What is a tropical cyclone?
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 terms hurricane and typhoon are regionally-specific names for a strong tropical cyclone.
Tropical cyclones with maximum sustained surface winds of less than 39 m.p.h. are called tropical depressions. Once the winds around the tropical cyclone reach at least 39 m.p.h. it is called a tropical storm and is assigned a name. If winds reach 74 m.p.h., then it is called a:
hurricane - in the North Atlantic Ocean and the north-east Pacific Ocean, east of the dateline;
typhoon - in the north-west Pacific Ocean, west of the dateline;
tropical cyclone - in other regions, including the Indian Ocean and South Pacific Ocean.
How do they get their names?
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 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.
Full list of names
How do they form?
Tropical cyclones require a number of conditions to develop, these include:
a source of warm, moist air derived from tropical oceans with sea surface temperatures normally in the region of, or in excess, of 27 °C;
winds near the ocean surface blowing from different directions converging and causing air to rise and storm clouds to form;
winds which do not vary greatly with height - known as low wind shear. This allows the storm clouds to rise vertically to high levels;
spin induced by the rotation of the Earth
The formation mechanisms vary across the world, but once a cluster of storm clouds starts to rotate it becomes a tropical depression. If it continues to develop it becomes a tropical storm, and later a hurricane or typhoon.
Hurricanes are synonymous with the Caribbean and the USA, but where else do they occur and do they happen here in the UK and Europe?
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.
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 do sometimes experience the remnants of old hurricanes coming out of the Tropics; as they get entrained into the Atlantic westerly flow at higher latitudes they can reach the UK and Western Europe as "extra-tropical storms". Although the Great Storm of October 1987 may have been enhanced by warm moist air originating in the Tropics, it was not by any definition a hurricane.
How are hurricanes graded?
In many parts of the world the Saffir-Simpson scale is used to grade hurricanes.
Category 1 - sustained wind speeds of 74 to 95 m.p.h.
Category 2 - sustained wind speeds of 96 to 110 m.p.h.
Category 3 - sustained wind speeds of 111 to 130 m.p.h.
Category 4 - sustained wind speeds of 131 to 155 m.p.h.
Category 5 - sustained wind speeds greater than 155 m.p.h.
While hurricanes of the high categories can cause catastrophic damage due to their wind speed, it should be noted that weaker hurricanes, and even tropical storms, can cause major problems due to heavy rainfall. The storm surge associated with tropical cyclones can also be very damaging.
When are the tropical cyclone seasons?
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.
Deadliest tropical cyclones
The most deadly tropical cyclone occurred in 1970, and caused an estimated 300,000 deaths in Bangladesh. The most deadly hurricane to hit the USA was the Galveston hurricane in 1900 which claimed 8,000 lives.
Some USA hurricane facts
2005 - Hurricane Katrina was the most costly on record, causing an estimated $80 billion of damage. It also claimed 1,300 lives - the greatest number for a USA hurricane since the 1928 Florida hurricane
1992 - Hurricane Andrew was a Category 5 hurricane which hit south- east Florida and south-east Louisiana, causing $45 billion of damage
1969 - Camille was a Category 5 hurricane with winds of 190 m.p.h. It hit Mississippi; south-east Louisiana, and Virginia, causing damage totalling $15 billion
1935 - The Labor Day Storm was a Category 5 hurricane which hit the Florida Keys
Is climate change affecting tropical cyclones?
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.
Statement on tropical cyclones and climate change
Does the Met Office forecast tropical cyclones?
"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.
Info from http://www.metoffice.gov.uk/corporate/pressoffice/hurricanes/
