Hurricanes are the most ferocious storms on earth. They have the power to destroy nations and create refugees in the world’s richest country. These spinning storms stretch for hundreds of miles, spewing rain, whipping up tornados and blowing gusts of up to 180 miles per hour that can re-contour hills, alter the course of rivers and obliterate towns and cities.

Three weeks ago, Katrina was just a name on a list to be assigned to a future hurricane. Today, it is the name associated with one of the most devastating natural disasters in the history of the United States. In her wake, she left up to one million people homeless, submerged 80 per cent of New Orleans in fetid water, took countless lives and created a disaster zone roughly the size of the UK.

Katrina hit at the height of Atlantic Hurricane Season, which runs from the beginning of June until the end of November. Meteorologists had warned that the 2005 hurricane season would be particularly intense and that the Atlantic basin region, which encompasses not only the Gulf of Mexico, but the eastern seaboard of the US and islands in the Caribbean, would be in for a rough ride. Even before Katrina howled across the states of Louisiana, Mississippi and Alabama, the predictions were coming true.

Hurricane Dennis hit Jamaica, Cuba and Florida in early July and was the strongest storm to form, before August, since records began. Less than two weeks later, Hurricane Emily swept across the Winward Islands, Grenada and Mexico, snatching that record away.

Sea monster

Like all hurricanes that form in the Atlantic Basin, Katrina began life as an unremarkable cluster of thunderstorms off the coast of West Africa. As they rolled westwards across the Atlantic these thunderstorms found perfect hurricane forming conditions around 280km (175 miles) east of Nassau in the Bahamas.

All hurricanes (called typhoons in south-east Asia and tropical cyclones if they occur in the Indian Ocean and south of the equator) need the same specific conditions for creation - sea temperatures of 26.5C (80F) or more and depths of least 50 metres (150 feet). Combine with warm, moist air, light, consistent upper-level winds and a location within 5 LESS THAN to 30 LESS THAN north or south of the equator.

Warm air from the thunderstorms and moisture from the ocean surface mix and rise. The rising air creates low pressure at sea level and more moist air is sucked upwards, generating higher wind speeds. As the warm vapour rises it condenses to form water droplets. The droplets accumulate and form cumulus and cumulonimbus clouds, which rise high into the atmosphere. Trade winds initiate the spin.

Due to the Coriolis Effect, hurricanes north of the equator spin counter-clockwise and south of the equator they spin clockwise.

These thunderstorms continue to grow, fuelled by large-scale evaporation and the area of low pressure expands.

On August 23, what started out as a collection of thunderstorms began to spiral and develop into Tropical Depression Number 12. On August 24, the depression evolved into Tropical Storm Katrina. Tropical storms can reach speeds of up to 74mph, but when the speed tipped over that point, this whirling mass of cloud became Hurricane Katrina.

As a storm reaches hurricane strength, the eye, its defining feature, develops. Typically measuring 30-50km (20 to 40 miles) across, it’s an area of extremely low atmospheric pressure, with few clouds, little or no rain and relatively light winds.

Outside this calm interior lies the eyewall and the furious winds associated with all hurricanes. Tightly-packed thunderstorms make up the constantly changing eyewall and beyond that lie the spiral rain bands, which can be 80-480km (50 to 300 miles) long. The average hurricane is around 480km (300 miles) long.

She’s unpredictable By the time Katrina became a hurricane on August 25, she was classed as category one. Eleven people died as she swept across the southern tip of Florida, but Katrina was still in her infancy.

Hurricanes don’t travel in straight lines. They can be pushed off course by other strong winds and spin back on themselves. They zigzag and they dawdle, intensify and lose strength. After leaving Florida, Katrina headed south-west and refuelled over the Gulf of Mexico, which had been warmed by a long hot summer. Here she grew to become a catastrophic category five storm, with sustained wind speeds of 175mph and gusts reaching 215mph.

Curving north, she headed towards one of the most vulnerable cities on the Gulf Coast - New Orleans.

For people in the path of Hurricane Katrina, landfall was the start of the drama. But for the hurricane itself, it was the beginning of the end. After travelling hundreds of miles, she finally has no more warm ocean to sustain her. However, a dying hurricane is still a formidable thing.

One of the biggest dangers it brings with it is a sea surge. As a hurricane hits a coastline, sea levels rise rapidly as the high winds push the sea forward. During Hurricane Katrina, storm surge spread as far away as Florida. In Biloxi, Mississippi, a surge of up to 7.6m (30ft) - the highest ever recorded in the US - was measured. In some places along the Mississippi coast the waves of water travelled 1.6km (1 mile) inland.

