Tropical cyclone

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Schematic cross-section through a tropical cyclone
The trajectories of the tropical cyclones from 1985 to 2005

A tropical cyclone is a low pressure system with organized convection , severe thunderstorms, and closed ground wind circulation around the center. Tropical cyclones , as the name suggests, usually only occur in the tropics or subtropics . Due to the Coriolis force , they rotate cyclonally . That is, tropical cyclones turn clockwise in the southern hemisphere and counterclockwise in the northern hemisphere. This rotation is also the cause of the typical, spiral-shaped cloud bands of such storms. The wind speed of a tropical cyclone can reach over 300 km / h. However, the speed of travel of the gravure system is only 15–30 km / h.

Tropical cyclones can stretch hundreds of kilometers in diameter. They usually exist at sea for a few days to two weeks. If they hit land, they can cause catastrophic damage to thousands of square kilometers.


Names of the tropical cyclones in the different regions: 1) Hurricane ( hurricane ) 2) Typhoon 3) Cyclone

Tropical cyclones with a wind speed that corresponds to a hurricane - wind force 12 on the Beaufort scale (this corresponds to more than 64 knots or 118 km / h) - have different names depending on their area of ​​origin:


As hurricanes are tropical cyclones in the Atlantic , the North Pacific east of 180 ° longitude and in the South Pacific east of 160 ° East, the Caribbean Sea and in the Gulf of Mexico referred to when they reach a maximum middle wind speeds of over 64 knots. Storms that resemble tropical cyclones are occasionally observed in the Mediterranean Sea . Such a storm is also medicane called a combination of the terms Medi terranean Sea (English for the Mediterranean ) and Hurri cane (English for hurricane ).


As typhoons , tropical cyclones in East and Southeast Asia as well as in the northwestern part of are the Pacific Ocean , west of the international date line and north of the equator called.


A cyclone is a violent hurricane in the Indian Ocean and the southern Pacific Ocean , more precisely in the Bay of Bengal and the Arabian Sea . The violent hurricanes that occur in the Indian Ocean south of the equator in the area of Mauritius , La Réunion , Madagascar and the African east coast as well as in the Australian region are called cyclones.

The classification of a tropical cyclone in various strengths is based on the Saffir-Simpson hurricane wind scale , which is officially only valid north of the equator and only for the Atlantic and the North Pacific east of the date line.


Schematic structure of a tropical cyclone

Especially in late summer and autumn, large amounts of water evaporate from the sea surface and rise with the warm air. They begin to rotate due to the Coriolis force , creating a huge vortex. In the middle is the “ eye ”, a largely wind-free, precipitation-free and cloud-poor zone in the center of the hurricane. The passage of the eye was often confused with the end of the storm; People who went outside in the meantime were often surprised by the storm that set in again quickly.

Directly around the eye is the “ eyewall ”, which consists of high-reaching clouds and in which the highest wind speeds generally occur. The wind direction in the eyewall is influenced by the centrifugal force ( gradient wind ). Multiple eyewalls can form in strong hurricanes. An outer eyewall may replace the inner eyewall. In this context, one speaks of a " cyclical new formation of eyewall " (English: eyewall replacement cycle ).

If a tropical cyclone reaches the coast and arrives overland, the eddy becomes weaker because there is no supply of warm, humid air.

Development conditions

Friction and humidity
Tropical cyclones only form over large water surfaces, as the friction here is much lower than on land and the humidity is high enough. It plays an essential role in the formation of the cyclones, as it contains energy ( latent heat ). This heat is slowly withdrawn from the ocean when it evaporates and quickly released again when it condenses . This heat of condensation is the main driving force behind a tropical cyclone.
Sea surface temperature
According to previous doctrines, the surface temperature of the water must be at least 26 ° C down to a depth of 50 m. But it depends on the temperature difference between the water and the air in higher layers. If the air is colder above, water temperatures of 20 to 24 degrees can be enough for the storm to form.
Low wind shear
The wind shear between different layers of air must not be too great, otherwise no vortex can form.

Places of origin

Formation areas and trajectories of tropical cyclones

Most tropical cyclones occur because of the favorable water temperatures within a zone that lies around the equator between southern and northern 30th parallel . Since the Coriolis force , the deflecting force of the earth's rotation, is only strong enough from 5 degrees north and south latitude to initiate a rotational movement of the cyclones, the equatorial area itself is almost impossible as a formation zone for tropical cyclones, but this does not mean that it is there do not occur. In these zones between the northern and southern 5th and 30th degrees of latitude, most tropical cyclones arise from wave disturbances in the trade wind, the " Easterly Waves ". In addition, the emergence of a tropical cyclone is supported by the intertropical convergence zone (ITC): The ITC ensures rising air masses and strong convection , because the two trade winds collide near the surface ( convergence ). At an altitude of approx. 12 - 15 km, the air masses strive apart again after ascending ( altitude divergence ).


