As Hurricane Isabel hits the US eastern seaboard, Nick Morrison looks at what causes hurricanes - and recalls the damage some of them have wreaked.

THE year started with Ana, but so far it's only really been Fabian that's attracted much attention, although Claudette provoked a bit of a stir. That is, until Isabel came along. She looks likely to cause the most disturbance of the lot. As for Wanda, well, she might not get a look in at all.

Hurricanes are one of the most dangerous and destructive forces in nature. Every year, thousands of people are killed or made homeless by hurricanes, which cause millions of pounds of damage, but attempts to combat their power - including dropping silver crystals into them - have so far proved futile. Yet, for all their devastating capability, they play an essential role in regulating the Earth's atmosphere.

Yesterday, thousands of US government employees in Washington DC were told to stay at home in preparation for the arrival of Hurricane Isabel. Flags and awnings were taken down at the White House. Tens of thousands more fled their homes along the US east coast. A state of emergency was declared in Maryland, Virginia and North Carolina, with the National Guard placed on full alert.

Although Isabel had weakened from a rare maximum Category Five hurricane to a Category Two, its top winds were still gusting at around 105mph - enough to cause considerable damage, even without the expected flooding as it releases its rain.

Isabel is the first major hurricane to hit the eastern seaboard of the US since Floyd came ashore in 1999. The Category Two hurricane caused an estimated $4.5bn of damage and claimed 56 lives, making it the third costliest in US history.

Hurricanes are large rotating storms, centring around an area of very low pressure, and with winds blowing at an average speed of more than 74mph. The storm may be up to ten miles high, and on average is 500 miles wide. It moves at speeds of up to 20mph - you cannot outrun a hurricane.

The formation of a hurricane depends on a number of factors all being in place, with the most important being a source of warm, moist air, typically from tropical oceans, and sufficient spin from the rotating Earth. As the rotation is greatest in equatorial regions, hurricanes occur between five and 30 degrees latitude, roughly between Guyana in South America and Jacksonville in Florida. Strictly speaking, hurricanes do not occur over the British Isles. The storm of October 1987 was a deep depression which was a remnant of a hurricane, and although the winds gusted to hurricane strength, the average speed was sufficient to class it only as a severe gale.

As the warm sea heats the air above it, a current of moist air rises up quickly, creating low pressure at the surface of the water. Winds coming towards this low pressure are sent spiralling upwards, releasing heat and moisture before falling back down. As the Earth rotates, the rising column of air starts to twist anti-clockwise, until it takes on the form of a cylinder around still air, the eye of the hurricane.

Water from the seas is taken up by the hurricane in evaporated form, and as the air rises, the water is released as condensation, causing huge clouds and rain. This heat energy also warms the air nearby, producing a drop in pressure, and as air rises to fill this area of low pressure, it brings more moist, warm air with it, drawn off the sea. This brings further energy into the storm, and the result is a ball of swirling wind which has its own energy source. As long as it is above tropical water, it can sustain itself, feeding off the warmth of the sea.

Even then, just a fraction of the heat in a hurricane, about three per cent, is contained in the circulating winds, although this in itself is the equivalent of six months electrical energy for the entire United States.

Easterly winds mean that hurricanes initially move westward and towards the poles, eventually drifting far enough to be hit by westerly winds, causing them to reverse direction. Along with their winds, hurricanes also cause waves up to 15 metres high and huge ocean swells, as well as rain from the two billion tons of moisture picked up in a day.

But as a hurricane finds its energy from the sea, when it is over dry land it is divorced from this supply. Even as it is wreaking its greatest destruction, the hurricane is heading towards its own death, further hindered by friction from passing across the land, disturbing the air flow and filling the eye with cloud until it dissipates.

They may have the potential to cause immense devastation, but hurricanes do have a purpose: they transfer heat and energy from the equator to the cooler areas towards the poles, helping to regulate the imbalances in the Earth's atmosphere.

Tropical cyclones - the strongest of which turn into hurricanes - may be as old as the oceans, but they have only been regularly named since the Second World War, when US military meteorologists gave them women's names. From 1950 to 1952, they were given names in the phonetic alphabet - Able, Baker, Charlie and so on - until 1953, when they again reverted to women's names. From 1979, the list changed to alternate between men's and women's names, with the list maintained by the World Meteorological Organisation. Names are considered to be less confusing than the alternative, which is to refer to storms by their latitude and longitude.

Six lists are used in rotation, so the 2001 list will be used again in 2007, but names are periodically dropped, usually if a storm is so destructive it would be considered insensitive to use it again. Thus, recent years have seen Marilyn, Keith, Georges and Mitch replaced with Michelle, Kirk, Gaston and Matthew. In this year's storms, Claudette escalated into a hurricane, which hit Texas, and Fabian was the worst hurricane to hit Bermuda in years, causing millions of pounds of damage and four deaths, when the Category Three storm struck the British territory earlier this month.

After Isabel, we can expect Juan, Kate and Larry, but with the hurricane season coming to an end next month, we may never see Sam, Teresa or Victor, let alone Wanda.

HURRICANE FACTS

* 'Hurricane' comes from Hurican, the Carib god of evil, who was derived from Hurakan, the Mayan creator god who blew across the water to create dry land.

* Hurricanes occur from July to October in the northern hemisphere, and from November to March in the southern hemisphere.

* Only Atlantic tropical storms are called hurricanes. In the Pacific, they are known as typhoons, and in the Indian Ocean as cyclones.

* Atlantic tropical storms are given alternate men's and women's names.

* On average there are six Atlantic hurricanes a year. Over a three year period, five hurricanes strike the US mainland.

* 'Major hurricane' is used for storms where the surface winds reach at least 111mph for a minimum of one minute.

* The eye of the hurricane is an area of light winds at the centre of the storm.

* An average hurricane can travel around 300-400 miles a day, and about 3,000 miles before it dies out.

* The National Hurricane Centre in Miami uses satellites to detect hurricanes.

* Research has been carried out into destroying hurricanes by dropping silver iodide crystals to freeze the condensed water, taking away the heat and weakening the storm's power.