Tropical cyclone scales
Encyclopedia
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.
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 wind
s 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 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.
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 wind
s 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 33 knots (61 km/h, 38 mph). A tropical depression is upgraded to a tropical storm should its sustained wind speeds exceed 34 knots (62 km/h, 39 mph). Tropical storms also receive official names from RSMC Tokyo. Should the storm intensify further and reach sustained wind speeds of 48 knot (89 km/h, 55 mph) then it will be classified as a severe tropical storm.
Once the system's maximum sustained winds reach wind speeds of 64 knots (118 km/h, 74 mph), the JMA will designate the tropical cyclone as a typhoon—the highest category on its scale. The JMA also unofficially divides typhoons into three further classifications—strong (強い, 64–84 knots), very strong (非常に強い, 85–104 knots), and intensive (猛烈な, ≥105 knots).
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.
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).
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 wind
s 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).
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).
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).
s worldwide:
s 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.
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.
Tropical Cyclone Warning Centres
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
are officially ranked on one of several tropical cyclone scales according to their maximum sustained winds and in what oceanic basin
Tropical cyclone basins
Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the north Atlantic Ocean, the eastern and western parts of the northern Pacific Ocean, the southwestern Pacific, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean. The...
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
Accumulated cyclone energy
Accumulated cyclone energy is a measure used by the National Oceanic and Atmospheric Administration to express the activity of individual tropical cyclones and entire tropical cyclone seasons, particularly the North Atlantic hurricane season. It uses an approximation of the energy used by a...
, the Power Dissipation Index, the Integrated Kinetic Energy Index, and Hurricane Severity Index
Hurricane Severity Index
The Hurricane Severity Index is a hurricane rating system which defines the strength and destructive capability of a storm. The HSI uses equations which incorporate the intensity of the winds and the size of the area covered by the winds...
.
Should a tropical cyclone form in the North Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
or the North-eastern Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, it will be classified using one of the categories in the Saffir-Simpson Hurricane Scale
Saffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
. In the Western Pacific, tropical cyclones will be ranked using the Japan Meteorological Agency
Japan Meteorological Agency
The or JMA, is the Japanese government's weather service. Charged with gathering and reporting weather data and forecasts in Japan, it is a semi-autonomous part of the Ministry of Land, Infrastructure and Transport...
's scale. The Regional Specialized Meteorological Centre
Regional Specialized Meteorological Centre
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
(RSMC) in New Delhi
New Delhi
New Delhi is the capital city of India. It serves as the centre of the Government of India and the Government of the National Capital Territory of Delhi. New Delhi is situated within the metropolis of Delhi. It is one of the nine districts of Delhi Union Territory. The total area of the city is...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
also uses a different scale to assess the maximum sustained winds of a tropical cyclone. In the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
, the Météo-France
Météo-France
Météo-France is the French national meteorological service.The organisation was established by decree in June 1993 and is a department of the Ministry of Transportation. It is headquartered in Paris but many domestic operations have been decentralised to Toulouse...
forecast center on La Reunion
Réunion
Réunion is a French island with a population of about 800,000 located in the Indian Ocean, east of Madagascar, about south west of Mauritius, the nearest island.Administratively, Réunion is one of the overseas departments of France...
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
Maximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...
recommended by the World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
(WMO) and used by most weather agencies is that of a 10-minute average at a height of 10 m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
(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 ScaleSaffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
is the classification system used for tropical cyclones in the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
and in the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
east of the anti-meridian
180th meridian
The 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian passing through the Royal Observatory, Greenwich. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes...
. In these oceanic basins, tropical cyclones with maximum sustained wind
Maximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...
s below 34 kt (65 km/h, 39 mph) are labelled as tropical depressions by either the National Hurricane Center
National Hurricane Center
The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of the National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th...
(if it is in the North Atlantic or North-east Pacific Basin) or the Central Pacific Hurricane Center
Central Pacific Hurricane Center
The Central Pacific Hurricane Center of the United States National Weather Service is the official body responsible for tracking and issuing tropical cyclone warnings, watches, advisories, discussions, and statements for the Central North Pacific Basin...
