george

Hurricanes blow themselves out as they reach the edge of the first of the three cells that move warm air towards the poles: at the boundary between the Hadley cell and the Ferrel cell.

The universal meteorological term for violent storms that start life over tropical waters is “tropical cyclone”, but they have different names around the world: “typhoon” in the northern Pacific and a “cyclone” in India and Australia are exactly the same phenomenon, and in the Atlantic it has the more familiar name of “hurricane”, which originates from the Central American Taino word huracan, meaning “god of evil”.

Fran 1996

Donna Camille Elena Bob Andrew 1960 1969 1985 1991 1992

• For easy reference, tropical cyclones, or hurricanes, have been given names since the 1950s. Each year an international committee creates an alphabetical list of alternating male and female names for the major hurricane regions.

• Names can be reused in a subsequent hurricane season, except those of especially damaging hurricanes, which are retired so as to avoid confusion.

Hurricanes are major atmospheric hazards in the low latitudes. They are massive tropical storms with winds exceeding 119kmh.

In an average year, about 80 tropical cyclones are recorded worldwide with two-thirds located in the northern hemisphere. Fifteen per cent of the world’s population is at risk.

The greatest threats exist for three geographical situations:

• Densely populated deltas, eg Bangladesh

• Isolated island groups, eg Philippines, Japanese islands and the Caribbean

• Highly populated coasts in MEDC areas, eg Florida

• Hurricanes can be detected on satellite images and their progression is often well forecast. Given suitable warning time, populations can be evacuated and lives saved.

• However, even with well-forecast hurricanes a problem arises in less economically developed regions, where due to poverty and the sheer numbers at risk, complete evacuation and natural or engineering solutions are too costly.

Tropical cyclones affect a very large part of the Earth’s surface. They vary in scale from tropical disturbances to hurricanes and are classified according to windspeed.

Hurricanes in the Caribbean and Atlantic Ocean

Typhoons in the western Pacific

Cyclones in the Indian Ocean

• Hurricanes develop as intense low-pressure systems over tropical oceans. Winds spiral rapidly around a calm central area known as the eye.

• The diameter of the whole hurricane may be as much as 800km, although the very strong winds, which cause most damage, are found in a narrower belt up to 300km wide.

In a mature hurricane, pressure may fall to as low as 880-970 millibars.

This, and the strong contrast in pressure between the eye and the outer part of the hurricane leads to strong winds.

Since tropical cyclones require heat and moisture, they always form over oceans with warm sea surface temperatures (at least 260C). As a consequences, they originate mainly over the western parts of the main ocean basins, where no cold ocean currents exist.

Hurricanes occur only in tropical latitudes, but not in all tropical latitudes. They are absent from the east and south Atlantic and the eastern south Pacific. Why?

As well as requiring high sea temperatures, the low pressure area has to be far enough away from the equator so that the Coriolis force (the force caused by the rotation of the Earth) creates rotation in the rising air mass. If it is too close to the equator there will be insufficient rotation and a hurricane will not develop.

Why do hurricanes not develop on the equator itself?

Because of the thermal requirements for formation, hurricanes are a hazard to specific locations at specific times of the year (eg from June to November in the west Atlantic).

A hurricane derives its energy from warm ocean water, but it loses power as it crosses land and quickly dries out. It may regenerate if it passes over the ocean again.

Hurricanes may move in straight lines, curves and loops influenced by the large-scale air masses they encounter. Their path is erratic; hence it is not always possible to give more than 12 hours notice. Why do hurricanes die out over land?

This meant coastal areas in several Gulf states had to be evacuated. Then Elena slowed and was pushed eastward by local air masses towards the central Florida coast. It stopped for a while offshore, causing evacuation along the length of coastal Florida. Elena then turned westward and northward again, causing the evacuation again of coastal Gulf states. It finally made landfall near Biloxi in Mississippi and caused US$1.4 billion in damage (1989 dollars).

The erratic nature of hurricanes is well illustrated by Hurricane Elena. This was a category 3 storm that caused considerable damage in the Gulf of Mexico in 1985. It headed west, rounded Florida then turned northward, towards an expected landfall in Mississippi.

• Hurricanes threaten people because of the strong winds, heavy rains, and storm surges.

• Storm surges occur as a result of wind-driven waves and the rise in sea surface due to low atmospheric pressure above. This amounts to a 26cm rise for every 30mb fall in pressure.

• The Saffir-Simpson scale of hurricane intensity is a useful way of describing hurricanes and identifying characteristics and damage.

In the USA deaths have dropped dramatically due to the advance warnings that can be broadcast before a hurricane makes landfall. However, damages in the USA continue to rise as Americans move to the coast, building larger and more expensive houses filled with costlier possessions. Minimising impacts:

• Prediction

• Warning

• Evacuation

• Better construction of buildings

• Sea wall construction or flood barrier

• Raising of buildings, rail and road

• Maintenance of a wide beach with high dunes

• Zoning laws to prevent building on low-lying land subject to storm surge flooding

Robin:Hurricanes are the tropical cyclones of the Atlantic.

• They tend to develop: 1. over warm tropical oceans with sea surface temperatures above 26°C.2. in autumn when sea surface temperatures are at their highest3. in the trade wind belt where the surface winds as they blow towards the equator4. between latitudes 5° and 20° north or south of the equator (the Coriolis force is weaker and insufficient to ‘spin’ the hurricane nearer to the equator) • The formation of hurricanes is not yet fully understood but they appear to originate when strong vertical movement of air draws with it water vapour from the ocean below.

• As the air rises, in a spiral movement, it cools and condenses - a process that releases vast amounts of latent energy and produces heavy rainfall.

• Once formed they move westwards often on erratic and difficult to predict courses.

• The hurricane rapidly declines once the source of heat and moisture is removed, i.e. when it moves over colder water or a land surface.• The average lifespan of a hurricane is 7-14 days.

• Hurricanes can cause damage in four main ways:

1. High winds which often exceed 160 kph.2. Storm (tidal) surges formed by the high winds and low pressure may inundate low lying coastal regions.3. Flooding can result from torrential rain or a storm surge.4. Landslides can occur from heavy rain where buildings have been erected on steep unstable slopes.• Hurricanes have the advantage of predictability. There are distinct ‘hurricane seasons’ within which more precautions can be taken.• Hurricanes can be easily spotted using satellite imagery which allows several days’ warning.