It may be pointed out that, because of their more widespread fetch size and duration of the zone of strong wind , the violent storms in temperate regions in the "roaring" forties or the "howling" fifties in particular , generate waves and swells that may be even higher than those generated by tropical cyclones.
In the Southern hemisphere, the swells known as "polar swells", formed by the deep depressions of the Far South, come from the southwest, while the cyclone swells come mostly from the north or the east, at least as far as the Mascareignes Islands and the east coast of Madagascar are concerned. Although far less well-known, the storm surge is the most lethal and devastating phenomenon that comes with cyclones.
It is particularly dangerous in the low level coastal regions with a bay and a large and shallow continental plateau where an enormous quantity of water lifted by the cyclone can penetrate the area up to several kilometres inland and drown and destroy everything on the way, generating tidal waves similar to tsunamis from seismic origin earthquakes , but for which they should not be mistaken. The storm surge is an anomalous rise of the average sea level associated with the passage of a cyclone.
This single rise "solitary wave" lasts several dozens of minutes up to one or two hours. It may be from a few centimetres to a few metres high its very variable magnitude may reach 4 to 8 metres for powerful cyclones and in the areas with a wide continental plateau. It may spread over an area over 10 to several dozens of km wide.
The storm surge comes to a climax just in front of the cyclone: in the left forward sector according to the direction in which the cyclone moves in the Southern hemisphere, and in the right forward sector in the Northern hemisphere. Just like the swell, the storm surge amplifies as it approaches the coasts. The storm tide is the sum of the normal astronomical tide and of the "storm wave", also called "surge", which is the rise of the average sea level due to the combined action of the wind and of the pressure fall in the core of the cyclone.
The rise due to the pressure fall, or "inverted barometer effect" can be explained as follows: in the core of the cyclone, the weight of the column of air above the surface is lighter and the sea is "sucked up" so to speak.
An easy to remember rule of thumb is that a drop of 1 hPa corresponds to a rise of one centimetre. Therefore, keeping in mind that the average sea-level pressure is about hPa, and that the central pressure of the most powerful cyclones is about hPa minimal record measured : hPa for super typhoon Tip , one can come to the conclusion that the sucking up effect due to the pressure fall is around several dozens of centimetres, up to a little more than one metre at the very most.
In fact, most of the storm surge is due to the action of the strongest winds at the periphery of the eye, which mechanically push the water in front of the cyclone. These winds generate a very strong current called "drift current" by friction on of the ocean's surface. On the high seas, this current is partly compensated, beyond a depth of 50 to 60 metres, by a counter-current in the opposite direction.
The amplitude of the storm wave is then relatively small. When the cyclone reaches a continental plateau or comes near land, this counter-current disappears and the surface water accumulates towards the shore all the more so as the continental plateau is extensive.
Consequently, the amplitude of the storm tide depends on the intensity and of the structure the size especially of the tropical cyclone or the storm, as this phenomenon can also be noticed in temperate zones , but also on other independent factors, such as the configuration of the coast, the topography of the sea bed, the trajectory of the phenomenon, the astronomical tide: a high tide of great amplitude spring tide , a straight trajectory of the storm, perpendicular to the coast, an estuary, a bay or a delta in which the water will be trapped as in a marine "cul de sac" such as the bottom of the Gulf of Bengal , shallow sea beds, are so many factors which will considerably increase the rise of the water.
Many testimonies at that time confirm this estimation, which might seem questionable. Its diameter is typically around to km, but can reach km. A tropical cyclone brings very violent winds, torrential rain, high waves and, in some cases, very destructive storm surges and coastal flooding. The winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Tropical cyclones above a certain strength are given names in the interests of public safety. More information on the classification of tropical cyclones is available on the WMO Community Platform here.
Tropical cyclones are associated with a variety of hazards. The combination of wind-driven waves and the low-pressure of a tropical cyclone can produce a coastal storm surge — a huge volume of water driven ashore at high speed and with immense force that can wash away structures in its path and cause significant damage to the coastal environment. Torrential rainfall results in flash-flooding, flooding, and potential landslides and mudslides.
