Tropical Cyclones

Different terminology is used for this weather phenomenon depending on the location:

• In the Caribbean Sea, the Gulf of Mexico, the North Atlantic Ocean and the eastern and central North Pacific Ocean, it is called "hurricane"
• In the western North Pacific, it is called "typhoon"
• In the Bay of Bengal and Arabian Sea, it is called "cyclone"
• In western South Pacific and southeast Indian Ocean, it is called “severe tropical cyclone”
• In the southwest Indian Ocean, it is called “tropical cyclone”

Depending on the maximum sustained wind speed, tropical cyclones will be designated as follows:

• Tropical depression is when the maximum sustained wind speed is less than 63 km/h.
• Tropical storm is when the maximum sustained wind speed is more than 63 km/h. It is then also given a name.
• Hurricane, typhoon, tropical cyclone, very severe cyclonic storm - depending on the basin - when the maximum sustained wind speed exceeds 116 km/h or 63 knots.

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. Damaging or destructive winds may reach speeds in excess of 300 km/h in the most intense systems. 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. Among these, a little more than half (45) become tropical cyclone/hurricane/typhoon. Out of the 85 tropical storms, 72% form in the northern hemisphere, and 28% in the southern hemisphere.  

Average annual cycle of tropical cyclone occurrence for each ocean basin. The abscissa spans the 13 months, December through January of the following year; the ordinate is the number of storms per hundred years. For each day, the graph shows the number of years that a cyclone was present (normalized per 100 years). The blue line represents all tropical cyclones (surface winds greater than 17 m s-1 or 34 knots); shading represents tropical cyclones of hurricane strength (surface winds greater than 33 m s-1 or 64 knots).

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. From the perspective of service users, this would include communities most vulnerable to disasters contributing to the information system. Working closely together, the suppliers of the services and the beneficiaries of those services would provide an integrated, authoritative, unified voice that everyone could resonate with, and in turn could take effective action.

Some of the concepts that underpin the idea of impact forecasting go beyond the terms traditionally used in weather forecasting. 

Hazard - A hazard is defined as a hydrometeorological-based, geophysical or human-induced element that poses a level of threat to life, property or the environment.

Hyydrometeorological Forecast Uncertainty - Hydrometeorological forecast uncertainty refers to the limits of predictability imposed by the state of the science and inherent randomness of the hydrometeorological system.

Exposure - Exposure refers to who and what may be affected in an area in which hazardous events may occur. If the population and economic resources were not located in (exposed to) potentially dangerous settings, no disaster risk would exist. Exposure is a necessary, but not sufficient, determinant of risk. It is possible to be exposed, but not vulnerable, for example, by living on a floodplain but having sufficient means to modify building structure and behaviour to mitigate potential loss. However, to be vulnerable to a hazard, it is also necessary to be exposed. Exposure is time (t) and space (x) dependent. An example of exposure related to geographic location would be a vehicle driving across a bridge during a windstorm. To highlight a situational example, a crane would have a much higher exposure during the same windstorm than would a car at street level. Exposure due to timing may exist at various timescales. For example, a windstorm that strikes during the peak of an urban rush hour would result in a much higher exposure factor than if it were to hit an unpopulated rural area in the middle of the night.

Vulnerability - Vulnerability refers to the susceptibility of exposed elements, such as human beings and their livelihoods and property, to suffer adverse effects when affected by a hazard. Vulnerability is related to predisposition, sensitivities, fragilities, weaknesses, deficiencies, or lack of capacities that favour adverse effects on the exposed elements. Vulnerability is situation specific, interacting with the hazard to generate risk. Therefore, vulnerability may also be time and space dependent. For example, as flood defences are built they protect the population in low-lying areas. As another example, the State of Florida in the United States implemented stricter building codes following Hurricane Andrew in 1992.

Risk -  Risk is defined as the probability and magnitude of harm attendant on human beings and their livelihoods and assets because of their exposure and vulnerability to a hazard. The magnitude of harm may change due to response actions to either reduce exposure during the course of the event or reduce vulnerability to relevant hazard types in general. Risk:

• Can be connected to each other and their effects can be compounded. Several or many risks can occur simultaneously within the same area. This requires an ability to compare them and to make trade-offs, assessing the relative importance of one risk in comparison with another, which may not necessarily be hydrometeorological in character.
• Are not always easy to identify, quantify and categorize, and sometimes identification occurs long after serious adverse consequences have been felt.
• Are evaluated differently in social terms. Thus, a risk considered serious in one location may be considered less so in another, or there is flexibility in accepting the risk.

These three separate forecasting paradigms are defined for clarity. There is a subtlety in the distinctions amongst them.

Paradigm 1 – Weather forecasts and warnings (hazard only): These types of forecasts and warnings contain information that refers only to atmospheric variables and how they are expected to change. In the case of weather warnings, the focus is on forecasting weather-based hazards only.

• Example 1: “Bora winds are expected tonight with wind speeds of 20 metres per second.”
• Example 2: “Severe thunderstorms are expected today with wind gusts exceeding 60 mph.”

Paradigm 2 – Impact-based forecasts and warnings (hazard and vulnerability only): These types of forecasts and warnings are designed to express the expected impacts as a result of the expected weather.

• Example 1: “Bora winds are expected tonight which may result in delays or cancellation to ferry services.”
• Example 2: “Severe thunderstorms with gusts over 60 mph will result in damage to trees and power lines.”

Paradigm 3 – Impact forecasts and warnings (hazard, vulnerability and exposure): These types of forecasts and warnings are designed to provide detailed information down to the individual, activity or community level. For many NMHSs, these types of forecasts will be the purview of partnering agencies rather than the role of the NMHS itself.

• Example 1: “Ferry services for the island of Brač will most likely be cancelled tonight due to Bora winds.”
• Example 2: “Extensive traffic delays in Kensington may occur due to the risk of large trees downing power lines and blocking roads as a result of severe thunderstorms.” Important to note is that knowledge of local exposure (that is, known ferry routes and neighbourhoods with particularly - Risk is defined as the probability and magnitude of harm attendant on human beings and their livelihoods and assets because of their exposure and vulnerability to a hazard. The magnitude of harm may change due to response actions to either reduce exposure during the course of the event or reduce vulnerability to relevant hazard types in general. 

All the materials in the sub-section were sourced in the above publication.

For completeness, these Guidelines describe the ultimate step of forecasting actual impacts, although it is recognized that this is a highly sophisticated exercise, requiring strong collaboration with partner agencies and significant research into exposure and vulnerability. For many WMO Members this step will not be the responsibility of the NMHS but rather that of the relevant DRCPA and other partners. Consult the Guidelines here.