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 Northeastern Pacific Ocean, then it will 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 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 in France uses a scale that covers the whole of the South West Indian Ocean. Both the Australian Bureau of Meteorology and the Regional Specialised Meteorological Centre 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 the same 10 m (33 ft) above the surface.[1][2] 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
| Category | m/s | knots | mph | km/h |
|---|---|---|---|---|
| 5 | ≥ 70 | ≥ 137 | ≥ 157 | ≥ 252 |
| 4 | 58–70 | 113–136 | 130–156 | 209–251 |
| 3 | 50–58 | 96–112 | 111–129 | 178–208 |
| 2 | 43–49 | 83–95 | 96–110 | 154–177 |
| 1 | 33–42 | 64–82 | 74–95 | 119–153 |
| TS | 18–32 | 34–63 | 39–73 | 63–118 |
| TD | ≤ 17 | ≤ 33 | ≤ 38 | ≤ 62 |
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 antimeridian.[3] In these oceanic basins, tropical cyclones with maximum sustained winds below 34 kn (65 km/h, 39 mph) are labelled as tropical depressions by either the National Hurricane Center (if it is in the North Atlantic or Northeast Pacific Basin) or the Central Pacific Hurricane Center (if located in the North Central Pacific Ocean). Should a tropical depression should reach 35 kn (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 kn (119 km/h, 74 mph) then the tropical storm will be designated as a hurricane.[4]
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 (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 windspeeds of at least 96 kn (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.[5] If a hurricane's maximum sustained winds reach 114 kn, (210 km/h, 131 mph), it will be catalogued as a Category 4 hurricane. Storms with winds that surpass 136 kn (250 km/h, 156 mph) are of Category 5 intensity.[5]
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.[5]
Historically, the term great hurricane was used to describe storms that possessed winds of at least 110 kn (200 km/h, 125 mph), large radii (over 100 mi / 160 km) and that caused large amounts of destruction. This term fell into disuse after the introduction of the Saffir-Simpson scale in the early 1970s.[6]
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). The NHC, CPHC and JTWC, define sustained winds based on 1-minute average speed, also measured 10 m (33 ft) above the surface.[1][2]
West Pacific
| Category | Sustained winds |
|---|---|
| Violent typhoon | ≥105 knots ≥194 km/h |
| Very strong typhoon | 85–104 knots 157–193 km/h |
| Typhoon | 64–84 knots 118–156 km/h |
| Severe tropical storm | 48–63 knots 89–117 km/h |
| Tropical storm | 34–47 knots 62–88 km/h |
| Tropical depression | ≤33 knots ≤61 km/h |
Any tropical cyclone that forms to the west of 180° and east of 100°E in the Northern Hemisphere is monitored by the Regional Specialized Meteorological Center in Tokyo, Japan.[7] 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.[7]
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).[7] 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.[7] 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.[7] Once the system's maximum sustained winds reach windspeeds of 65 knots (70 mph 120 km/h), the JMA will designate the tropical cyclone as a typhoon—the highest category on its scale.[7]
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.[8] 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.[9]
North Indian Ocean
| Category | Sustained winds (3-min average) |
|---|---|
| Super Cyclonic Storm | ≥120 kt ≥221 km/h |
| Extremely Severe Cyclonic Storm |
90–119 kt 166–220 km/h |
| Very Severe Cyclonic Storm |
64–89 kt 118–165 km/h |
| Severe Cyclonic Storm |
48–63 kt 89–117 km/h |
| Cyclonic Storm | 34–47 kt 63–88 km/h |
| Deep Depression | 28–33 kt 51–62 km/h |
| Depression | 17–27 kt 31–50 km/h |
Any tropical cyclone that forms to the west of longitude 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.[10] RSMC New Delhi uses six different categories to measure the wind speed of a tropical cyclone based on the maximum sustained winds over a 3-minute averaging period.[10]
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 kn (51 km/h, 31 mph).[11] A depression is classified as a deep depression when it has maximum sustained winds between 27 kn (51 km/h, 31 mph) and 33 kn (61 km/h, 38 mph).[11] Should a deep depression intensify further, it will be classified as a cyclonic storm if its sustained winds reach 34 kn (62 km/h, 39 mph).[11] When a tropical system is classified as a cyclonic storm, it is assigned a name by the IMD.[10]
In cases where cyclonic storms possess windspeeds greater than 48 kn, (88 km/h, 55 mph), they are classified as severe cyclonic storms.[10] A severe cyclonic storm is labelled as a very severe cyclonic storm when it reaches windspeeds greater than 64 kn, (118 km/h, 74 mph).[10] 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 kn, (222 km/h, 138 mph).[11]
Southwest Indian Ocean
| Category | Sustained winds |
|---|---|
| Very Intense Tropical Cyclone |
>115 kt >212 km/h |
| Intense Tropical Cyclone |
90–115 kt 166–212 km/h |
| Tropical Cyclone | 64–89 kt 118–165 km/h |
| Severe Tropical Storm |
48–63 kt 89–117 km/h |
| Moderate Tropical Storm |
34–47 kt 63–88 km/h |
| Tropical Depression |
28–33 kt 51–62 km/h |
| Tropical Disturbance |
<28 kt <50 km/h |
Any Tropical Cyclone that forms to the west of 90°E in the Southern Hemisphere, is monitored by Météo-France who run the Regional Specialized Meteorological Center in La Reunion.[12] 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.[12]
