 km/h)||>120 knots (140 mph; 220 km/h)||Category 5 hurricane|
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- 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
- Australian Bureau of Meteorology (TCWC's Perth, Darwin & Brisbane). – 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