(if located in the North Central Pacific Ocean). Should a tropical depression reach 35 kt (65 km/h, 40 mph), it will receive a name
Tropical cyclone naming
Tropical cyclones have officially been named since 1945 and are named for a variety of reasons, which include to facilitate communications between forecasters and the public when forecasts, watches, and warnings are issued. Names also reduce confusion about what storm is being described, as more...
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
Kilometre
The kilometre is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second...
/ 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°180th meridian
The 180th meridian or antimeridian is the meridian which is 180° east or west of the Prime Meridian passing through the Royal Observatory, Greenwich. It is common to both east longitude and west longitude. It is used as the basis for the International Date Line because it for the most part passes...
and east of 100°E
100th meridian east
The meridian 100° east of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, Asia, the Indian Ocean, the Southern Ocean, and Antarctica to the South Pole....
in the Northern Hemisphere is officially monitored by the Regional Specialized Meteorological Center
Regional Specialized Meteorological Center
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
in Tokyo
Tokyo
, ; officially , is one of the 47 prefectures of Japan. Tokyo is the capital of Japan, the center of the Greater Tokyo Area, and the largest metropolitan area of Japan. It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family...
, Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. The Japan Meteorological Agency
Japan Meteorological Agency
The or JMA, is the Japanese government's weather service. Charged with gathering and reporting weather data and forecasts in Japan, it is a semi-autonomous part of the Ministry of Land, Infrastructure and Transport...
, 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 wind
Maximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...
s produced by the storm averaged over a 10-minute interval.
A tropical depression is the lowest category that the Japan Meteorological Agency
Japan Meteorological Agency
The or JMA, is the Japanese government's weather service. Charged with gathering and reporting weather data and forecasts in Japan, it is a semi-autonomous part of the Ministry of Land, Infrastructure and Transport...
uses and is the term used for a tropical system that has wind speeds not exceeding 33 knots (61 km/h, 38 mph). A tropical depression is upgraded to a tropical storm should its sustained wind speeds exceed 34 knots (62 km/h, 39 mph). Tropical storms also receive official names from RSMC Tokyo. Should the storm intensify further and reach sustained wind speeds of 48 knot (89 km/h, 55 mph) then it will be classified as a severe tropical storm.
Once the system's maximum sustained winds reach wind speeds of 64 knots (118 km/h, 74 mph), the JMA will designate the tropical cyclone as a typhoon—the highest category on its scale. The JMA also unofficially divides typhoons into three further classifications—strong (強い, 64–84 knots), very strong (非常に強い, 85–104 knots), and intensive (猛烈な, ≥105 knots).
The United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
' Joint Typhoon Warning Center
Joint Typhoon Warning Center
The Joint Typhoon Warning Center is a joint United States Navy – United States Air Force task force located at the Naval Maritime Forecast Center in Pearl Harbor, Hawaii...
(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
Saffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
—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
National Hurricane Center
The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of the National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th...
and Central Pacific Hurricane Center
Central Pacific Hurricane Center
The Central Pacific Hurricane Center of the United States National Weather Service is the official body responsible for tracking and issuing tropical cyclone warnings, watches, advisories, discussions, and statements for the Central North Pacific Basin...
. 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.
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).
North Indian Ocean
Any tropical cyclone that forms between longitudeLongitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
45°E and 100°E in the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...
is monitored by the India Meteorological Department
India Meteorological Department
The India Meteorological Department , also referred to as the Met Office, is an agency of the Ministry of Earth Sciences of the Government of India. It is the principal agency responsible for meteorological observations, weather forecasting and seismology...
(IMD), who run the Regional Specialized Meteorological Center
Regional Specialized Meteorological Center
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
in New Delhi
New Delhi
New Delhi is the capital city of India. It serves as the centre of the Government of India and the Government of the National Capital Territory of Delhi. New Delhi is situated within the metropolis of Delhi. It is one of the nine districts of Delhi Union Territory. The total area of the city is...
, India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
. Since 1998, RSMC New Delhi has used six different categories to measure the wind speed of a tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
based on the maximum sustained wind
Maximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...
s 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
Tropical cyclone naming
Tropical cyclones have officially been named since 1945 and are named for a variety of reasons, which include to facilitate communications between forecasters and the public when forecasts, watches, and warnings are issued. Names also reduce confusion about what storm is being described, as more...