A Tropical Disturbance is the lowest category on the Southwest Indian Ocean Tropical Cyclone scale, and has wind speeds 28 knots (32 mph, 50 km/h).[12] 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 (40 mph, 65 km/h.) then it will be classified as a Moderate Tropical Storm and assigned a name by either the Sub Regional Center in Mauritius or Madagascar.[13]
Should the named storm intensify further and reach winds speeds of 48 knots (55 mph, 89 km/h), then it will be classified as a Severe Tropical Storm.[13] A Severe Tropical Storm is designated as a Tropical Cyclone when it reaches wind speeds of 64 knots (74 mph, 118 km/h).[12] Should a Tropical Cyclone intensify further and reach wind speeds of 90 knots (103 mph, 166 km/h), it will be classified as an Intense Tropical Cyclone.[12] A Very Intense Tropical Cyclone is the highest category on the Southwest Indian Ocean Tropical Cyclone scale, and has winds of over 115 knots (132 mph 212 km/h).[13]
Australia
| Category | Sustained winds |
Gusts |
|---|---|---|
| Five | >107 kn (198 km/h; 123 mph) | >151 kn (280 km/h; 174 mph) |
| Four | 86–107 kn (159–198 km/h; 99–123 mph) | 122–151 kn (226–280 km/h; 140–174 mph) |
| Three | 64–85 kn (119–157 km/h; 74–98 mph) | 90–121 kn (167–224 km/h; 104–139 mph) |
| Two | 48–63 kn (89–117 km/h; 55–72 mph) | 68–89 kn (126–165 km/h; 78–102 mph) |
| One | 34–47 kn (63–87 km/h; 39–54 mph) | 49–67 kn (91–124 km/h; 56–77 mph) |
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.[14] Both warning centres use the Australian Tropical Cyclone Intensity Scale which measures tropical cyclones using a six category system.[14] 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 than the Saffir-Simpson Hurricane Scale which uses 1-Minute Maximum Sustained winds.[1]
When a Tropical Cyclone that has wind speeds below 35 knots (40 mph, 65 km/h) forms east of 160°E it is labelled as either a Tropical Disturbance or a Tropical Depression by RSMC Nadi.[14] If it forms to the west of 160°E it is labelled as a Tropical Low by the Australian Bureau of Meteorology.[14] 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.[15]
If a tropical depression should reach 35 knots (40 mph, 65 km/h), it will be named by the TCWC or RSMC and be classified as a Tropical Cyclone.[16] Should the cyclone intensify further reaching maximum sustained winds of 65 knots (75 mph 145 km/h) then the cyclone will be designated as a Category Three Severe Tropical Cyclone.[16] A Severe Tropical Cyclone will be classified as a Category Five Severe Tropical Cyclone should the cyclones maximum sustained wind speeds be greater than 110 Knots (130 mph, 200 km/h) and the gusts be above 150 knots (175 mph, 280 km/h).[16]
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 Centers worldwide:
| Tropical Cyclone Classifications (all winds are 10-minute averages) | ||||||||
|---|---|---|---|---|---|---|---|---|
| Beaufort scale[17] | 10-minute sustained winds (knots) | N Indian Ocean IMD |
SW Indian Ocean MF |
Australia BOM |
SW Pacific FMS |
NW Pacific JMA |
NW Pacific JTWC |
NE Pacific & N Atlantic NHC & CPHC |
| 0–6 | <28 | Depression | Tropical Disturbance | Tropical Low | Tropical Depression | Tropical Depression | Tropical Depression | Tropical Depression |
| 7 | 28-29 | Deep Depression | Tropical Depression | |||||
| 30-33 | ||||||||
| 8–9 | 34–47 | Cyclonic Storm | Moderate Tropical Storm | Tropical Cyclone (1) | Tropical Cyclone (1) | Tropical Storm | Tropical Storm | Tropical Storm |
| 10 | 48–55 | Severe Cyclonic Storm | Severe Tropical Storm | Tropical Cyclone (2) | Tropical Cyclone (2) | Severe Tropical Storm | ||
| 11 | 56–63 | |||||||
| 12 | 64–72 | Very Severe Cyclonic Storm | Tropical Cyclone | Severe Tropical Cyclone (3) | Severe Tropical Cyclone (3) | Typhoon | Typhoon | Hurricane (1) |
| 13 | 73–85 | Hurricane (2) | ||||||
| 14 | 86–89 | Severe Tropical Cyclone (4) | Severe Tropical Cyclone (4) | Major Hurricane (3) | ||||
| 15 | 90–99 | Intense Tropical Cyclone | ||||||
| 16 | 100–106 | Major Hurricane (4) | ||||||
| 17 | 107-114 | Severe Tropical Cyclone (5) | Severe Tropical Cyclone (5) | |||||
| 115–119 | Very Intense Tropical Cyclone | Super Typhoon | ||||||
| >120 | Super Cyclonic Storm | Major Hurricane (5) | ||||||
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 organisations, 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.[18]
Accumulated Cyclone Energy (ACE), is used by the National Oceanic and Atmospheric Administration and other agencies, to express the activity of individual tropical cyclones which are above tropical storm strength and entire tropical cyclone seasons.[19] 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.[19] 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.[19] As well as being squared ACE can also be cubed, and this version is known as the Power Dissipation Index (PDI).[20]
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.[21] 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.[21] Points are awarded on an exponential scale, with the majority of points reserved for hurricane force and greater wind fields.[21]
See also
References
- ^ a b c Tropical Cyclone Weather Services Program (June 1, 2006). "Tropical cyclone definitions" (PDF). National Weather Service. Retrieved 2006-11-30. Cite error: The named reference "NWSM Defs" was defined multiple times with different content (see the help page).
