Do we need a new way to rate hurricanes?

Behind the beast
Ocean and atmospheric temperatures work together to determine the maximum wind speed attainable. This value is known as the "maximum potential hurricane intensity" and is calculated using a formula developed in 1998 by Emanuel, the MIT climatologist.

Based on ocean and atmospheric conditions on Earth nowadays, the estimated maximum potential for hurricanes is about 190 mph.

This upper limit is not absolute, however. It can change as a result of changes in climate. Scientists predict that as global warming continues, the maximum potential hurricane intensity will go up. They disagree, however, on what the increase will be.

Emanuel and other scientists have predicted that wind speeds — including maximum wind speeds — should increase about 5 percent for every 1 degree Celsius increase in tropical ocean temperatures.

Chris Landsea, a meteorologist at the National Hurricane Center, disagrees.

Landsea believes that even in the worst-case global warming scenarios, where global temperatures ratchet up by an additional 1-6 degrees Celsius, there would be about a 5 percent change, total, by the end of the 21st Century. That means that hurricane-force winds are unlikely to exceed 200 mph, Landsea said.

The fastest "regular" wind that's widely agreed upon was 231 mph, recorded at Mount Washington, New Hampshire, on April 12, 1934. During a May 1999 tornado in Oklahoma, researchers clocked the wind at 318 mph.

Time for a new scale?
Some scientists believe that the Saffir-Simpson scale is too simplistic and that it should either be extended or replaced.

"A rational scale would have equal increments of either the wind speed squared or the wind speed cubed," Emanuel said today. "There's nothing like that [with the Saffir-Simpson scale], it's all over the place. I think it will ultimately be revised."

Other critics have pointed out that the Saffir-Simpson scale doesn't take into account a hurricane's size or the amount of rainfall.

The rains associated with some hurricanes can lead to flooding that causes just as much or more death and damage than wind.

A hurricane's size can also make a large difference in the amount of damage it inflicts. Hurricane Katrina, which was a Category 5 storm before weakening prior to landfall, caused much more damage than Camille — another Category 5 hurricane that struck in 1969. Katrina was a much larger. Katrina's hurricane-force winds extended 105 miles from its center while Camille's only extended 60 miles out.

Emanuel says a new hurricane rating system will need to have at least three numbers, describing not only wind speed, but also rainfall and storm size.

"It will also be continuous, so you can have a category 4.6 or 4.7, and it will be open-ended, so that the categories just keep going up," Emanuel said.

Saffir says: Keep it simple
Adding too many variables into a rating system would make it too complex, Saffir said. Part of the reason that the Saffir-Simpson scale has lasted so long is because it is easy for the public to grasp.

"Every hurricane is different," Saffir said today, "so you really couldn't categorize every type of hurricane as far as size and extent. As far as rainfall goes, we already have a scale for rainfall; it's measured in inches and I think that's really all that's needed."

But some critics argue that the simplicity of the scale often comes at the price of accuracy.

A new hurricane rating system might indeed become too complex for the public to easily understand, but in a way, the public doesn't have to understand it, Emanuel said.

"If you just think about it, the public's not directly involved in the decision to evacuate based on weather forecasts. In the case of Katrina, the mayor of New Orleans said, 'Get out.' It's important that the mayor and his associates or emergency managers understand the three numbers, but it's not so important that the public does."

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