Traditionally, seven areas of distribution are distinguished:

  • Northwest Pacific Ocean:
    The tropical cyclones in the region west of the date line and north of the equator are called typhoons . Typhoons often affect China , Japan , South Korea , Hong Kong , the Philippines and Taiwan , Vietnam, and parts of Indonesia . There are also numerous islands of Oceania . This basin is the most active, a third of the world's tropical cyclones occur here. There is no other coast in the world where more tropical cyclones land on land than in the People's Republic of China . In second place is the Philippines with 6–7 tropical storms annually.
  • Northeast Pacific Ocean This basin, north of the equator and east of the date line, is the second most active. The storms are called hurricanes here and often have an impact on western Mexico and, less often, on California or the northwestern part of Central America . The database does not record any hurricane that hit the mainland in the US state of California . However, historical records state that a storm in the city of San Diego caused winds in excess of 65 knots in 1858 . However, the exact course of this minimal hurricane is unknown. In 1939, 1976 and 1997, however, winds in California were measured at hurricane strength.
  • Northern Atlantic Ocean
    This region consists of the Northern Atlantic Ocean , the Caribbean Sea and the Gulf of Mexico . The number of cyclones in this area varies from one to more than twenty storms annually; an average of about ten storms form each year. The east coast of the United States , the Gulf Coast of the United States, Mexico , Central America , the Caribbean Islands, and Bermuda are primarily affected by these storms . Venezuela, the Atlantic provinces of Canada and the Macaronesian Islands are occasionally caught in the path of storms. According to the current state of knowledge, the position of the Azores high is decisive in the long term . At the current position, held by the Azores high since 1000 BP and previously between 5000 and 3400, hurricanes hit both the Atlantic and Gulf coasts. Between 3400 and 1000 modern chronology, the Azores high lay further to the south-west, for example above Bermuda, and therefore directed significantly more hurricanes into the Gulf of Mexico. Paleotempestological studies showed that 3 to 5 times more hurricanes reached the Gulf coast during this time, but only half as many reached the Atlantic coast. Most of the more intense hurricanes are so-called Cape Verde hurricanes , which form off the West African coast near Cape Verde . More rarely, hurricanes develop into extra-tropical systems that reach western Europe , such as Hurricane Gordon , which affected the Iberian Peninsula and Great Britain . Tropical cyclones in Spain are just as rare as in California. From the past two hundred years, only two tropical cyclone systems are known to have reached the Iberian Peninsula as such: Hurricane Vince during the 2005 season and a hurricane from 1842.
  • Northern Indian Ocean:
    In this basin, the storms, here called cyclones , form in two areas, the Bay of Bengal and the Arabian Sea , with activity five to six times higher in the Bay of Bengal. In this basin, the season has two high points: in April and May, before the monsoons set in, and then again in October and November, immediately afterwards. The hurricanes with the most fatalities raged here, for example the cyclone in East Pakistan in 1970 , which probably killed more than 300,000–500,000 people. In this basin, cyclones mainly affect India , Bangladesh , Sri Lanka , Thailand , Myanmar and Pakistan . Occasionally, however, a cyclone hits the Arabian Peninsula .
  • Southwest Pacific Ocean:
    Tropical activity in this area primarily affects Australia and Oceania . Tropical storms rarely reach Brisbane and New Zealand , usually during or after the transition to an extra-tropical system.
  • Southeast Indian Ocean:
    In this region, tropical cyclone activity mainly affects Australia and Indonesia . The storms, referred to here as cyclones, mostly hit the Australian north coast between Exmouth and Broome in Western Australia .
  • Southwest Indian Ocean:
    Although the area has almost half a century of data, cyclone research only became a priority in this area in 1999 when Météo-France deployed additional research personnel in La Reunion. Cyclones in this area affect Madagascar , Mozambique , Mauritius , Réunion , the Comoros , Tanzania and Kenya .

South Atlantic, East South Pacific

Tropical cyclones are very rare in the southern Atlantic and the southeastern Pacific, as the cold Benguela and Humboldt currents cool the tropical oceans significantly, so that the required water temperature of at least 26 ° C is rarely reached. In addition, there are often strong and therefore unfavorable high-altitude winds over the South Atlantic. On March 26, 2004 off Brazil, Cyclone Catarina was the only known tropical cyclone in the South Atlantic with sustained wind speeds that reached hurricane strength. A few other tropical storms are known from earlier years, but these did not reach wind speeds of more than 63 km / h.

