Image: courtesy NOAA
Tropical cyclones are one of the most dangerous natural hazards to people. Every year, they cause considerable loss of life and do immense damage to property. However, tropical cyclones are essential features of the Earth's atmosphere, as they transfer heat and energy between the equator and the cooler regions nearer the poles.
A tropical cyclone is the generic term for a low pressure system over tropical or sub-tropical waters, with organised convection (i.e. thunderstorm activity) and winds at low levels circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere). The whole storm system may be five to six miles high and 300 to 400 miles wide, although sometimes can be even bigger. It typically moves forward at speeds of 10-15 m.p.h., but can travel as fast as 40 m.p.h. At its very early and weak stages it is called a Tropical Depression. When the winds reach 39 m.p.h. it is called a Tropical Storm. If the wind should reach 74 m.p.h. or more the tropical storm is called a Hurricane in the Atlantic and the north-east Pacific or a Typhoon in the north-west Pacific. In other parts of the world, such as the Indian Ocean and South Pacific the term Cyclone or Tropical Cyclone is used.
In the tropics there is a broad zone of low pressure which stretches either side of the equator. The winds on the north side of this zone blow from the north-east (the north-east trades) and on the southern side blow from the south-east (south-east trades).
Within this area of low pressure the air is heated over the warm tropical ocean. This air rises in discrete parcels, causing thundery showers to form. These showers usually come and go, but from time to time, they group together into large clusters of thunderstorms. This creates a flow of very warm, moist, rapidly rising air, leading to the development of a centre of low pressure, or depression, at the surface.
There are various trigger mechanisms required to transform these cloud clusters into a tropical cyclone. These trigger mechanisms depend on several conditions being 'right' at the same time. The most influential factors are:
The Coriolis force caused by the rotation of the Earth helps the spin of this column of rising air. The development of the surface depression causes an increase in the strength of the trade winds. The spiralling winds accelerate inwards and upwards, releasing heat and moisture as they do so.
As the depression strengthens it becomes a tropical storm and then a hurricane or typhoon. A mature hurricane or typhoon takes the form of a cylinder of deep thundercloud around a centre that is relatively free from clouds. There is a relatively small area of intense horizontal winds at the surface, often well over 100 m.p.h., while air rises strongly above, maintaining the deep cumulonimbus clouds.
Further aloft at about six miles, the cloud tops are carried outwards to give thick layer clouds due to the outward-spiralling winds leaving the tropical cyclone core. At the centre of the tropical cyclone, air is subsiding, which makes it dry and often cloud free, and there is little or no wind at the surface. This is called the eye of the storm.
Structure of a tropical cyclone
Great amounts of energy are transferred when warm water is evaporated from tropical seas. This energy is stored within the water vapour contained in moist air. As this air ascends, 90% of the stored energy is released by condensation, giving rise to the towering cumulus clouds and rain. The release of heat energy warms the air locally, causing a further decrease in pressure aloft. Consequently, air rises faster to fill this area of low pressure, and more warm, moist air is drawn off the sea, feeding further energy to the system. Thus, a self-sustaining heat engine is created.
As little as 3% of the heat energy may be converted into mechanical energy of the circulating winds. This relatively small amount of mechanical energy equates to a power supply of 1.5x1012 Watts - equivalent to about half the world-wide electrical generating capacity!
Tropical cyclones are named to provide ease of communication between forecasters and the general public, regarding forecasts, watches and warnings. Since the storms can often last a week or even longer, and more than one can be occurring in the same region at the same time, names can reduce the confusion about what storm is being described.
Names were first used widely in World War II and were subsequently adopted by all regions. In most regions pre-determined alphabetic lists of alternating male and female names are used. However, in the north-west Pacific the majority of names used are not personal names. While there are a few male and female names, most are names of flowers, animals, birds, trees, foods or descriptive adjectives. The names are also not allocated in alphabetical order, but are arranged by the name of the Asian country which contributed the name.
Tropical cyclones form between approximately 5° and 30° latitude and initially move westward (owing to easterly winds) and slightly towards the poles. Many tropical cyclones eventually drift far enough from the equator to move into areas dominated by westerly winds (found in the middle latitudes). These winds tend to reverse the direction of the tropical cyclone to an eastward path. As the tropical cyclone moves polewards it picks up forward speed and may reach 30 m.p.h. or more. An average tropical cyclone can travel about 300 to 400 miles a day, or about 3,000 miles before it dies out.
Tropical cyclones which occur in the Atlantic region and affect the Caribbean and USA usually comprise less than 15% of global tropical cyclone activity. Tropical cyclones also occur in various parts of the Pacific Ocean, and can affect coastal regions of Mexico, south-east Asia, north-east Australia and the south Pacific islands. Those that form in the Indian Ocean can affect India, Bangladesh, north-west Australia, some parts of east Africa and Indian Ocean islands such as Mauritius and Madagascar.