Often lurking in the hurricane are tornados, which form while it is blowing over land. They measure up to 250m across and they usually have a lifespan of minutes, while a hurricane can live for up to two weeks. Average tornado speeds reach 200mph and where the funnel touches the ground, they create narrow trails of destruction. As Katrina swept northwards, tornados were reported as far away as Georgia.

Finally, there are the torrential rains - 15-30cm (6 to 12 inches) of rain is typical. Hurricanes actually move quite slowly (10-30mph) and the slower they move the more rain that falls. As the aftermath of Katrina showed, it is not the collapsing buildings or flying debris that leaves the most casualties during a hurricane, but the subsequent flooding caused by a combination of surge and heavy rain. In Mississippi and Alabama, the waters receded relatively quickly. However, in New Orleans, a city that sits seven to eight feet below sea level, Katrina was the storm that the city had long dreaded.

Planning ahead

In 1965, New Orleans took a direct hit from a category two hurricane called Betsy. Seventy-two people died as the 10ft storm surge overwhelmed the levees that protected the city from Lake Pontchartrain in the north and the Mississippi in the south. Four years later Hurricane Camille, one of only two category five hurricanes to hit the US, killed some 260 people when it hit Mississippi, Alabama and Louisiana. Yet 33 years later, with improved warning systems and better communication, a category four storm kills as many as 10,000 people in the same three states.

A growing coastal population ensured there were more people living in the path of the storm. Only 200,000 people evacuated for Camille, compared to the half a million who streamed out of New Orleans ahead of the storms.

The destruction of the barrier islands and wetlands that used to provide some natural protection for Louisiana was cited as another reason for the high death toll. Then there were the levees built to cope only with a category three storm. However, the biggest killer was lack of planning and the failure to get more people out of harm’s way. While hurricanes are unstoppable, the damage they cause can be reduced with resources, effective emergency planning and accurate forecasting.

Eyes on the sky

The first line of defence for the Atlantic region is the National Hurricane Centre (NHC) in Miami, Florida. Long before a tropical storm develops into a hurricane, scientists will be watching their screens, analysing their satellite images and data feeds from instruments on ocean buoys and direct observation from ships. They will be tracking the direction of the storm and looking for signs of it strengthening. They will also be using information collected by Hurricane Hunters sent out to fly into and above the storms to provide detailed real-time information (see panel).

The data from all these sources is fed into various storm modelling programs and the combination of direct observation, computer analysis and human expertise provide the forecasts.

Despite the increasing complexity of technology, short-term forecasting is still an inexact science. Scientists are still refining techniques that will improve short-term forecasts able to pinpoint the precise time and location of landfall days, rather than hours in advance.

Research is also focused on long-term forecasting, and one of the leading organisations in the field is based not in Florida, but in the UK. Tropical Storm Risk (TSR) has combined the expertise of meteorologists, climate physicists and statisticians from University College London and the Met Office to create computer models that can predict the intensity of a hurricane season months in advance.

They were able to predict back in December that the 2005 Atlantic hurricane season would be a particularly busy one. In their last bulletin in August they predicted “record-breaking activity levels ... at 150 per cent above average”.

Professor Mark Saunders, lead scientist at TSR explains how it works: “We have two main predictors. The first being how warm or cold the sea waters are in AugustSeptember - the main hurricane season - and how warm it is in the regions where hurricanes usually form, between the west coast of Africa and the Caribbean.

“The second predictor, which we feel is more important, is the speed of the trade winds, which blow from east to west across the tropical north Atlantic, from Africa to the Caribbean. Sea temperatures are at their warmest on record, and the forecast is for trade winds to be strongly enhancing, so we anticipate that the 2005 Atlantic season will be exceptionally above average.”

As they count the dead and bulldoze the wreckage in Mobile, Gulfport, Biloxi and New Orleans, it is sobering to think that the 2005 season still has six weeks to run.

Within 12 hours of hitting Louisiana, Katrina was downgraded to a tropical storm. As she raced northwards, she lost her potency and within a few days she could no longer be detected on satellite and radar screens. In her place were images of the dispossessed, shattered landscapes and economies.

She may be gone, but she will never be forgotten.

Useful websites

* National Oceanic and Atmospheric Administration: http:hurricanes.noaa.gov

* National hurricane Centre: www.nhc.noaa.gov

* CNN: www.cnn.comspecials2005hurricanes

* Wikipedia: http:en.wikipedia.orgwikiHurricanes

* Animated hurricane cycle: http:news.bbc.co.uk1hiscitech4588149.stm

* Tropical Storm Risk: http:tropicalstormrisk.com

* Caribbean Hurricane Network: http:stormcarib.com

* Hurricane information for children: www.ns.ec.gc.caweatherhurricanekids.html