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).
France and the 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 CenterRegional Specialized Meteorological Center
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
in La Reunion
Réunion
Réunion is a French island with a population of about 800,000 located in the Indian Ocean, east of Madagascar, about south west of Mauritius, the nearest island.Administratively, Réunion is one of the overseas departments of France...
. 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
Saffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
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
Mauritius
Mauritius , officially the Republic of Mauritius is an island nation off the southeast coast of the African continent in the southwest Indian Ocean, about east of Madagascar...
or Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
.
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
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
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 cycloneTropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
that forms to the east of 90°E in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
is monitored by either the Australian Bureau of Meteorology and/or the Regional Specialized Meteorological Center
Regional Specialized Meteorological Center
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
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
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
, which are a further 30-40% stronger than the 10-minute average sustained winds. This is different from the Saffir-Simpson Hurricane Scale
Saffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
, 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
Jakarta
Jakarta is the capital and largest city of Indonesia. Officially known as the Special Capital Territory of Jakarta, it is located on the northwest coast of Java, has an area of , and a population of 9,580,000. Jakarta is the country's economic, cultural and political centre...
, Indonesia
Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
.
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 CentreRegional Specialized Meteorological Centre
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
s worldwide:
| Tropical Cyclone Classifications (all winds are 10-minute averages) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Beaufort scale Beaufort scale The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:... |
10-minute sustained winds | N Indian Ocean(IMD) | SW Indian Ocean(MFR Météo-France Météo-France is the French national meteorological service.The organisation was established by decree in June 1993 and is a department of the Ministry of Transportation. It is headquartered in Paris but many domestic operations have been decentralised to Toulouse... ) |
Australia(BoM) | SW Pacific(FMS Fiji Meteorological Service The Fiji Meteorological Service is a Department of the government of Fiji responsible for providing weather forecasts and is based in Nadi. Since 1995, FMS has been responsible for naming and tracking tropical cyclones in the Southwest Pacific region... ) |
NW Pacific(JMA Japan Meteorological Agency The or JMA, is the Japanese government's weather service. Charged with gathering and reporting weather data and forecasts in Japan, it is a semi-autonomous part of the Ministry of Land, Infrastructure and Transport... ) |
NW Pacific(JTWC Joint Typhoon Warning Center The Joint Typhoon Warning Center is a joint United States Navy – United States Air Force task force located at the Naval Maritime Forecast Center in Pearl Harbor, Hawaii... ) |
NE Pacific &N Atlantic(NHC National Hurricane Center The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of the National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th... , CHC Canadian Hurricane Centre The Canadian Hurricane Centre is a division of the Meteorological Service of Canada, an agency of Canada's Department of the Environment, which exists to advise Canadians on the threat of tropical cyclones such as hurricanes and tropical storms... , & CPHC Central Pacific Hurricane Center The Central Pacific Hurricane Center of the United States National Weather Service is the official body responsible for tracking and issuing tropical cyclone warnings, watches, advisories, discussions, and statements for the Central North Pacific Basin... ) |
||
| knots | km/h | mph | ||||||||
| 0–6 | <28 | <52 | <32 | Depression | Tropical Disturbance | Tropical Low | Tropical Depression | Tropical Depression | Tropical Depression | Tropical Depression |
| 7 | 28-29 | 52-56 | 32-35 | Deep Depression | Tropical Depression | |||||
| 30-33 | 56-63 | 35-39 | ||||||||
| 8–9 | 34–47 | 63-89 | 39-55 | Cyclonic Storm | Moderate Tropical Storm | Tropical Cyclone (1) | Tropical Cyclone (1) | Tropical Storm | Tropical Storm | Tropical Storm |
| 10 | 48–55 | 89-104 | 55-64 | Severe Cyclonic Storm | Severe Tropical Storm | Tropical Cyclone (2) | Tropical Cyclone (2) | Severe Tropical Storm | ||
