Do we need a new way to rate hurricanes?

By Ker Than

updated 7:04 p.m. ET Oct. 20, 2005

In a season that has included three Category 5 hurricanes for the first time on record in the Atlantic Basin, scientists are beginning to wonder if their rating system is adequate, LiveScience has learned.

On the Saffir-Simpson hurricane scale, there is no Category 6. But Hurricane Wilma this week brushed up against where a 6 would be if the scale were logically extrapolated to include another category.

And hurricanes are getting stronger, apparently fueled by global warming. Researchers expect that trend to continue.

Kerry Emanuel, a climatologist at the Massachusetts Institute of Technology, calls the Saffir-Simpson scale irrational, in part because it deals only with wind. "I think the whole category system needs serious rethinking," Emanuel told LiveScience.

But in a telephone interview, the 88-year-old co-creator of the scale, Herbert Saffir, defended it as simple and useful for the public.

"As simple as it is, I like the scale," Saffir said today. "I don't like to see it too complex."

The history of the scale
In 1967, the United Nations commissioned Saffir, a Florida consultant engineer, to study low-cost housing in regions of the world that were prone to tropical cyclones and hurricanes.

Saffir realized there was no way to describe the effects of a hurricane, so he developed his own five-category scale. Later, Robert Simpson, then director of the National Hurricane Center, modified Saffir's work, adding measurements for flooding and storm surge.

The result was the Saffir-Simpson Hurricane Rating scale.

Hurricane guide Birth of a hurricane Hurricane facts and figures Wind-proofing your property Hurricane health and safety Hurricane tracker Worst U.S. hurricanes Ten deadliest U.S. hurricanes Timeline of worst U.S. weather

A Category 1 storm begins at 74 mph and a Category 5 at 156 mph. On average, there is about a 20 mph increment in wind speed between the categories.

An extrapolation suggests that if a Category 6 were there, it would be in the range of 176-196 mph. Hurricane Wilma, which had maximum recorded wind speeds of 175 mph, would have been on the verge of breaking into this hypothetical new category.

The scale didn't include a Category 6 for two reasons.

First, it was designed to measure the amount of damage inflicted by a hurricane's winds, and beyond 156 mph, the damage begins to look about the same, according to Simpson.

"When you get up into winds in excess of 155 mph you have enough damage," Simpson said in a 1999 interview with the National Weather Log, a publication of the National Oceanic and Atmospheric Administration.

"If that extreme wind sustains itself for as much as six seconds on a building it's going to cause rupturing damages that are serious no matter how well it's engineered. So I think that it's immaterial what will happen with winds stronger than 156 miles per hour. That's the reason why we didn't try to go any higher than that," Simpson said.

Another reason is that Category 5 hurricanes are relatively rare, or at least they used to be.

"In general, I didn't expect that there would be too many hurricanes that went [above] 155 miles per hour for sustained winds," Saffir said. "The limit seems to be about 175 miles per hour and I don't know of anything that goes much over that."

Some scientists predict, however, that the intensity of hurricanes and their maximum wind speeds may be increasing and that Category 4 and 5 storms will become more common in the years to come.

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