Year of the hurricanes?

By Peter Gibbs
BBC Weather Centre

Hurricane Ivan is now a threat to Cuba and Florida

With hurricanes battering the Americas again, it is worth remembering that the 2003 season was also very active.

However, very few storms made landfall and so there was little media attention.

Hurricanes are natural phenomena and have been storming the globe for thousands of years.

They are important features of the Earth's atmosphere as they transfer heat and energy from the equator to the poles.

As to whether today's hurricanes are stronger and more destructive than in the past, many factors must be taken into account and it is often very hard to gauge.

Many factors

Global warming has recently been at the forefront of media attention and the likely rise in global temperatures has often been linked with the increasing frequency and ferocity of worldwide storms.

A hurricane is a spinning vortex of winds swirling round a eye of very low pressure Warm, moist air is drawn upwards around the eye Cooler dry air is sucked downwards by the low pressure centre Banks of thunderstorms surround the edges

It could be said that a rise in sea surface temperatures will surely increase the area where these storms can form, therefore it is likely to increase the frequency of such hazards.

However, there are many areas which currently have sea temperatures of over 26C but which do not spawn hurricane development.

There are in fact so many other factors which influence the development of hurricanes - the influence of El Nino, seasonal Saharan rainfall and wind shear to name but a few - that it is very difficult to pinpoint any effect that global warming might have.

What we do know is that for a hurricane to form, several conditions must be fulfilled:

• Sea surface temperatures greater than 26C

• Rapidly cooling air above

• A sufficient spin from the rotating Earth.

A hurricane's strength is measured on a scale of 1 to 5. Five is the strongest wind strength around the eye.

It is defined as a large rotating storm centred on an area of very low pressure, with wind speeds in excess of 119km/h (74mph).

A look back into the history books shows that the number of category 5 storms in the Atlantic Ocean has not increased in recent years.

Stormy season

The 2004 season started rather quietly with no hurricanes and only one tropical storm - defined as having winds of 90-100km/h (55-63mph) - in June and July.

In 2003, two hurricanes and six storms had already occurred in the same period.

August, however, was a different story: four hurricanes and four tropical storms formed, with four of them hitting the Caribbean or the US.

Storms such as Charley and Frances had long, warm sea tracks allowing them to gain moisture and energy as they made their way towards land.

With Ivan and Jeanne bringing the hurricane count to six, it could be said that after a late start, this season is surely shaping up to be a stormy one, and there are still two months to go.

Historic hurricanes

In terms of loss of life, the deadliest hurricane occurred in 1900, when 8,000 people were killed, mainly in Texas.

This was mostly due to the lack of preparedness and technical advancement in understanding such storms.

The costliest of hurricanes, however, occurred in 1992. Hurricane Andrew (a category 5 storm) swept through one of the most densely populated areas of Florida.

More than $25bn worth of damage was caused, but because of advances in forecasting, computer and satellite technology, timely warnings meant that fewer than 100 deaths were recorded.