Their potential for wreaking havoc caused by those associated hazards is exacerbated by the length and width of the areas they affect, their intensity, frequency of occurrence and the vulnerability of the impacted areas. About 85 tropical storms form annually over the warm tropical oceans of the globe. Meteorologists around the world use modern technology, such as satellites, weather radars and computers, to track tropical cyclones as they develop.
Tropical cyclones may be difficult to forecast, as 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 forecast 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. The 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.
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 and Hydrological Services. Each year the impacts of tropical cyclones and other weather, climate and water extremes around the Earth give rise to multiple casualties and significant damage to property and infrastructure, with adverse economic consequences for communities that can persist for many years.
All this happens in spite of the fact that many of these severe events have been well forecast, with accurate warning information disseminated in a timely fashion by the responsible National Meteorological and Hydrological Service NMHS. Put simply, while there is a realization of what the weather might be, there is frequently a lack of understanding of what the weather might do. It is no longer enough to provide a good weather forecast or warning — people are now demanding information about what to do to ensure their safety and protect their property.
If this gap is to be closed, then an all-encompassing approach to observing, modelling and predicting severe hydrometeorological events, and the consequent cascade of hazards through to impacts, needs to be developed. Tackling this problem will require a multidisciplinary and highly integrated and focused endeavour. This is essential to ensure access to the best possible science, and the optimum services, to manage multi-hazard events today, and to provide the best possible evidence base on which to make the costly decisions on infrastructure needed to protect the population in the future as climate changes.
Improving the understanding of the potential impacts of severe hydrometeorological events poses a challenge to NMHSs and their partner agencies, particularly disaster reduction and civil protection agencies.
Progressing from weather forecasts and warnings to multi-hazard impact-based forecast and warning services represents a paradigm shift in service delivery for many NMHSs. Successful impact-based forecasting requires collaboration with others who have the additional necessary expertise, resources and knowledge such as demographic data, crowd-sourcing techniques, geographical information systems GISs , interoperability, and third-party data integration and usage to deliver impact services that NMHSs cannot do on their own.
Dangers of tropical cyclones. Tropical cyclones are classified by the strength of the wind associated with them, but there are other phenomena which can be just as damaging as the wind that frequently accompany tropical cyclones: high seas - large waves of up to 15 metres high are caused by the strong winds and are hazardous to shipping storm surge - a surge of water of up to several metres can cause extensive flooding and damage in coastal regions heavy rain - the tropical cyclone can pick up two billion tons of moisture per day and release it as rain.
This also leads to extensive flooding - often well inland from where the tropical cyclone hit the coast tornadoes - tropical cyclones sometimes spawn many tornadoes as they hit land which can cause small areas of extreme wind damage.
Cyclone monitoring is probably the area where weather forecasting has the most significant economic and social consequences.
A crucial aspect concerns the transmission of meteorological information observed state of the cyclone, expected trajectory and evolution to the security authorities and the public. Warning procedures must have been well defined in advance, including the different levels leading to graduated reactions from the authorities and the population.
The interruption of economic and civil activities, the accommodation of displaced persons, the provision of emergency relief and the subsequent restoration of services are major operations in terms of cost and social impact. It is therefore necessary to avoid both the underestimation of risks with potentially dramatic consequences and the overestimation of warnings that erodes the confidence that populations and officials have in forecasts.
On these islands, it is not possible to evacuate populations far from threatened areas. The focus is on the protection of people and property.
After a hurricane event, devastation requires rapid action and significant state support , which is only possible with some effectiveness in developed countries.
Elsewhere, the scars left by a cyclone can last for years. The considerable damage that must be reimbursed can put insurance companies in difficult situations. Given the scale of the claims hundreds of millions to tens of billions euros , these companies also reinsure themselves with other companies. Through a game of financial dominoes, the passage of a cyclone over a tropical country can trigger economic storms in the quieter markets of London, Zurich, New York or Tokyo.
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The strongest winds on Earth are due to tornadoes, causing devastation unmatched by any other…. Thunderstorms are made up of clouds called cumulonimbus. A combination of phenomena updraft of moist…. Tropical cyclones extract their energy from the heat stored in the tropical oceans and transform…. Impacts of tropical cyclones Every year, cyclones, typhoons and hurricanes affect dozens of countries around the world.
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