- ^ a b Federal Emergency Management Agency (2004). "Hurricane Glossary of Terms". Archived from the original on 2005-12-14. Retrieved 2006-03-24. Accessed through the Wayback Machine.
- ^ Atlantic Oceanographic and Meteorological Laboratory, Hurricane Research Division (2007-07-12). "Frequently Asked Questions: What regions around the globe have tropical cyclones and who is responsible for forecasting there?". NOAA. Retrieved 2008-12-24.
- ^ National Hurricane Center (2005). "Glossary of NHC/TPC Terms". National Oceanic and Atmospheric Administration. Retrieved 2008-12-24.
- ^ a b c National Hurricane Center (2006-06-02). "Saffir-Simpson Hurricane Scale Information". National Oceanic and Atmospheric Administration. Retrieved 2007-02-25.
- ^ Doehring, Fred (1994). "Florida Hurricanes and Tropical Storms: 1871–1993: An Historical Survey" (PDF). Florida Institute of Technology. pp. 53–54. Retrieved 2008-12-26.
{{cite web}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ a b c d e f Typhoon Committee (2008). "Typhoon Committee Operational Manual" (PDF). World Meteorological Organization. Retrieved 2008-12-23.
- ^ Joint Typhoon Warning Center (2008-03-31). "What are the description labels used with tropical cyclones by JTWC?". Joint Typhoon Warning Center Frequently Asked Questions (FAQ). United States Navy. Retrieved 2008-12-22.
- ^ Joint Typhoon Warning Center (2008-03-31). "How are JTWC forecasts different than forecasts issued by tropical cyclone warning centers (TCWCs) of other countries?". Joint Typhoon Warning Center Frequently Asked Questions (FAQ). United States Navy. Retrieved 2008-12-26.
- ^ a b c d e "Tropical Cyclone Operational plan for the Bay of Bengal and the Arabian Sea" (PDF). World Meteorological Organization. 2008. Retrieved 2008-12-23.
- ^ a b c d "IMD FAQ:How are low pressure system classified in India? What are the differences between low, depression and cyclone?". India Meteorological Department. 2008-06-24. Retrieved 2008-12-23.
- ^ a b c d e "Tropical Cyclone Operational Plan for the South West Indian Ocean 2006" (PDF). World Meteorological Organization. 2006. Retrieved 2008-07-03.
- ^ a b c "Tableau de définition des cyclones". Météo-France. 2008. Retrieved 2009-01-14.Template:Fr icon
- ^ a b c d "Tropical Cyclone Operational plan for the South Pacific & Southeast Indian Ocean" (PDF). World Meteorological Organization. 2008. Retrieved 2008-07-03.
- ^ "Extreme Weather Warning 20-04-08 00z". Indonesian Meteorological and Geophysical Agency. 2008. Retrieved 2008-04-20.
- ^ a b c "Frequently Asked Question 3 - How is a severe tropical cyclone different from a non-severe cyclone?". Australian Bureau of Meteorology. 2009. Retrieved 2009-01-14.
- ^ Walter J. Saucier (1955). Principles of Meteorological Analysis. Retrieved on 2009-01-09.
- ^ "Integrated Kinetic Energy". Atlantic Oceanographic and Meteorological Laboratory. 2008. Retrieved 2009-01-18.
- ^ a b c Tropical Cyclone Weather Services Program (06-01-2009). "Background Information: The North Atlantic Hurricane Season" (PDF). National Oceanic and Atmospheric Administration. Retrieved 2008-01-16.
{{cite web}}: Check date values in:|date=(help) - ^ Kerry Emanuel (2005). Increasing destructiveness of tropical cyclones over the past 30 years. Retrieved on 2009-01-17.
- ^ a b c "Background Information: The North Atlantic Hurricane Season". American Meteorological Society. 2008-12-19. Retrieved 2009-01-16.
{{cite web}}: CS1 maint: date and year (link)