Mediterranean Sea

Hurricane-like lows have already occurred several times in the Mediterranean region, and the formation of an eye has even been observed in individual systems. In November 2011, the American Weather Service (NOAA) classified a Mediterranean low as a tropical storm for the first time. It was named 01M . It was not given a conventional hurricane name , however, as the responsibilities for the Mediterranean have not yet been clarified.

Stages of development

The tropical cyclone Epsilon over the Atlantic on December 3, 2005
The Eye of Typhoon Odessa , Pacific Ocean, August 1985
If a large-area, convection-inducing flow, for example an Easterly Wave or an extra-tropical low pressure area over sufficiently warm water, meets sufficiently moist air masses and favorable shear conditions , a self-sustaining process can be triggered.
The latent heat released by the condensation leads to the fact that the rising air is additionally accelerated. This creates a negative pressure on the water surface, i.e. a low. The air masses flowing in from below meet the same criteria and are also accelerated. This alone does not trigger the chain process, otherwise large thunderstorms would cause tropical cyclones in our latitudes.
Due to the Coriolis force , the air masses (Low Level Inflow) flowing from all sides towards the convection area characterized by low pressure begin to circulate around a rotation center on the relatively frictionless water surface: An LLCC (Low Level Circulation Center) is created. This circulation also organizes and supports the convection; the rotation allows even more air to rise. It also prevents the inflowing air masses from balancing out the negative pressure in the center. The rotation thus supports the self-preservation of the gravure printing in the rotation center. The faster the cyclone rotates, the more warm, moist air is condensed out. When the air has given off enough moisture, it does not rise any further and moves up and sideways away from the center of rotation (high level outflow).
The resulting system continues to intensify as long as the conditions allow it. If the development conditions are optimal, this system intensifies up to a certain upper limit. Among other things, the surface friction prevents it from being exceeded, as it has a braking effect. The record is held by the Taifun Tip (Northwest Pacific, 1979) with a core pressure of 870  hectopascals and a diameter of 2200 km. On average, tropical cyclones reach a diameter of 500 to 700 km. This makes them significantly smaller than extra-tropical low pressure systems.
If the tropical cyclone rotates fast enough, an eye can form. The eye is a relatively cloud-free, almost windless area around the center of rotation, in which cold, dry air sinks from above. The eye is surrounded by high-reaching cumulus clouds , the eyewall. The tropical cyclone reaches the highest wind speeds in this area. Since the storm also has an additional proper motion that is added to the rotation speed, the main wind field is always on the side on which the rotation and the proper motion point in the same direction. Example: If a cyclone with a rotation speed of 200 km / h on the northern hemisphere moves counterclockwise with an own speed of 30 km / h to the north, the result is a total speed of 230 km / h on the eastern eyewall. On the western side, on the other hand, only 170 km / h is reached, as the natural movement counteracts the rotational movement here.
Tropical cyclones move at different speeds: in lower latitudes with 8 to 32 km / h, in higher latitudes with up to 80 km / h. In both hemispheres , the cyclones mostly move in a westerly direction and then turn parabolically to the east. In the northern hemisphere usually as follows: W NW N NE. At some point they leave the range of favorable conditions and weaken; either due to land contact, too cold water, dry air masses or too high shear. The record for longevity is held by tropical cyclone John, which swept the Pacific for 31 days in 1994 . When tropical cyclones reach the frontal zone of the mid-latitudes, they can transform into an extra-tropical low pressure system (extratropical transition).


Tropical cyclones are among the natural phenomena that can develop into natural hazards when people, nature and property are threatened. If a damaging event actually occurs, the natural event turns into a natural disaster . The effects of wind and rain can cause severe damage. On the one hand by the mechanical force of the wind itself, on the other hand by the very heavy persistent precipitation and thirdly by storm surges. A tropical cyclone often rages for hours in one place due to its large spatial extent. The different threats posed by tropical cyclones are explained in more detail below:

Continuous high wind speeds, up to over 250 km / h, in gusts even over 350 km / h, are possible close to the eye. This strong wind endangers shipping in the sea areas in particular and can damage even massive buildings and vehicles or trees on land through wind power directly or indirectly through being thrown up and moving objects. The wind weakens quickly when the tropical cyclone crosses onto land because it is cut off from its energy source (condensation heat, see above) and the increased friction of the land surface also weakens the storm.
Storm surge
A tropical cyclone pushes a flood mountain in front of it. In the northern hemisphere, where low pressure areas rotate counterclockwise, this is particularly pronounced in those quadrants to the right of its trajectory. In the southern hemisphere, due to the clockwise rotation, the particularly endangered areas are to the left of the train track. In these areas, the vectors of the direction of pull and the circulating winds of the storm add up. When reaching the mainland, the most severe flooding is to be expected in these quadrants. The high wind speeds can lead to a rising tide of up to 10  m above normal high tide. This can lead to widespread flooding.
Due to the continuous evaporation of warm surface water and condensation in the cloud system, there are large amounts of water in the storm system, so that the precipitation amounts caused by a tropical cyclone can reach over 500 mm (that is 500 liters per square meter). These enormous amounts of precipitation do not cause problems over the ocean, but often trigger floods on land and can also lead to landslides .
Waves over 20 m high endanger shipping as well as coasts and islands and their inhabitants. They often lead to coastal erosion .
These small-scale air eddies are a frequent accompaniment to tropical cyclones. They form in the thunderstorms that orbit the tropical cyclone. Most of these are waterspouts, but tornadoes also occur over land. Even at the edge of the eye of a strong tropical cyclone, tornadoes can briefly form due to cold air masses falling from above into the center, but their lifespan is usually limited to seconds or minutes; their destructive potential is nevertheless very strong.