In the northern hemisphere most tropical cyclones occur between June and November with a peak in September. However, in the north-west Pacific it is not unusual to have the occasional tropical cyclone outside of this period. In the southern hemisphere the season usually lasts from November to April. Tropical cyclones are occasionally observed in the South Atlantic, but this is a very rare occurrence. The map above shows the areas of the world affected by tropical cyclones.
Although developed in the USA, the Saffir-Simpson hurricane wind scale is used to rank tropical cyclone wind strength in many parts of the world.
Other phenomena which can be just as damaging than the wind frequently accompany tropical cyclones:
These phenomena can cause major destruction, especially when the tropical cyclone's path takes it over land. However, a path over land also causes the destruction of the tropical cyclone itself. As it moves over land, its energy source is depleted and friction across the land surface distorts the air flow. This leads to the eye filling with cloud and the tropical cyclone dies.
Due to advances in numerical weather prediction computer models, forecasting the initial development of tropical cyclones has improved greatly in recent years.
There are certain areas over the oceans that are particularly favourable for tropical cyclone development, but it is usually certain characteristics in a cluster of thunderclouds that leads forecasters to recognise them as tropical depressions. This is done by people at specialist tropical cyclone forecasting centres around the globe such as the National Hurricane Center in Miami, Florida who are constantly studying satellite images, instruments and other weather data to detect and track them through their life-cycle. Once detected, their track is forecast using a combination of numerical forecasting models, synoptic forecasting and statistical methods, which have been developed from the study of the behaviour of past storms.
Observations from ships at sea are always very useful, although, once the existence of a storm and its forecast track have been broadcast, ships tend to move out of their way! Reinforced aircraft, fitted with instruments, fly through and over tropical cyclones, and weather radar can locate storms within 200 miles of the radar station. In addition, tropical cyclones are tracked by satellites, which provide very useful information both to forecasters and numerical forecast models.
"Official" responsibility for forecasting tropical cyclones in the areas affected lies with the regional centres such as the National Hurricane Center in Miami. However, the Met Office does produce tropical cyclone forecasts from its global model, which are disseminated to the regional centres. They then use these forecasts, along with other forms of guidance, to produce their forecasts. Met Office forecasts of tropical cyclones show considerable skill and are among the best models available to agencies such as the National Hurricane Center.
The USA has the most sophisticated tropical cyclone warning system. Once a tropical depression has been identified, a series of forecast advisories, which detail the expected track and likely strength of the tropical cyclone, are broadcast. When there are definite indications that a tropical cyclone is approaching land, watches and warnings along coastal regions are raised, which aim to give information to the local authorities of places likely to be in the tropical cyclone's path, so that they can make preparations to protect public safety.
Specially designed shelters in which people can ride out the worst of the weather may have been built in advance. Coastal regions immediately in a tropical cyclone's path are particularly dangerous and people are usually advised to move away from these. In the USA, this can involve the mass evacuation of a million or more people. Warnings are broadcast on TV and radio. NOAA Weather radio is available on a variety of frequencies and gives up-to-date information and advice. There is also a lot of information on a variety of web sites, the most authoritative being those maintained by NOAA and the National Hurricane Center.
Hurricanes are tropical features and generally require sea temperatures much higher than those around the UK, even in the summer. Hence, hurricanes cannot form at our latitudes. However, we are sometimes affected by deep depressions that are the remnants of tropical cyclones. Intense mid-latitude depressions, even those which do not originate from a tropical cyclone can produce winds of a strength equivalent to a tropical cyclone.
The most widely publicised such depression occurred on 16 October 1987, known as The Great Storm. Some gusts were over 100 m.p.h., but this was not the strongest wind recorded over the British Isles. That was on 13 February 1989 when a deep depression passing just to the north of Scotland produced a gust of 142 m.p.h. (123 knots) at Fraserburgh. The reason that The Great Storm of October 1987 gained such publicity was that it occurred in a densely populated area and caused enormous damage to trees.
The impact of climate change - specifically global warming caused by the burning of fossil fuels - on tropical cyclone activity is the subject of ongoing debate and research in the scientific community. Recent catastrophic events, such as Hurricane Katrina, have given the debate a higher profile. While some evidence has been presented in order to indicate a recent upturn in tropical cyclone activity in some parts of the world, others have argued that natural variations in tropical cyclone activity - on a regional, annual and inter-decadal scale - mask any signal from the impact of global warming, which remains relatively small.
Although there is no clear consensus on whether global warming is currently having any measurable impact on tropical cyclones, climate models indicate that there may be an increase in tropical cyclone intensity in the future, under continued global warming. However, the models also indicate that tropical cyclone frequency will either remain unchanged or decrease.
A number of tropical cyclone scientists came together in 2006 to issue a statement on the status of understanding and research into tropical cyclones and climate change, which was presented to the World Meteorological Organization.
The following links provide further information on tropical cyclones.
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Last updated: 22 November 2013