Marilyn (1995): Landfall can bring devastation

Until now forecasters have been able to predict the path of hurricanes with increasing accuracy, but their strength has often been unknown until landfall.

This is because the the factors controlling the intensity of a hurricane have been poorly understood.

Now Dr Kerry Emanuel, from the Massachusetts Institute of Technology, says the strength of a hurricane depends on three things

• the initial intensity of the storm
• temperature changes in the air
• heat exchange with the ocean

His new forecasting model looks at the thunderstorms surrounding the eye of the storm, and the hurricane's interaction with the ocean.

Once the path of the hurricane is known, Dr Emanuel says he can now forecast its strength reliably.

"We think we can understand most of the physics that are critical to the intensity of hurricanes," he says.

Significant effect

With a computer program he has designed for use on ordinary PCs, Dr Emanuel says the strength of storms "can be predicted as far in advance as an accurate track prediction can be made".

Floyd was one of the strongest Carribbean hurricanes since records began

Hurricanes gather energy from warm water at the ocean surface, he says. Cooler water churned up in choppy seas takes the strength out.

Even a 1C drop in water temperature can have a significant effect by decreasing the amount of evaporation into the eye of the storm.

Previous, less reliable methods of forecasting hurricane strength did not take the ocean-atmosphere exchange into account, according to Dr Emanuel.

Accurate reassessment

The model has been welcomed by Hugh Willoughby from the National Oceanic and Atmospheric Administration (NOAA).

He is impressed by its accuracy in assessing - retrospectively - the maximum wind strengths of Hurricanes Hugo in 1989, Andrew in 1992, and Opal in 1995.

The impact of Hurricane Mitch

Hurricane Opal provided a good example of the meteorologists' inability to predict the strength of a storm. After blowing up suddenly, Opal killed nine people and caused $3bn worth of damage.

Hurricanes are tropical storms brought about by heavy evaporation of water from the ocean's surface. The hot wet air is sucked into a tropical depression, and rises in a spiral in which winds can reach 300km/h (200mph).