Tropical Cyclone (Cyclonic Wind, Rain [Storm] Surge)

Unique identifier / Notation

MH0057

Synonyms

Typhoon,

Hurricane,

Cyclone,

Severe tropical cyclone.

A tropical cyclone is a cyclone of tropical origin of small diameter (some hundreds of kilometres) with a minimum surface pressure in some cases of less than 900 hPa, very violent winds and torrential rain; sometimes accompanied by thunderstorms. It usually contains a central region, known as the ‘eye’ of the storm, with a diameter of the order of some tens of kilometres, and with light winds and a more or less lightly clouded sky (WMO, 2017).

Alternative definition: A tropical cyclone is a warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organised deep convection and closed surface wind circulation about a well-defined centre (WMO, 2017).

Primary reference(s)

WMO, 2017. Manual on Codes, International Codes, Volume I.2, WMO-No. 306. World Meteorological Organization (WMO). Accessed 25 November 2019.

Additional scientific description

Depending on the maximum sustained wind speed, tropical cyclones are designated as follows (WMO, no date):

A tropical depression when the maximum sustained wind speed is less than 63 km/h.
A tropical storm when the maximum sustained wind speed is more than 63 km/h*. It is then also given a name.
Depending on the ocean basin, either a hurricane, typhoon, severe tropical cyclone, severe cyclonic storm or tropical cyclone when the maximum sustained wind speed is more than 119 km/h*.

*The designation thresholds for storm and hurricane are based on the Beaufort Scale. Tropical cyclones can be hundreds of kilometres wide and can bring destructive high winds, torrential rain, storm surges and occasionally tornadoes (WMO, no date).

The typhoon season in the western North Pacific region typically runs from May to November. The Americas/Caribbean hurricane season runs from 1 June to 30 November, peaking in August and September. The cyclone season in the South Pacific and Australia normally runs from November to April. In the Bay of Bengal and Arabian Sea, tropical cyclones usually occur from April to June, and September to November. The East Coast of Africa normally experiences tropical cyclones from November to April (WMO, no date).

Note: Typhoon, hurricane, cyclone, and tropical cyclone are different terms for the same weather phenomenon in different geographical regions (WMO, no date):

In the western North Atlantic, central and eastern North Pacific, Caribbean Sea and Gulf of Mexico, such a weather phenomenon is called a ‘hurricane’.
In the western North Pacific, it is called a ‘typhoon’.
In the Bay of Bengal and Arabian Sea, it is called a ‘cyclone’.
In the western South Pacific and southeast India Ocean, it is called a ‘severe tropical cyclone’.
In the southwest India Ocean, it is called a ‘tropical cyclone’.

Metrics and numeric limits

Strength thresholds for tropical cyclone intensity vary according to the geographical regions indicated above. For the hurricane, the Saffir-Simpson Hurricane Wind Scale is used, where hurricane strength varies from Category 1 to 5: Category 1 (maximum sustained wind speeds of 119–153 km/h), Category 2 (maximum sustained wind speeds of 154–177 km/h), Category 3 (maximum sustained wind speeds of 178–209 km/h), Category 4 (maximum sustained wind speeds of 210–249 km/h) and Category 5 (maximum sustained wind speeds exceeding 249 km/h) (NOAA, no date).

Key relevant UN convention / multilateral treaty

Not identified.

Examples of drivers, outcomes and risk management

Meteorologists around the world use modern technology such as satellites, weather radars and computers etc. to track tropical cyclones as they develop. Tropical cyclones are often difficult to predict, because they can suddenly weaken or change their course. However, meteorologists use state-of-art technologies and develop modern techniques such as numerical weather prediction models to predict how a tropical cyclone evolves, including its movement and change of intensity, when and where one will hit land and at what speed. Official warnings are then issued by the National Meteorological Services of the countries concerned (WMO, no date). The impact of a tropical cyclone and the expected damage depend not just on wind speed, but also on factors such as the moving speed, duration of strong wind, storm surge and accumulated rainfall during and after landfall, sudden change of moving direction and intensity, and the structure (e.g., size and intensity) of the tropical cyclone, as well as human response to tropical cyclone disasters (NOAA, no date).

The World Meteorological Organization (WMO) framework allows the timely and widespread dissemination of information about tropical cyclones. As a result of international cooperation and coordination, tropical cyclones are increasingly being monitored from their early stages of formation. The activities are coordinated at the global and regional level by the WMO through its World Weather Watch and Tropical Cyclone Programmes. The Regional Specialised Meteorological Centres with the activity specialisation in tropical cyclones, and Tropical Cyclone Warning Centres, all designated by the WMO, are functioning within its Tropical Cyclone Programme. Their role is to detect, monitor, track and forecast all tropical cyclones in their respective regions. The Centres provide, in real-time, advisory information and guidance to the National Meteorological Services (WMO, no date).

The health impacts of tropical cyclones depend on the number of people living in low-lying coastal areas in the storm’s direct path, the built environment including building design, and whether there is sufficient time for warning and evacuation (WHO, 2020).

Tropical cyclones, may directly and indirectly affect health in many ways, for example by: increasing cases of drowning and other physical trauma; increasing risks of water- and vector-borne infectious diseases; increasing mental health effects associated with emergency situations; disrupting health systems, facilities and services, leaving communities without access to health care when it is needed most; and damaging basic infrastructure, such as food and water supplies and safe shelter (WHO, 2020).

When tropical cyclones cause floods and sea surges, the risk of drowning and water- or vector-borne diseases increases. In addition, flood waters may contain sewage and chemicals, hide sharp objects made of metal or glass and electrical lines, or host dangerous snakes or reptiles, which can result in diseases, injuries, electrocution and bites. The greatest damage to life and property is not from the wind itself, but from secondary events such as storm surges, flooding, landslides and tornadoes (WHO, 2020).

The World Health Organization (WHO) works with Member States to build resilient and proactive health systems that can anticipate the needs and challenges during emergencies so that they are more likely to reduce risks and respond effectively when needed. During disasters, such as tropical cyclones, the WHO helps to restore primary care services so that facilities can deliver essential services, including immunisation, basic treatment for common illnesses, acute malnutrition and maternal care while ensuring the ongoing supply of medications for people living with HIV, tuberculosis or diabetes. As the health cluster lead for global emergencies, the WHO also works with partners to ensure appropriate food supplementation; to assemble mobile health teams and outreach; to conduct epidemic surveillance, early warning and response; and to call for emergency funding to support health action (WHO, 2020).