| 11 | 56–63 | 104-119 | 64-74 | |||||||
| 12 | 64–72 | 119-135 | 74-84 | Very Severe Cyclonic Storm | Tropical Cyclone | Severe Tropical Cyclone (3) | Severe Tropical Cyclone (3) | Typhoon | Typhoon | Hurricane (1) |
| 13 | 73–85 | 135-159 | 84-99 | Hurricane (2) | ||||||
| 14 | 86–89 | 159-167 | 99-104 | Severe Tropical Cyclone (4) | Severe Tropical Cyclone (4) | Major Hurricane (3) | ||||
| 15 | 90–99 | 167-185 | 104-115 | Intense Tropical Cyclone | ||||||
| 16 | 100–106 | 185-198 | 115-123 | Major Hurricane (4) | ||||||
| 17 | 107-114 | 198-213 | 123-132 | Severe Tropical Cyclone (5) | Severe Tropical Cyclone (5) | |||||
| 115–119 | 213-222 | 132-138 | Very Intense Tropical Cyclone | Super Typhoon | ||||||
| >120 | >222 | >138 | Super Cyclonic Storm | Major Hurricane (5) | ||||||
Alternative scales
There are other scales that are not officially used by any of the Regional Specialized Meteorological CentreRegional Specialized Meteorological Centre
A Regional Specialized Meteorological Centre is responsible for the distribution of information, advisories, and warnings regarding the specific program they have a part of, agreed by consensus at the World Meteorological Organization as part of the World Weather Watch.-Tropical...
s or the Tropical Cyclone Warning Centres. However they are used by other organizations, such as the National Oceanic and Atmospheric Administration
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...
. 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
Accumulated cyclone energy
Accumulated cyclone energy is a measure used by the National Oceanic and Atmospheric Administration to express the activity of individual tropical cyclones and entire tropical cyclone seasons, particularly the North Atlantic hurricane season. It uses an approximation of the energy used by a...
(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
Hurricane Severity Index
The Hurricane Severity Index is a hurricane rating system which defines the strength and destructive capability of a storm. The HSI uses equations which incorporate the intensity of the winds and the size of the area covered by the winds...
(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
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
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 windsMaximum sustained wind
The maximum sustained winds associated with a tropical cyclone are a common indicator of the intensity of the storm. Within a mature tropical cyclone, they are found within the eyewall at a distance defined as the radius of maximum wind, or RMW. Unlike gusts, the value of these winds are...
recommended by the World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
(WMO), and used by most weather agencies, is that of a 10-minute average at a height of 10 m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
(33 ft). However, RSMC Miami
National Hurricane Center
The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of the National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th...
and RSMC Honolulu
Central Pacific Hurricane Center
The Central Pacific Hurricane Center of the United States National Weather Service is the official body responsible for tracking and issuing tropical cyclone warnings, watches, advisories, discussions, and statements for the Central North Pacific Basin...
, 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.
See also
- Tropical cyclogenesisTropical cyclogenesisTropical cyclogenesis is the term that describes the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which mid-latitude cyclogenesis occurs...
- Tropical cyclone basinsTropical cyclone basinsTraditionally, areas of tropical cyclone formation are divided into seven basins. These include the north Atlantic Ocean, the eastern and western parts of the northern Pacific Ocean, the southwestern Pacific, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean. The...
- Tropical cyclone namingTropical cyclone namingTropical cyclones have officially been named since 1945 and are named for a variety of reasons, which include to facilitate communications between forecasters and the public when forecasts, watches, and warnings are issued. Names also reduce confusion about what storm is being described, as more...
External links
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 BengalBay of BengalThe Bay of Bengal , the largest bay in the world, forms the northeastern part of the Indian Ocean. It resembles a triangle in shape, and is bordered mostly by the Eastern Coast of India, southern coast of Bangladesh and Sri Lanka to the west and Burma and the Andaman and Nicobar Islands to the...
and the Arabian SeaArabian SeaThe Arabian Sea is a region of the Indian Ocean bounded on the east by India, on the north by Pakistan and Iran, on the west by the Arabian Peninsula, on the south, approximately, by a line between Cape Guardafui in northeastern Somalia and Kanyakumari in India... - 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