Hurricane Scale Revamped for 2010

May 14, 2010

The commonly-recognized five-category rating system that describes the strength of a hurricane, and estimates the havoc its winds could cause has been updated for the 2010 storm season, which begins in the Atlantic on June 1.

The system, more properly known as the Saffir-Simpson Hurricane Wind Scale no longer includes estimates for storm surge or inland flooding due to rainfall. The scale was changed in order to ease confusion over storm surge and flooding predictions that didn’t match real-life situations, said Chris Landsea, science operations officer at the National Hurricane Center, and leader of the team that made the change.

The new scale still classifies hurricanes by their maximum sustained wind speeds beginning with Category 1 (74 mph), and ascending to Category 5 (155 mph or greater). Any storm rated at Category 3 or above is considered a major hurricane.

More importantly, the scale also tells people how strong a storm it will take in order to knock down fences, trees, power lines, or even entire buildings. The damage descriptions to homes, shopping centers and industrial buildings have been updated as well, reflecting a greater amount of coastal development.

Landsea explained, “In coastal areas of Florida, there are a lot more high-rises where the windows are susceptible to damage. That broken glass wasn’t covered at all in the old one.”

According to the report in The Insurance Journal:

In a Category 1 hurricane, shingle or metal roof coverings could peel off mobile homes, stone chimneys can topple and large tree branches will snap.
By Category 3, most mobile homes, fences and unprotected windows face certain destruction, and people should expect to go without electricity or running water for days after the storm passes.
A Category 4 storm will cause severe structural damage even to well-built homes.
Category 5 damage is catastrophic: total roof failure, blown-out windows throughout high-rises, neighborhoods isolated by fallen trees and power poles, water shortages and other widespread suffering.

In Florida and the Carolinas, however, the damage described may be less severe, because those states have the best building codes in the country, according to Landsea.

The revised Saffir-Simpson scale eliminates references to flooding, storm surge, or the amount of seawater pushed by a hurricane’s winds, and, according to Florida’s state meteorologist Amy Godsey, this simplified version of the scale also shows that forecasters have a better understanding of how storm surge works. She said the problem with the previous version of the scale was that it was extremely inconsistent with regard to storm surge.

2004’s Hurricane Charley, for example, made landfall in southwestern Florida with Category 4 winds, but brought the storm surge of a much weaker storm. 2008’s Hurricane Ike, however, made landfall just outside Galveston, Texas as a Category 2 storm, but brought a storm surge of 15 to 20 feet – a range generally associated with old-scale Category 4 or 5 storms.

Landsea said that if Ike had made landfall in Daytona Beach, Florida, its storm surge would have been only around eight feet, because the deep offshore waters of the Atlantic coast produce smaller surges than the shallower Gulf of Mexico.

Explained Landsea, “It’s like the difference between having a plate full of water and a bowl full of water. Put a fan next to them, and the water will be pushed off the plate, but the water will just swirl around in the bowl.”

Landsea says the number of variables in a storm – the size, and speed, the depth of water along the coast, and the topography of the place where it makes landfall – all combine to make the development of a separate storm surge scale impractical.

Instead, storm surge and flooding forecasts will remain in hurricane advisories and statements issued by the National Hurricane Center and local National Weather Service offices. In addition, the hurricane center is considering adding storm surge warnings to its existing list of watches and warnings. Sometimes, places outside the cone of a hurricane warning can still be vulnerable to storm surge, said Bill Read, hurricane center director. No decision on such warnings will be made for another two or three years.

The revision to the Saffir-Simpson scale took more than a year. Five wind engineers submitted updated descriptions of wind damage, and a draft was released for public comment last year, making this the first update to the scale since the removal of barometric pressure readings about ten years ago, said Landsea. Storms intensify as pressure drops, so those readings once helped forecasters assign a Saffir-Simpson category.

“Back in the ’70s it was easy to measure the pressure but difficult to measure the winds. Nowadays we have much better aircraft that fly into the winds and can measure them,” Landsea said.

Herbert Saffir, a Miami-area structural engineer, created a scale for the damage an approaching hurricane could bring, in 1969. The five-category scale was subsequently expanded to include storm surge and flooding in the early 1970s by Robert Simpson, then director of the hurricane center.

Before the Saffir-Simpson scale, hurricanes were simply described as major or minor.