Fearsome and destructive forces of nature

By MICHAEL McCARTHY

Is 2004 the Year of the Hurricane? It depends on where you're considering it from.

If you live in the United States you'll certainly think so, because Florida has been struck by three in a month, and as America dominates the world's media, the story has had huge attention right around the globe. Now a fourth storm has devastated parts of the Caribbean.

However, although it is certainly unusual for three tempests of the intensity of hurricanes Charley, Frances and Ivan to burst through one state in four weeks, the fact that all three made landfall so close together in time and space may well be pure chance.

Hurricanes sweep the oceans every year. So here are some of the questions that this year's hurricane season throws up, with some of the answers:

Q: What is a hurricane?

A: A hurricane is a tropical cyclone, an area of intense low pressure in the tropics surrounded by a violent rotating storm. It is called a hurricane in the north Atlantic, the northeast Pacific east of the dateline, and the south Pacific east of 160E; west of the dateline it is called a typhoon, and in the Indian Ocean, a cyclone.

It officially becomes a hurricane if its wind speeds reach 121km/h, or force 12 on the Beaufort scale; below that it is a tropical storm. Every year, there are about 100 tropical storms and about 50 of them reach hurricane strength. The name comes from "Hurican", the Carib god of evil.

Q: How is a hurricane formed?

A: The sea surface temperature needs to be above 26.5C for hurricanes to form.

The air above warm tropical water rises quickly as it is heated by the sea, and as it does so it rotates or spins, creating an area of very low pressure, which becomes the eye of the storm. Winds grow around the eye with great velocity, generating violent seas.

Q: Why are its effects so severe?

A: First, hurricanes produce the highest wind speeds, up to 320km/h in the most extreme cases, which only the strongest structures can withstand.

Second, they produce absolutely enormous amounts of rain, which can lead to catastrophic flash floods.

But third - and sometimes most seriously - they produce a phenomenon known as a storm surge. This is a huge raising of the sea level, caused jointly by the huge winds and the very low atmospheric pressure. In the most extreme cases it can be as much as 7.6m above normal.

The hurricane pushes this heightened sea along in front of it and when it hits the coastline, especially low-lying coasts, it can cause disastrous inundations, especially when the surge combines with torrential rain.

Britain experienced something like this on January 30, 1953, when a violent gale combined with low pressure produced a storm surge in the North Sea, which breached the sea defences of Lincolnshire and East Anglia and drowned 307 people.

Once a hurricane reaches land, it tends to die out fairly quickly as there is no more warm water to supply heat. But out in the open ocean it can last for a fortnight or more.

Q: How are hurricanes graded?

A: Hurricanes are now measured on the Saffir-Simpson scale, formulated in 1969 by Herbert Saffir, a consulting engineer, and Dr Bob Simpson, the director of the US National Hurricane Centre.

They devised the scale, from one to five, following Hurricane Camille in 1969, the most violent storm ever to hit the continental US.

Its categories run like this: Category one (minimal), winds 121-153km/h, minor flooding, slight structural damage, storm surge up to 1.5m.

Category two (moderate): winds from 154-177km/h, roof and tree damage, storm surge 1.8-2.4m.

Category three (extensive): winds from 178-209km/h, houses damaged, severe flooding, storm surge 2.7-3.7m.

Category four (extreme): winds of 210-250km/h, major structural damage to houses and some roofs destroyed, storm surge 4-5.5m.

Category five (catastrophic): winds above 250km/h, many buildings destroyed, smaller ones blown away completely, severe inland flooding, storm surge of more than 5.5m.

Q: How bad are the 2004 hurricanes?

A: The three storms that hit Florida this summer were pretty bad, but not among the worst on record: they were not as intense as was feared.

Frances was a category two/three and did the least damage; Charley was a three/four, and Ivan a category four, occasionally touching five.

But they did not compare in destruction with Hurricane Andrew, the category four/five storm that struck Florida in August 1992, which caused US$25 billion ($37.8 billion) worth of damage at today's prices; or in sheer power with Hurricane Camille, which struck Mississippi in 1969 leaving 256 dead; or the Labour Day Hurricane of 1935 which hit the Florida Keys, killing 423.

These latter two storms, full category fives, had winds that approached 323km/h and there has been nothing else like them in the US meteorological record.

Q: Are they worse or more frequent?

A: Although global warming is confidently expected to produce more violent storms, scientists cannot yet prove a link between hurricane rates and climate change.

There does not seem to have been an increase in the number of category five hurricanes worldwide. This year appears to be more active than 2003 and 2002 but less active than the four years before that.

Q: Why and how are hurricanes named?

A: All tropical cyclones are named, to ease communication between forecasters and the general public about forecasts, watches, and warnings.

Since the storms can often be long-lasting and more than one can occur in the same region at the same time, names can reduce the confusion about which storm is being described.

Before the 20th century, especially in the Caribbean, hurricanes were sometimes named after the saint's day on which they struck land. During World War II, US Navy meteorologists gave them the female names of wives and loved ones, but by 1950 a formal naming strategy was in place for North Atlantic cyclones, based on the phonetic alphabet of the time (Able, Baker, Charlie and so on.)

In 1953 the US Weather Bureau decided to switch to female first names, and with the agreement of the World Meteorological Association, included male first names in the list in 1979.

Each meteorological region of the world now has an agreed list of names. The letters Q, U, X, Y, and Z are not used because few names begin with these letters. Quite a few hurricane names - including Andrew, Betsy, Bob, Camille, Hugo and Hilda - have been officially retired because the storms concerned caused damage on a scale unlikely to be repeated. About 50 names have been retired: a country can request retirement.

- INDEPENDENT