The cyclone in East Pakistan claimed the most deaths in Bangladesh in 1970 , when around 300,000 people died. Hurricane Katrina , on the other hand, caused the greatest financial damage in 2005, with damage totaling over 100 billion US dollars . The strongest storm on reaching the coast in 1969 was Hurricane Camille , which reached a steady wind speed of 306 km / h on the US Gulf Coast. The longest-lived tropical cyclone observed to date was Hurricane John , which moved across the Pacific for around 31 days in 1994. The largest recorded tropical cyclone in 1979 was Typhoon Tip with a diameter of 2,200 km.

Global warming

Tropical cyclones get their energy from the warm surface water of the oceans. With the slight warming of the surface temperature observed in the last few decades, there is in principle more energy available for the evaporation of water. The subject of scientific debate is whether the rise in the surface temperature of the world's oceans associated with global warming could result in a growing proportion of severe tropical cyclones. This connection is currently the subject of controversial scientific discussions.

See also

Web links

 Wikinews: Tropical Storms  - In The News
Commons : Tropical Cyclone  - Collection of images, videos, and audio files

Worldwide observation services:

Interesting links:

Individual evidence

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  2. Weyman, James C. and Linda J. Anderson-Berry: Societal Impact of Tropical Cyclones ( English ) In: Fifth International Workshop on Tropical Cyclones . Atlantic Oceanographic and Meteorological Laboratory . December 2002. Retrieved June 4, 2008.
  3. Shoemaker, Daniel N .: Characteristics of Tropical Cyclones Affecting the Philippine Islands ( English , PDF; 660 kB) Joint Typhoon Warning Center. 1991. Retrieved June 4, 2008.
  4. a b Chenoweth, Michael and Christopher Landsea : The San Diego Hurricane of October 2, 1858 ( English , PDF; 294 kB) American Meteorological Society . November 2004. Retrieved June 4, 2008.
  5. ^ Atlantic Oceanographic and Meteorological Laboratory , Hurricane Research Division: Frequently Asked Questions: What are the average, most, and least tropical cyclones occurring in each basin? ( English ) NOAA . Retrieved June 4, 2008.
  6. ^ Liu, Kam-biu; Fearn, Miriam L. (2000). "Reconstruction of Prehistoric Landfall Frequencies of Catastrophic Hurricanes in Northwestern Florida from Lake Sediment Records". Quaternary Research 54 (2): 238-245. doi : 10.1006 / qres.2000.2166
  7. ^ Scott, DB; et al. (2003). "Records of prehistoric hurricanes on the South Carolina coast based on micropaleontological and sedimentological evidence, with comparison to other Atlantic Coast records". Geological Society of America Bulletin 115 (9): 1027-1039. doi : 10.1130 / B25011.1
  8. Blake, Eric S .: Tropical Cyclone Report: Hurricane Gordon: September 10-20, 2006 ( English , PDF; 1.8 MB) National Hurricane Center . November 14, 2006. Archived from the original on May 10, 2009. Retrieved June 4, 2008.
  9. ^ Joint Typhoon Warning Center : 1.2: North Indian Tropical Cyclones . In: 2003 Annual Tropical Cyclone Report . 2004. Retrieved June 4, 2008.
  10. ^ Sinclair, Mark: How often is New Zealand hit by tropical cyclones? Archived from the original on August 6, 2008. (PDF) In: NIWA Science (Ed.): Water & Atmosphere . 10, No. 1, March 2002. Retrieved June 4, 2008.
  11. Bureau of Meteorology : Tropical Cyclones in Western Australia - Climatology ( English ) Archived from the original on 22 September 2012. Retrieved on 4 June of 2008.
  12. World Meteorological Organization : Tropical Cyclone RSMC / South-West Indian Ocean (DOC) Archived from the original on September 8, 2006. Retrieved June 4, 2008.